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The optimum air/fuel ratio for car engines is 14.7:1, more gas doesn't always equal more power. One of the most common mistakes on any high-performance motor is to over-carburete. Luckily the stock 240Z Hitachi SU is a great carb for both street and performance and automatically meters the gas depending on the airflow through it. So whether you are stock or have a cam with large duration, with the proper needle the SU carb tries to keep the correct mixture from idle to redline. A simple, accurate design.

The "good" SU's were from 1970-1972 and will have 4 or 3 screws holding down the domed piston depending on which year it was made. The year makes no difference for performance as the nozzles and needles from different years interchange. If you have carbs with the square choke flap in the inlet or round (NOT Domed) flat tops then you have poorly performing (can one say "smudge pot") emission carbs. Exchange them for earlier SU models or use other options.
For street performance and autocrossing stick with SUs.


The '70-71 240 used "N27" needles and related nozzles.

The '72 manual trans car used N54 needles, the '72 automatic used N58, both with the same '72 nozzles.

The '73 nozzles/needles were emissions designs, not recommended for performance.

                    SM needles from British SUs are excellent for performance.

Always mount an electric pump as close to the gas tank as you can. It's better to "push" the gas rather than "pull".

- Remove the little copper mesh filters in the SU float bowl inlets. These can restrict flow.

- Retain the water cooling through the intake manifold.

- Don't block off the PCV hoses. It's important that the crankcase be vented to the intake so the motor stays clean. Using a little K&N breather on the valve cover is fine but can stink the car up with fumes if you have ring blowby.

- On 240s keep the "flow-valve" on the driver fender-well connected. This pulls fumes from the gas tank and burns them in the carbs.

We've all spent afternoons syncing the carb airflow, topping up the damper oil and making sure the jet needle/fuel nozzle levels are set properly (2 - 2 1/2 turns down, etc). But how many of us have checked to make sure the gas float levels are identical on each carb? This is actually a very big deal. If the fuel levels are different in each float bowl, even a small bit, one carb will always be lean or rich compared to the other.

The issue is that the SU uses a combination of incoming gas and the float closing the fuel-valve to create a steady fuel level at the tip of each fuel nozzle. If this fuel level varies between carbs then its also at a different level on each jet needle.

This has been covered in several places online. Be careful since you're dealing with gasoline(!) Mark the front of the float bowl 23mm down from the top edge of the bowl (not the top of the lid). These measurements are done with the carbs on the car.

Get some small diameter clear plastic hose from the hardware store, cut a piece about 18" long.

Connect one end to a float bowl's outlet and run the other end up over the carb. Some people connect the hose to the drain plug on the float bowl but the fuel hose outlet is a lot easier.

Bring your fuel pressure to what it would be while idling. This means if you have a mechanical fuel pump turn the engine over a few times with the coil wire off. With an electric fuel pump just turn the ignition on to pump up the fuel pressure.

The fuel level will equalize in the hose and should line up with the 23mm mark on the float bowl. By sighting the level in the hose and bending the tab on each float you can get the float fuel levels to match within a millimeter.

                                                 Carb fuel level  001.jpg (67038 bytes)  << (Click to enlarge) 

HOWEVER: Even though adjusting the floatbowl levels to 23mm down is a factory setting, it doesn't guarantee that the fuel levels in the carbs are the same.

TIP: We did some measuring with a straightedge and found that on my 4-screw SUs a 23mm fuel level in the float bowl corresponds to exactly 1 centimeter (10 millimeters) below the fuel nozzle "ledge" in the carb. But how do you see to measure fuel 1 centimeter down inside the fuel nozzle? See below.

Remove the dome and main piston from each carb so you can look down the tip of both fuel nozzles.

Screw each mixture nut exactly 10 turns down from fully up. Each full turn drops the nozzle tip 1 mm, so 10 turns puts the tip of the fuel nozzle 10 mm (1cm) down....which happens to be the 23mm float bowl level.

Then look down the tip of each fuel nozzle and adjust each float to set the gas level at the fuel nozzle tip.

We ended up taking 9 turns down and set the fuel at the meniscus of the tip. Then when I went to 10 turns the fuel was exactly at the tip. A bit of a juggling act but after a few minutes you will get the hang of it. When you get them to match it's a great feeling.

NOTE #1:
If your mixture nut won't go down more than a few turns, it's because the factory "stopper" next to the nut is still in place. Unless originality is a concern, you can permenantly remove both stoppers from the bottom of the carb.

if the fuel level is too high and overflows the nozzle tip, you will need to drain each float bowl a bit before starting again.

The result was a lower, more solid idle and less choke needed at startup. I get no stumbling off idle and part-throttle acceleration is smoother, acceleration more powerful. Cruising on the interstate requires a lighter touch on the accelerator. Interesting.

"Tuning" and adjusting SUs:

Centering the jets: The metering needle and jet must not come into contact. If they did, wear would surely result to these precision parts, which should stay precise! Watch the centering of the carb jets.

"Synchronizing" the carbs. This impressive sounding term, which sounds as though it was borrowed from NASA, is the source of much confusion, profit to purveyors of unusual looking "special" tools, and more discussion than British weather. I don't see the big seems intuitive that the two carbs need to be doing the same thing at the same after assuring that the linkages act upon the throttles at the same time, the carbs should be dynamically "synched" (an even more impressive term!), that is, throttle idles adjustment screws should be adjusted such that at idle, the induction of the carbs is as close to equal as possible. This will result in a nice even, smooth idle. A number of devices are available to help with this equalization, and I too own a couple of them...the Unisync and Tuning Tool Set...but I find that these NEVER make it out of the toolbox anymore! I prefer instead to use my own original equipment for tuning...and I have this with me ALL THE TIME! I use my eyes and ears! I look at the height which the dashpot is raised, and I might stick my head down between the two carbs to better listen and compare the wonderful sucking noises.

For more "Depth" see below... "Ready to Really Tune and Synchronize Your Carburetors!"

Hypothetical questions: What's the difference between a visual indication by a little indicator button being sucked up the tube of a Unisync, for instance, vs. the dashpot of the carb being sucked up? Why would you trust a separate indicator tool more than the actual carb?) Just kidding .... I resort to the above then verify with my trusty Unisync Tool. Strangely I must be blessed... ....I’m usaly spot on!

Synchronizing Tools

Unisync..... A little red "float" is sucked up the graduated indicator tube as a function of the amount of air pulled past its own variable venturi.

Maintenance SU carbs, once set for satisfactory operation don't really need a lot of attention...sure the linkages should be clean and (must be!) lubed, and the damping oil should be checked occasionally. I also find that a half turn richer during the cold (and flu) season also gives better throttle response, but owner/mechanics who insist on needlessly fiddling with their SUs should consider getting a girlfriend, or maybe taking up video games, or knitting, or some other hobbies less harmful to the way their vintage Datsun Z runs.

Dampener Oil or Dampening or Damping Oil: Function of the damping oil is three-fold. First, to lubricate the precision fit and the never-ending movement of the dashpot. Second, to dampen rapid pulsing of the dashpot at low RPMs, and thirdly, to slow dashpot response when throttle opening is increased, effectively enrichening the air-fuel mixture (an accelerator pump effect).  

                                       (see our offering ) >>>     SU Oil Label 1.5.jpg (10439 bytes)

"Topping up" is a misleading term...topping up suggests that the reservoir should be filled a lot more than it should be...the damper tube filled to the top is way too full...about a thimble full should be enough (or a teaspoon & a 1/4, a less than a quarter of a shot glass etc. ...whatever's handy!)...the important point is to to have the one-way-valve (those brass parts on the bottom end of the rod coming down from the Screw-On cap) totally submerged in it at all times and at all vertical locations of travel as the dashpot moves. And to prevent those telltale "blood-spots" on the underside of the hood from overfilled oil shooting out of the vent, I just snug on the modified Screw-on cap, hold a rag over the vent-hole, and (air filter removed) lift the dashpot as high as it will go...excess oil will be shot out into the rag...if none shoots out, I add just a bit more and repeat until it does (kind-of like: lather, rinse, repeat...except: add oil, check for squirt, repeat)...and by checking the damping oil level in this manner, we are also checking for probably the most important function of an SU carb: The free motion of the dashpot throughout its entire range of travel.

Common SU problems:

Overflowing fuel bowls The fuel feed is a low pressure (2-3.5 PSI) system with a simple float and needle valve controlling the flow into the fuel bowl. Unfortunately, and more often than just occasionally, almost every owner has seen the bowls overflow (caused by contaminants or possibly moon phase, holding open the bowl valves)'s hell on fuel mileage when this is pumped out onto the roadway, to say nothing of if some of the vapors are ignited by sparks at the brushes of the alternator etc.

Fuel feed pressure, that the carbs are exposed to, is (indirectly) lowered by spacer under the mechanical fuel decreases the excursion of the pump-lever by the camshaft elliptic, so it is critical to replace this spacer when replacing fuel-pump. Caution!...spacer is made of brittle bakelite. When installing a fuel pump, torque fuel-pump bolts evenly, and do not over-torque to prevent cracking this spacer.

Fuel flow blockage due to blocked jet tube. Even with a pre-pump and post-pump fuel filters installed, I've seen enough miniscule particles of (???p) in the bottom of the fuel-filter, that I don't know for certain, but swear there was enough contamination to affect the (vacuum motivated only) fuel delivery, as the vehicle exhibited fuel delivery symptoms only at high fuel demands, and the symptoms were cured after replacing the filter/s.

False air: Worn throttle shafts can cause "false air". False Air is defined as air which enters the engine by a path other than through the carb and past the venturi, where it is "measured" such that the corresponding correct amount of fuel may be dispensed. High mileage SUs typically develop wear-gaps when the shafts bear and wear against their light alloy bodies. This allows false air to be ingested. False air causes a lean mixture at idle...less so at elevated RPMs, it also causes inconsistent idle and difficulties in tuning (typical compensation is to richen up the mixture at idle).

I recently bought a quality carb rebuild kit which contained pair of long/short valves for each carb (with no mention in the instructions). Different year SUs came with different length float "ears", but which length valve went with each year carb and why? The stock valve is labeled 1.7.

Our stock 1970 SU's did have a 2mm longer valve in the front and short one in the rear. But the valve lengths aren't mentioned in any of the Datsun manuals and when we spoke to several informed sources I got conflicting answers. Some people suggested I use short valves in both carbs but couldn't explain why. It surprised us that on a 40 year old car that no one knew the answer to what must be a common question.

The real reason: It's because of the direction that the float arm pivots between the front and rear carbs. Under acceleration gas flows away from the needle in the front carb but towards the needle in the rear carb. The factory determined a need to compensate for this, too bad they never shared it with us. Regardless of which ones you use, the more important issue is to make sure afterwards you get the fuel levels at the tip of each fuel nozzle as mentioned above.

Use a K&N filter in the stock 240 air cleaner. This keeps all the PCV hoses and vacuum lines active, reduces intake noise and gas smell, and makes the engine bay look stock.The air horns on the 240Z baseplate are actually better than aftermarket airhorns and flow more than the carbs require so no anxiety is needed when using the stock housing. Our second choice would be the individual round K&N filters that use nuts to hold the filters on. If you want to repaint your 240 air cleaner use "Chevrolet Orange" engine enamel (it's close in colour). Available from the auto parts store.

Other SU Upgrades:

Polished Dashpot domes. Shiny is always faster!!!

"Velocity Stacks": Velocity stacks decrease the turbulence of the air ingested by the carbs...and presumable improve the atomization at the venturi...air filters should still be prevent sucking in random debris (with decreased turbulence)! Stacks really only make a measurable improvement in the upper RPM range and wide open throttle (WOT), at low revs, so it seems to me that they follow the "silicon breast-implant theory" explanation of their existence (don't do much, but look great). 


The gasoline sold in the Northeast US and California is formulated differently than fuel in the rest of the country. While leaded fuel was phased out in 1996 in the United States, little is said about the widespead use of "Reformulated Gasoline".

RFG contains a high percentage of "oxygenates" which allow fuel to vaporize more quickly, especially in cold weather during engine warmup, to reduce emissions. Mandated by the EPA in the early 1990s, it was introduced quietly and began appearing at gas stations in large metropolitan areas...which had higher air pollution. The first time I filled up with RFG in 1994 I remember it had a peculiar acetone smell. MMT used to be a key oxygenate but is now mostly illegal because it started showing up in public water supplies. Ethanol is now the common additive, listed as a maximum of 10%.

Not mentioned is that RFG contains less energy than "old" gas because of these oxygenates, hurting gas mileage by about 3+%. This also means less horsepower. The EPA said years ago there is "Summer" and "Winter" RFG which is rotated in and out of circulation starting about October (transitioning to Winter) and May (switching back to Summer). Summer RFG has the lowest amount of oxygenates and most "power" while the Winter gas has the most oxygenates and poorer performance because it vaporizes quickly. This means if you're running Winter RFG and suddenly get a warm day you can find your engine pinging, so don't dyno or race your car on Winter RFG.

Interestingly, RFG doesn't produce chocolate-tan spark plug deposits the old tune-up books have been showing for decades. Instead, plugs can tend to have a white appearance as though they are running lean which makes plug reading more difficult. We have also heard a rumor that RFG gas pings (detonates) very quietly compared to old gas which tended to sound like there were marbles in the cylinders. Push 10:1 compression with aggressive timing on your Z and never hear pinging should worry some. I'd don't want to push my engine into detonation to prove RFG pings silently but I believe it to be true.


If you are using carburetors you can remove the stock mechanical fuel pump from the head to reduce drag from the cam and replace it with an electric pump. There are two types of aftermarket fuel pumps: Rotary Vane, and Oscillating. The square Purolator/Facet brand electric pumps found in auto stores are the oscillating type. Reliable I'm told, but the design is very noisy with a loud tapping that's audible through the rear deck, especially if you have a quiet exhaust, unless you use quality rubber insulators. A rotary vane pump produces a quieter whirring sound and is the style used on newer cars.

Our rotary vane choice is the Carter CRT-P4070. It can produce 72 gallons per hour and 6psi max and is considered very reliable which is the goal. The perfect spot for it is just under the mustache bar against the brace that holds the differential. Pushing fuel through a stock 240Z fuel rail, this pump produces about 8 psi at the carbs, while the factory pressure required of SU carbs is 3.5 psi. Even though it seemed to run fine we didn't like the mismatch so we have recently installed an adjustable Holley fuel pressure regulator between the pump and fuel rail to dial down the pressure to 5psi. With the extra fuel demand of a L28 I think the tiny extra bit of pressure will help keep the fuel supply up without overpowering the valve needles. We have to admit it's a satisfying feeling to dial in my own fuel pressure by using an allen wrench.

2010 NOTE: this pump appears to have been replaced by the GP-4070 which Carter calls a "competition series" pump. The specs and design appear identical to the CRT-P4070. One wonders what qualifies the GP-4070 as a competition pump and the P4070 not?

The balance tube between the intake manifolds performs the simple task of creating a spot for the brake booster hose, vacuum hoses, and PVC hoses to vent to. We're not sure why people think blocking it off from the manifold will somehow increase horsepower, all that does is ruin the brake vacuum. Additionally, you want the PCV hoses to vent into the balance tube to keep crankcase by-products and blowby from dirtying up the block. Make sure the gasket under each end of the balance tube is sealed tight as a drum as they get squashed after 30 years and can leak air into the manifolds. Permatex makes a great roll of gasket material you can use to make cut your own.

How we are .....Ready to Really Tune and Synchronize Those Carburetors!

Once you have completed the pre tune check list and corrected all problems you are ready to tackle the intricacies of tuning and synchronizing the multiple carburetor setup on you Z. How difficult is this? Not difficult at all, even for a first time novice you should be able to precisely tune your carburetors in 15 minutes or less - only IF you have completed the pre-tune check list and corrected all problems.

If you are adjusting an existing carburetor set.

If the engine is running reasonably well you may skip these first steps. If you are having problems or if the car has sat for a long time first set the carbs to a base line. Remove the dashpots and pistons. With a soft cloth and some carburetor cleaner clean the deposits from the inside of the piston chamber and the both the small and large diameters of the piston. Clean the steel rod in the center of the piston and the bore inside the piston chamber. DO NOT USE ANY ABRASIVE such as sand paper - never sand blast or glass bead! Lightly coat the steel center rod with WD40 or similar light oil. The piston and side wall of the chamber should be clean and dry. Be careful to avoid bending or damaging the needle - avoid removing it, they bend easily and often stick in the piston.

Raise the jet until the top of the jet is even with the bridge. Back out the idle adjusting screw until it is clear of its stop, turn it back until it just touches the stop and then 1 .5 - 2 turns. Replace the piston, spring and dashpot being sure to put the same piston back into the dashpot that it came out of - do one at a time to be safe.

Raise the piston using the lift pin on the underside of the dashpot mounting flange, when released, the piston should drop cleanly to the bridge with a distinctive 'clank'. If the piston does not drop cleanly you may need to center the jet. If the piston drops cleanly lower the jet adjusting nut 2 complete turns. You now have a base line adjustment and are ready to tune your carburetors.

After you have a base line or if installing new carburetors

Bring the engine to normal operating temperature.

Loosen the screws on the interconnecting linkage - each carburetor should operate independently of the other disconnect the choke cable loosen the cable stop screw.

Using a tachometer, adjust the idle speed to factory specifications. Make your adjustment incrementally, turn the adjusting screw 1/4 turn at a time, adjusting each one equally.

Raise the jet adjusting nut, on one carburetor at a time, until the engine RPM just begins to drop. Depending on where your base line settings were you may first notice a rise in RPM before it begins to fall. Lower the jet adjusting nut until the RPM begins to rise and then 1/6 turn (one flat).

Readjust idle to manufacturers specifications.

Test fuel mixtures - using the lift pin on the underside of the dashpot mounting flange, press up until you feel contact with the piston and then lift slightly (specs are 1/32 of an inch - good luck). If the engine RPM increases when the pin is lifted, and remains elevated, your mixture is rich - raise the mixture adjusting nut. If the engine RPM drops when the pin is lifted your mixture is too lean, lower the mixture adjusting nut. If the engine RPM initially raises as the pin is lifted, and then settles back to the original RPM or very slightly above, your mixture is right.

Readjust idle speed if necessary. To synchronize the carburetors "redneck style" use a length of fuel hose. Hold one end of the hose just in front of the carburetor inlet - not so that it obstructs air flow - and listen to the 'hiss' of the air as it enters the carb. You will easily notice any difference in airflow by the intensity of the 'hiss'. Alternatively reduce the idle speed of the 'fast' carb and increase the idle speed of the 'slow' carb until both produce the same 'hiss'. Evenly adjust both idle speed screws to reach the desired engine idle speed. Recheck fuel mixture, and carburetor synchronization. I verify with a synchronizer tool.

Adjust the pegs on the ends of the interconnecting linkage until there is approximately 1/16 inch clearance (free play) before the peg contacts the slot on the carburetor tab. Tighten all screws on the interconnecting linkage. Tighten the cable stop, accelerator linkage and choke cables.

Pull the choke until the jets are just at the point of moving but have not moved. Push the choke back in - put on your air filter/filters - close the hood - You are done!

Assuming that you have followed the instructions of the preceding, your should now be enjoying the renewed performance and economy of your S.U carbs. The fact that you are reading this page.... further...... maybe things have not gone quite as planned. Below I have listed some of the most common problems, and how to correct them. If these do not cure your problem please do not hesitate to call me. Even if you did not purchase a set of carburetors from me, I am an enthusiast and hope to help you get your "Z" running to its highest potential.
NOTE: I realize that inspite of my tendency (tendency???!!!) to be a bit longwinded ....I base all of the following on what has been brought to my attention to be my four (4) decades + of working on these little beasts in one variant or another. You may notice my growth and use of wit to keep most of the marbles in "their row. With that .....Do well.....

Fuel flows out the vent pipe or pours out the jet

If fuel flows out the jet but not the vent pipes make sure the vent pipes are clear. Clogged vent pipes prevent venting above the fuel level in the float bowls. As fuel fills the bowls the air above the fuel is compressed, the pressure on the fuel causes it to be 'injected' out the jet. This is an uncommon problem but it does happen.

Fuel flowing out the vent pipes and trickling out the jet indicate a malfunction in the float/float valve assy. The most common cause is debris becoming lodged in the float valve (needle and seat) - this is in turn most generally caused by debris in the fuel line and/or fuel tank. Install a fuel filter as close to the carburetors as possible, use new fuel line after the filter. If you feel comfortable doing so, remove the float bowl lid, pull the float hinge pin, shake the float to see if it has fuel inside - if so you need a replacement float - now remove the needle from the float valve and wipe the tip with a towel - being sure to remove any debris. Blow compressed air or spray carburetor cleaner into the float valve seat and out the fuel inlet. Repeat until you are certain all debris is removed. Reassemble.

Fast idle - even with screws backed completely out

Make sure linkage is not holding the carbs open.

The most common cause of fast idle is vacuum leaks. Spray all joints and connections in the intake manifold with carb cleaner. If you notice an increase/decrease in RPM you have located a vacuum leak which must be repaired. Also check all vacuum hoses and the vacuum canister on the distributor.

Make sure the fast idle screws are clear of the fast idle cam.

Loosen the screws on the inter connecting linkage, one carburetor can easily hold the other carburetor open. With the linkage loose press down on each idle screw to make sure it is completely home. Infrequently the new shafts may be stiff in rotating (I work hard to make sure this does not happen), if this happens to be the case you may wish to add a stronger or additional springs.

Overly advanced ignition timing can cause fast idle as well, check and correct as needed.

One carburetor draw far more air than the other.

Make sure the pistons are moving freely, and when dropped hit the bridge with a distinctive clank.

Follow the instructions in the tuning guide for establishing a base line, make sure linkage is loose and reset idles making sure that the fast idle screws are clear of the fast idle cam. Synchronize as per previous instructions.

If after resetting idle, one carb still appears to draw more air, you most likely have an internal engine problem. Recheck valve adjustments, a valve not opening or not closing will effectively eliminate air flow through that cylinder. Check compression, very low compression will significantly reduce the air flow for that cylinder. Observe, and if possible measure, valve lift - a worn cam lobe will prevent the valve form opening completely and cause significant or complete reduction in air flow.

Backfire I.E. the motor fart!!

If most pronounced on acceleration check mixture, a lean mixture may cause backfire. Check also for vacuum leaks, a vacuum leak will lean the mixture. Check the level of the dashpot oil - use specific SU dashpot oil. The piston rising without restriction prevents enrichment necessary for acceleration.

                         Our Offering >>>     SU Oil Label 1.5.jpg (10439 bytes)

Ignition too far advanced or retarded - ignition occurs while valve is still open. Wrong firing order If static ignition timing is correct check again with an advance timing light.

Re check compression, use a bleed down tester if available, a burnt or non closing valve will allow burning fuels to ignite fuel in the intake stream.

Too rich mixture even with jet all the way to the top - each of these problems remedied in restored sets, included for reference only .

The shoulder of the needle should be even with the face of the piston - place too deep into the piston will cause rich running.

Wrong or missing piston spring will allow the piston to rise farther than intended. Too little clearance in the piston to dashpot will raise the piston farther than intended.

Float level too high.

Worn needle or jet - incorrect needle or jet .

Choke stuck open - jet stuck in bearing, not returning.

And now a few words on carb fluid. I've found the level and type of fluid in the carbs to be of the utmost importance. What concerns me is not the fact that it's critical, but the fact that it's seldom mentioned. I've corrected some pretty major-appearing drivability problems, just by adding the correct "Genuine S.U" oil to the carbs. I've heard of people running the carbs dry, and others who used water. I can't vouch for their success, only my own.

After passing this on countless times via email, I figured I'd post some info on setting the needle position. Many of these cars have been around the block a few times, so often times, the carbs are way out of whack - too far to straighten out with the above procedure. F'rinstance, when I got my latest "70" 240Z, the rear carb was about five turns out, whereas the front carb was at around two. Yikes! So, when you encounter something like that, or when you just can't get them lean enough, try this:


If you haven't noticed by now, SUs do need periodic, if simple, maintenance. Among the checks required to ensure top performance is the inspection of the damper oil levels.

The damper (the black plastic screwy thing on the tops of your SU domes) acts somewhat like a shock. The damper/oil combination helps to regulate the piston movement under driving conditions. With proper oil levels the dampers will be completely immersed in oil. You are here now because those oil levels are not correct or you'd like to experiment.


The main symptom of improper damper oil level is an off-idle/low-speed stumble in acceleration. This happens because, with oil levels that are too low, the distance between the oil level and the damper allow the piston to flutter up into the bore. This premature lifting of the piston disrupts the metered air and fuel flow into the engine and gives you a stumbling/lean acceleration. Upper rpms are not affected because the piston has raised enough to cover the damper in the remaining oil.

Inspection Interval:

The necessary inspection period for damper oil levels depends on your driving style. If you do mostly freeway commuting you can lengthen the inspection interval. If you have a heavy right foot and/or do a lot of driving in traffic, you'll have to inspect the oil levels more frequently.

The Datsun Factory Manual specifies an approximate inspection period of 3,000 miles, or every three months. I would say this is appropriate for the average driver in average conditions, and it also coincides with a typical Datsun owner's engine oil change. If, however, you're on the harder side of use, I'd recommend a monthly check of oil levels.

The easiest way to check the oil levels is to first remove your air filters. Raise each piston up into the bore with your finger. You should immediately feel resistance and it should start at the same point on each carb. If you feel any "free play" as you raise the piston (before the oil is raised in contact with the damper), then the level of oil in that carb is low.

Damper Oils:

(Also see Advanced Techniques, below)

There are as many different suggestions for what type of oil to use in your SU dampers as there are oils. Datsun recommends using actual "Damper Oil" not engine oil. (with good reason)...I've tried using motorcycle shock oil and ATF but I found that it was too light and didn't provide enough dampening. This was only evident after installing my air/fuel ratio gauge. This caused my air/fuel ratio to go lean whenever I opened the throttle. The proper "Damper" oil has cured that.

Adding Oil:

Datsun specs list the total volume of oil (per damper) at a teaspoon & a 1/4. If you don't have a way to accurately measure that amount of oil, you will just have to be aware of two concerns:

Use equal amounts of oil in each damper,

Have the same weight of oil in each damper

Always use "Genuine S.U" oil.

                                   SU Oil Label 1.5.jpg (10439 bytes)

You will have to remove the SU damper, dome, and piston spring from the carb body. You should only work on one carb body at a time so you won't mix up parts between carbs. Always keep each SU dome with its corresponding piston on the same carb body.

After you have removed the piston from the carb body, pour out any remaining old oil. Dispose of the oil properly according to your countrys/state's enviromental laws. Slowly pour the oil into the piston until it reaches approximately 2/3rds of an inch from the lip. Test-fit the damper all the way into the piston. I fill the oil until I can put the damper in all the way without spilling any oil. You'll get the idea when you try it.

Advanced Techniques/Observations:

Since I installed my Air/Fuel gauge, I have been able to see that the ATF fluid I'd been using was far too light a weight oil for my application.

For example, under cruise conditions I was getting a fairly rich reading on the air/fuel gauge. Any increase in throttle gave me a lean condition immediately. This was because the oil in the dampers was not heavy enough to keep the piston from opening rapidly. The sudden increase in vacuum from the motor draws up the SU piston (which allows more air into the motor). With a bigger hole from which to draw air through, vacuum over the jet bridge does not rise as quickly to pull more fuel into the stream (despite more air getting into the engine = lean air/fuel ratio). The low vacuum doesn't allow enough fuel to be sucked from the jet to compliment the increased air flow.

Then I purchased some motorcycle shock oil. Replacing the ATF with the oil somewhat improved the air/fuel readings. Then I found through my British contacts "The REAL Stuff’ and now even with heavy throttle application the fuel mixture does not go lean. Obviously the correct oil is not allowing the piston to rise as fast as it was before. The seat-of-the-pants differences are not overly dramatic, but at speed (say, on the freeway, and at higher revs) you can feel a little more urgency out of the car. Whatever the other benefits, the mixture is not going off-the-gauge lean as it was before, and this is good for my peace of mind. Without the air/fuel gauge, however, I wouldn't have known I had a problem til the engine gave up from the lean burn... can one say holes in the pistons...smoked ring lands... or..valve issues!!!!.

What change I have noticed has come mainly courtesy of the air/fuel gauge. Yet I’ve noticed much more of a change with time....good stuff. During this past weekend I noticed my air/fuel gauge going perfect under acceleration/load. This was most noticeable as I went up a few grades on my way up to Crater Lake (high altitude conditions). It wasn't terribly warm that morning, so it would seem that the proper damper oil is a must (at least with my motor),....... where I'll stay.

Tuning SU Carburetors.....One-Oh-One

I’ve been meaning to write this up for some time, ever since I did the SU Performance Tuning 101 a few months ago. This one is more like Basic SU Adjustment for Happy Driving.
The trick to tuning SU carbs is to understand that there are two things you need to get right: the air flow, and the fuel mixture. While they are interconnected, they are also independent, and need to be measured and adjusted independently.

Specialty Tools

You will probably need to arrange to buy or borrow a Unisyn flow meter. The Unisyn is the usual gauge for getting the air flow balanced between the two carbs. This is simple to use. It consists of an adjustable opening (same size circumference, but with a disc on a threaded rod that you can screw tighter or looser) that you use to set the level of a little float that rises or falls in a glass tube at the side of the gauge.

For the fuel mixture, I have become sold on a device called the Gunson ColorTune (or ColourTune, as it’s a British co.). This is a spark plug with a crystal pressure- and heat-resistant window in it that lets you see into the combustion chamber while the motor is running. The color of the flame indicates the mixture richness. While it’s not absolutely essential, it makes life so much easier that it’s worth the cost.

If you don’t have a Gunson, I’ve included the standard directions here for determining correct mixture (step 4 of the Adjusting Mixture procedure).

To tune SU carbs, first locate the following components:

Throttle linkage. These are the things that connect the throttle linkage (the bar connected to your foot through the means your car uses, rods) to the carburetors’ throttle levers.

Throttle stop screws. These set the idle speed for each carb, and are located typically behind the dashpot, on the same side of the carb to which the throttle linkage connects.

Mixture adjusting nut. This is the lower of the two nuts at the very bottom of the carburetor. SU carburetors have integral float chambers, on which the mixture is adjusted by turning a large nut. You’ll need to experiment (and I explain how) to see which way makes this richer and which way makes it leaner.

Lifting pins. These are little wobbly metal pins under the dashpot. When you push up on the pin, it raises the piston in the dashpot. Find these; they’re crucial if you don’t have a Colortune.

The bridge. This is the part inside the carburetor, where the gas jet opens into the airstream. You’ll see a needle inside the jet, and the jet itself should be a few fractions of an inch down from the bridge itself. The jet is the brass tube that sits in the center of the bridge, with a tapered needle poking down into it.

The choke linkage screws. Comparable to the throttle linkage screws (and usually the same size), but on the linkage that goes between the choke cable and the mixture adjustment mechanism. They make sure that both carbs are enriched when you pull on the choke.
Balancing The Air Flow

1. Start with the engine warmed up to operating temperature and perform your standard ignition tune-up (new points & points gap, new condenser (if so equipped), timing, spark plug gap, etc.) top off with "Genuine S.U" oil first. If you’ve got a timing light and a dwell meter, you can verify all is correct. When it’s warm, shut the motor off and remove the air filters.

2. Begin by balancing the air flow. To do this, first loosen the throttle linkage screws. Leave them connected, just loosen them.

3. Back out the throttle stop screws till you can see that they are just touching the throttle stop. Then open each carburetor (that is, lower the throttle stop screw) 1-1/2 turns of the throttle stop screw and start the engine. It will probably idle at about 2000 RPM; don’t worry.

4. Put the Unisyn over either carb and adjust the orifice in the Unisyn till the little float at the side rests at the middle of its graduated tube. (Pre-diagnostics: if the idle drops and the car wants to die when you slap on the Unisyn, the carb is too rich; if the idle soars upwards, it’s too lean.) Hold the Unisyn over the carb for only long enough to see the level of the float, then remove it.

5. Place the Unisyn on each carburetor in turn to check its flow, adjusting the throttle stop screws until both carburetors register the same position on the graduated tube of the Unisyn. (The float will probably move either up or down in the tube, which is why you want to center it in Step 4.) When both carburetors flow the same amount of air, tighten the throttle linkage screws, adjusting for the amount of free-play between the linkage and the throttle stops that your manual calls for (probably about 0.006�?). Your goal should be to achieve the lowest possible idle with both carbs balanced and the engine running smoothly. (Note that the idle speed will very probably rise as you get the mixture correct.)

If you’ve taken more than five minutes to do this, rev the engine to over 2500 RPM (assuming the idle isn’t already that high) for thirty seconds or so to clear the spark plugs. Then adjust the mixture.

Adjusting The Mixture:

Note: in the following procedure, one "flat" is the basic increment of adjustment, and refers to 1/6 of a turn of the mixture adjusting nut. This corresponds to the flat faces on the nut.

You can — with the motor shut off — peer down the throat of the carb and turn the mixture screw while watching the top of the jet. Remember that moving the top of the jet up will lean out that carb, while moving the top of the jet down will richen it.

1. Shut the car off and loosen the choke linkage screws.

2. Adjust the mixture nuts fully lean.

The idea is to zero out the jet: raise it all the way up in the bridge.

3. Now drop the jet an equal amount — two full turns for carbs, Then start the car.

Note: In the following step, you might want to consider adjusting the carburetors one-half a flat too lean, as the mixture will be enriched when you put the air filters (which restrict air flow) on at the end of the tuning process.

4. Raise the lifting pin so that the piston rises no more than 1/16�?. Listen to the engine’s exhaust note and compare it to the following conditions:

If the exhaust note rises and stays high till you drop the piston, this carburetor is adjusted too rich. Turn the mixture nut one flat (one-sixth of a turn) up, moving the jet toward the bridge, then repeat Step 4.

If the exhaust note falls and the car sounds as though it is going to stall, this carburetor is adjusted too lean. Turn the mixture nut one flat (one-sixth of a turn) down, moving the jet away from the bridge, then repeat Step 4.

If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carburetor is set correctly. When you have achieved this setting for both carburetors, continue with Step 5.

5. Tighten the choke linkage screws so that the choke cable will pull an equal amount on both mixture nuts when you pull the knob.
6. At this time, I find I usually have to adjust the idle again because getting the fuel mixture right usually changes the idle speed. Since you know you have the throttles synchronized, I normally just adjust the idle without loosening the throttle linkage. The easiest way is to screw one of the screws out till it doesn’t’ even touch the throttle stop, then use the other to get the idle speed right. When that’s done, you can screw the other stop screw down till it just touches the stop on that carb and you’re set.

7. Replace the air filter/s and go for a test drive!


SU carburetors are most fuel-efficient when slightly lean, and provide the most power when they are slightly rich. You can use this knowledge to provide a certain amount of tuning for the kind of driving you do. If you learn to read spark plugs, you can get a basic idea of what your engine’s condition is and make fine adjustments to the mixture nuts accordingly.

If you have a ColorTune, you simply install it in place of one of the plugs, then adjust the carburetor that feeds that cylinder (the front carburetor for 1,2 & 3, the rear for 4,5 & 6). The ColorTune will let you see the color of the flame. White flashes mean too lean; yellow flame means too rich. Blue (like a Bunsen burner) is correct, and blue with a faint orangish tinge is the best for power.

If you highly modify your engine, you will probably need to modify your needles, as it is the needle profile that determines the mixture curve for different air-fuel loads.

If you experience uneven idle, hunting, or an idle that changes (rises or falls) as the engine’s temperature climbs or drops, you probably have vacuum leaks. The most serious fault on most old SUs is wear in the throttle shaft area. To test for this, spray some non-combustible carburetor cleaner on the outside of the throttle shaft; and if the engine speed drops, it means some of this is getting into the air stream from outside the carburetor. You may also have leaks from the manifolds, from tubing such as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for locating those leaks as well.

Other problems that SU carbs experience involve dirt in the dashpot and occasionally in the float chamber. The dashpot is a precision piece of machining that involves very close tolerances so that the piston doesn’t stick or bind when it rises and falls. A little grit between the piston and the dashpot can make the car jerk and sputter. Take the dashpot off, wipe the insides down with carb cleaner and a lint-free, clean rag, then reinstall it, getting the screws down nicely. Also, don’t swap the pistons between dashpots; they’re matched to one another so that the clearance between the piston and the wall of the dashpot makes a tight seal but permits easy rising and falling.

Dirt in the float bowl basically shuts off that carburetor (or can make it flood open, depending on whether the dirt is wedging the valve open or closed). You can try rapping on the float bowl with the handle of a screwdriver, but your best bet is to take the cover off, clean out the valve fittings, and reinstall everything, with a new fuel filter for good measure.

SU’s have adjustable floats, in which you need to set the float height (which basically equals the fuel level in the float chamber) by bending a brass arm. New floats are fairly inexpensive and aren’t a bad idea if you’re doing a rebuild.

Grose-Jets are very popular with some people and a big pain for others. It appears — and this is just conjecture — that Grose-Jets work best in cars with adjustable floats, as they are longer than the stock SU float valves. The standard failure for Grose-Jets is to flood the carburetor. I have never had problems with the stock SU float valves or floats.

Tuning Your S.U. Carbs .......Twice-as-Nice

Well, it’s not really that hard to set up SU’s, just different. Of course it always gets more interesting when you have more than one…. Here is a layman's guide to adjusting SU’s (long version):

step 1-

Tune up the rest of the engine- REALLY! clean or replace, and set the points (if so equipped), set the timing, plugs, valve lash, and remove the air filter/filters. (have new one/s ready) All of these things can affect the setting of the carbs, which should be done LAST, (if at all). The carbs rarely need to be adjusted, once set. Also replace/install the gas filter. Of course, it helps if the carbs are in good mechanical condition as well. But you can consider a rebuild once you have gotten things working first!

step 2-

clean the carbs! use gum-out or similar stuff, clean all external linkages, shafts, and stuff.

step 3-

Remove the float bowl covers, clean the float bowls, remove old sediment, and check/adjust the float setting. (turn the cover upside down, and get a *1/8�? in drill bit, set the drill bit accross the cover, the float tab should just touch the bit.) Make sure the needle is moving and seating properly. This is just like most floats. Replace the cover.

Note: You can check for matching float settings, after setting the mixture, by removing the pistons, and peering down at the jets. The fuel level should be about the same on both carbs.(After car has been run only)

step 3b- Go get a pint of ale (Saki??? ....Purist!), or something close, and set it nearby.

step 4-

Remove one piston cover. CAREFULLY remove the piston, DO NOT BEND THE NEEDLE. Set the piston down on a clean wadded rag to prevent rolling. Clean the inside of the carb. Check operation of the throttle. Check the throttle shaft slop- this is the most common place for wear on an SU, and is often where air/vacuum leaks occur. The bushings and shafts can be replaced, but it requires some machining. A small amount of leakage can be tolerated, the car just won’t idle as evenly. Clean the piston. Stare in awe at the odd carburetor design, simple and effective, (constant velocity). Dump the old oil out of the damper if you haven’t already spilled it. clean. Reassemble, check piston movement, raise it, then let go, it should fall freely. If not, check assembly again, make sure the piston isn’t binding against the carb body, it should ride only on the damper shaft. Do not stretch the spring. When all is operating properly, fill the damper with "Proper"....."Genuine S.U" oil for damping..

step 5-

Start the car and warm it up, then turn off/disconnect/otherwise disable the choke mechanism. (Loosen the screws on the clamps so that the choke stuff isn’t doing anything) This will get set later.

step 6-

Check coarse throttle adjustments- make sure the throttle pulls on both carbs equally, and returns completely when released. This is adjusted by relaxing the set screws on the throttle shaft and matching the two sides. You can check the float adjustments now by removing the piston & cover, and looking at the fuel level in the needle seats. Both carbs should be . If not, readjust one or both floats to match the level.

step 7-

Synchronize the throttles- if you have a uni-syn, here’s your chance to use it, (or other air flow gauge), if not use a tube and listen to the airflow. The Uni-Syn is much easier to use, and can result in better balance. Alternately adjust the idle screw on each carb, attempt to set the idle as low as possible (~500 RPM). Adjust until the airflow is close to the same at each carb. The engine may now be running rough, just keep the idle speed high enough to keep running. Give the throttle a quick snap to make sure everything is settled, then check sync again. Periodically snap the throttle to make sure everything is seated. Large differences in where you can adjust the two carbs may indicate air/vacuum leaks, or other problems, such as a bad valve)

Magic Time- Relax, and shake your voodoo rattle…

step 8-

Adjust the mixture- this is done with the spring-loaded hex nut fitting under the carb, where the fuel supply tube enters from the float chamber. Turning the fitting raises and lowers the needle seat. Pick a carb, and turn the fitting 3 flats (1/2 turn), first in one direction, then back 3, then 3 in the other direction. Note where the engine runs better, idle speed should increase. Turn to the best setting. Repeat this procedure until you get the best operation you can, (highest idle speed), keeping track of flats turned will help you remember where you were. If you get lost, turn all of the way in, then back out 12 flats (2 turns) and start again. Periodically snap the throttle and push up on the fitting to make sure everything is seated. Note: When you think you’re close, stop, un-cramp your fingers, breath deep, and do the same to the other carb. Then retune the first carb, and then the second again. This serves to match the mixture of the 2 carbs, and prepare you for the beer (saki) sitting over there in the sun. (why do you think the British drink warm beer? And..... the Japanese drink hot saki????

step 9-

repeat step 7, setting the idle speed as low as possible, and re-syncing the idles. Now go back and readjust the mixtures. After a couple of reiterations, the engine should be running smoothly (controlled by mixture) and at a low idle. Repeat as necessary. Set the final idle to 750 + RPM, depending on the condition of the rest of the engine.

This is a standard mixture test, performed AT IDLE: Under operation, (air filter off) lift the carb piston by 1/16" with the lifting pin, which leans the mix a tad. If: -RPM’s rise and stay up, that carb is rich. -RPM’s rise briefly, then drop, mix is about right. -RPM’s fall, engine gets rougher- mix is lean.

[where is step 10????--Ed]

step 11-

Adjusting the choke:

The choke is supposed to do two things; the first half of travel moves a cam on each carb which opens the throttle, for warm up. The second half pulls down on the needle seat to enrichen the mixture, for starting. Start with the choke in the off position (knob/s forward). Adjust the so that the cam only starts moving the throttle after you start pulling out on the cable (adjust with shafts and adjusting screws). Try to get both carbs adjusted the same, so that both begin to hit the cam at the same time. This is not real critical, but you can use your Uni-Syn to match air-flow on both sides, with the choke partly engaged. After the cable is about halfway out, it should start engaging the lever which pulls down on the needle seats. Adjust the linkages so both carbs are acted on equally. You can do this by adjusting for even running of the engine. Of course, for a warm engine, the richness of this mixture will cause some roughness. Make sure the needle seats return freely when you release the choke.

step 12-

Drink that warm beer (Saki) ....(only one, no DUI now…) it will taste great at this point!, go wash up, and go for a ride.

Notes: These procedures assume that your engine/carbs are in reasonable operating condition. If something is malfunctioning/leaking, etc, this should still help, but the results may vary. For instance, if you have leaky carbs, worn needles, engine modifications, etc, you may find things work better if you tune for optimum performance at open throttle rather than idle.

The first time through carb adjustments can be confusing, once you’ve done it, all of the stuff in the manuals makes sense. Go back and read them again- As always, I recommend. (Original factory manuals are to be read in a clean environment, photo copies/repros are for smearing grease all over, use them!)

Tuning Your S.U. Carbs .......Third Time Is A ???

Warm the engine up, then shut it off:

1) Turn both carbs' adj. nuts all the way in (lean)
2) Turn both carbs' adj. nuts out three turns (richen)

Re-start, and:

3) Set balance with Uni-syn
4) Turn in one carb 1/2 turn, and listen for decrease in RPM
5) Turn the other carb in 1/2 turn, and listen for the same decrease in RPM

a) if the RPM difference isn't the same, you may need to turn one in more than the other to get them the same. Don't do more than about one full turn difference. The differences are pretty slight, and if your carbs are worn out a bit, you may be unable to detect a difference. If you can't, don't worry too much....

6) Repeat 4 & 5 until the car stalls

a) it may not stall, if the carbs are worn out. If it doesn't stall, turn the adjustment nuts all the way in, and continue

7) Turn both carbs out two full turns
8) Rebalance, and see how it runs

a) depending on the condition of your carbs, you may need to go richer than that. I.E., on my L28/E88/SU, with good carbs, I'm at about 2.5 turns out from stall. My friends L24/E88/SU, with worn-out throttle shaft bushings (soon to be re-built), has to be set at about 3.5 turns out from all the way in out to get off-idle performance acceptable. Although, if my car stalls at 1 turn out, our cars are set the same (1.0 + 2.5 = 3.5)

Additionally, elevation and temperature can affect tune. The best way to get them spot-on is to get the car running properly, then adjust the carbs as necessary to get the best performance. Also, the front or rear carb may need to be richened or leaned to match the other from step eight. Once you get it running properly, drive it for a while, then pull the plugs. You want all six plugs to look the same. If three of the plugs are darker (dull black - glossy black is probably oil), lean that carb out 1/2 a turn and recheck. Of course, any modification requires rebalanacing the carbs....

Here's some symptoms of incorrect adjustment. Of course, all these problems can be the result of other problems, so consider this chart when attempting to diagnose problems that appear after tuning the carbs.

Too Lean?

Too Rich?



Poor low RPM performance

Fouling plugs

Running hot

Poor mileage


And now a few words on carb fluid. I've found the level and type of fluid in the carbs to be of the utmost importance. What concerns me is not the fact that it's critical, but the fact that it's seldom mentioned. I've corrected some pretty major-appearing drivability problems, just by adding the correct "Genuine S.U" oil to the carbs. I've heard of people running the carbs dry, and others who used water. I can't vouch for their success, only my own.

After passing this on countless times via email, I figured I'd post some info on setting the needle position. Many of these cars have been around the block a few times, so often times, the carbs are way out of whack - too far to straighten out with the above procedure. F'rinstance, when I got my latest "70" 240Z, the rear carb was about five turns out, whereas the front carb was at around two. Yikes! So, when you encounter something like that, or when you just can't get them lean enough, try this:

Warm up the car.

Shut it off and remove the carb domes (take out the dipsticks first).

Remove spring from dashpot (the piston-thing that lives inside the dome).

CAREFULLY remove the dashpot - see the needle on the bottom?

Loosen the set-screw that holds the needle so that the needle can move a bit.

Turn the adjusting nuts all the way in (located underneath the carb throat, where the fuel line enters from the float bowl).

Pull the needles out about 1/8" or so, you should see the shoulder of the needle sticking out past the dashpot body.

CAREFULLY reinstall the dashpot, and push down ever-so-gently to seat the needle fully in the nozzle (that's the hole the needle sits in).

Pull the dashpot back out, snug down the needle set screws, and put them back together.

Set the balance and mixture as above.

                                The Real History & Reasoning of the Mighty SU

Invented by the British ... English to you Yanks. Probably because they are not electrical in any way (and therefore exempt from the "Lucas Curse" (see down stream a bit.... Lucas, Prince of Darkness...RIP.....), the SU carbs of a vintage Datsun Z are, in my opinion, one of the few things the British car industry got really right, bless them...(contrary to what you might hear in the automotive world)!

More History: SU carbs were allegedly developed working with plumbers (air and fuel "flow" through them, so that makes sense, sort of). It is well known that SU stands for Skinner's Union, my careful research has uncovered however, the less well known fact, that they were actually developed by British pork butchers, and that SU was actually shortened from SSU, standing for the Sausage Skinners Union. Below, an earlier development of the Sausage Skinners Union. The SSU, SG5, featuring "Spiral Venturi Design" (SVD). It was said to be the inspiration for the first SU side draft 1902...the family resemblance is apparent.

After a lot of "product development", this is what they came up with. It's a beautiful thing!

In a wonderfully simple SU carburetter., the varying exposure of a tapered "metering needle" is the mechanical equivalent of the electronic engine management system's lookup table or "engine map". Naturally, in the modern electronic engine management system, there is a lot more optimization going on, but the SU had the really important issue - correct metering and dispensing of fuel for the volume of air drawn through - very well under control.

The SU carburetter is of a constant Velocity, variable choke design...that is, by varying the position of the dashpot as a function of the volume of air the engine draws, the cross-sectional area of the throat of the carb is varied, which keeps the velocity at the venturi point constant...and by keeping the velocity constant, it is fairly simple to meter the fuel for a given volume of air induced, although this leads to somewhat of an oversimplification in fuel metering, since the fuel should be metered as a function of mass of the air being ingested and not just volume (as density of the air, which can vary due to things like weather, season and altitude, is not compensated for). That is why I like to adjust the mixture for summer and winter. Other than that, they seem to thrive on neglect, I honestly don't understand why some owners seem to continuously need to fiddle with them!

An exception to the constant velocity occurs during changes in the throttle Acceleration...more on this underDampener Oil (see our offering ) following.

                                      Proper Lubrication for SU Carbs

                                                    SU Oil Label 1.5.jpg (10439 bytes)  < Click Here To Order

                                                         We have sourced the REAL thing .....Order from our parts department

S.U - Operating Principle

SU carburettors featured a variable venturi controlled by a piston. This piston has a tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits fuel into the

airstream passing through the carburetor. Since the needle is tapered, as it rises and falls it opens and closes the opening in the jet, regulating the passage of fuel, so the movement of the piston controls the amount of fuel delivered, depending on engine demand.

The flow of air through the venturi creates a reduced static pressure in the venturi. This pressure drop is communicated to the upper side of the piston via an air passage. The underside of the piston is open to atmospheric pressure. The difference in pressure between the two sides of the piston lifts the piston. Opposing this are the weight of the piston and the force of a spring that is compressed by the piston rising. Because the spring is operating over a very small part of its possible range of extension, its force is approximately constant. Under steady state conditions the upwards and downwards forces on the piston are equal and opposite, and the piston does not move.

If the airflow into the engine is increased - by opening the throttle plate (usually referred to as the "butterfly"), or by allowing the engine revs to rise with the throttle plate at a constant setting - the pressure drop in the venturi increases, the pressure above the piston falls, and the piston is sucked upwards, increasing the size of the venturi, until the pressure drop in the venturi returns to its nominal level. Similarly if the airflow into the engine is reduced, the piston will fall. The result is that the pressure drop in the venturi remains the same regardless of the speed of the airflow - hence the name "constant depression" for carburettors operating on this principle - but the piston rises and falls according to the speed of the airflow.

Since the position of the piston controls the position of the needle in the jet and thus the open area of the

jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by

the profile of the needle. With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling. The well-controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions.

This self-adjusting nature makes the selection of the maximum venturi diameter (colloquially, but inaccurately, referred to as "choke size") much less critical than with a fixed-venturi carburettor. To prevent erratic and sudden movements of

the piston it is damped by light oil in a dashpot which requires periodic topping up.

The dampening is asymmetrical: it heavily resists upwards movement of the piston. This serves as the equivalent of an "accelerator pump" on traditional carburetors by temporarily increasing the speed of air through the venturi, thus increasing the richness of the mixture.

The beauty of the SU lies in its simplicity and lack of multiple jets and ease of adjustment. Adjustment is accomplished

by altering the starting position of the jet relative to the needle on a fine screw.

At first sight, the principle appears to bear a similarity to that of the slide carburetor, which was previously used on many motorcycles.

The slide carburetor has the same piston and main needle as an SU carburetor, however the piston/needle position is directly actuated by a physical connection to the throttle cable rather than indirectly by venturi airflow as with an SU carburetor. This piston actuation difference is the significant distinction between a slide and an SU carburetor.

The piston in a slide carburetor is controlled by the operator's demands rather than the demands of the engine. This means that the metering of the fuel can be inaccurate unless the vehicle is traveling at a constant speed at a constant throttle setting - conditions which are rarely encountered except on motor-ways. This inaccuracy results in the wastage of fuel, particularly as the carburetor must be set slightly rich to avoid a lean condition, which when performed repeatedly can cause significant engine damage. For this reason Japanese motorcycle manufacturers ceased to fit slide carbs and substituted constant-depression carbs which are essentially miniature Japanese SUs. It is also possible - indeed, easy to retro-fit an SU carburetor to a bike that was originally manufactured with a slide carburetor, and thereby obtain improved fuel economy and more tractable low-speed behavior.

SU Carb Applications Here's a handy chart relating an engine cc's and HP range with the size and number of SU carbs the case of a 2 liter Datsun Z engine (yes 2 liter .... as in Japan) with two 1 3/4", it would seem that we have plenty of carburetion from the factory!

                                                                Chart Coming Soon!!!! 

Lucas, Prince of Darkness...Rip.....

Since they, Lucas, are a major cause for the bad name British vehicles have received over the years (right up there with what I call "excessive maintenance"), I believe they deserve all of the criticism they can get. Even vintage Volvos got a taste of this "equipment". With early 1800 production taking place at Jensen in Britain, electrical harnesses and minor components were naturally procured in-country from guess who? Unfortunately, this "equipment" wound up easily being the least reliable components ever found in a Volvo! Luckily though, the drive-train and therefore major engine electrical components, which were installed after the bodies arrived back in Sweden, were retained as Bosch...phew!

In my informative years.... back when I’d take in MG’s, Triumph’s and Jaguars....I was actually given a good looking (clean, and appeared to have new guts...but we know that doesn't mean much!) Lucas generator in a box of spares one time...I wound up taking it to the next British car show/swap meet (I remember noting it the meet looked like the worlds biggest gathering of oil leaks!) and giving it away to the first guy who wanted it. I told him I didn't want it in the same shelf or even in the same garage, contaminating my quality Bosch & Hitachi components...he stomped away with it and refilled his glass of Chardonnay (but didn't argue about it either).

Here is my handy compendium of everything having to do with Lucas, purveyors of the least reliable automotive electrical equipment and systems known to man. It should be referred to anytime you are possibly considering the purchase of a vintage British vehicle, obviously a moment of weakness which one would hope you would get over rather quickly without succumbing.

It is said that many people who left Lucas went to work at Smiths gauges. The consistency of the products of both of those companies would seem to support that.

Caution: (More) Sarcastic Humor Follows!

Some Tidbits:

Possible explanation of Lucas quality philosophy: A gentleman does not motor about after dark.

All Parts Falling Off This Car (including switches crumbling behind the dashboard) are of the Finest British Workmanship.

Lucas lamps do not create light, they "suck dark".


Q: Why do the British drink warm beer? A: Because their refrigerators are made by Lucas.


Factory Motto: Home before dark.


Here's a hoot...I saw this offered on E-bay recently...oh goodie!...A Lucas Fault Diagnosis Manual ....seems a bit self-serving to me... if we didn't have Lucas, we wouldn't need that either... same thing with lawyers!


Happy Anniversary! I understand 2003 was the 100th anniversary of the death of Joseph Lucas. I wonder if anyone is going to have a special event celebrating this, other than to try to keep their lights working consistently and predictably for one entire year.


As a technician working in a MBZ Dealership it has come to my attention that Lucas supplies audio components for new MBZ vehicles. Please add "Lucas Prince of Silence" to your list.


Electrical Theory by Joseph Lucas

Positive ground depends upon proper circuit functioning, the transmission of negative ions by retention of the visible spectral manifestation known as "smoke". Smoke is the thing that makes electrical circuits work; we know this to be true because every time one lets the smoke out of the electrical system, it stops working. This can be verified repeatedly through empirical testing. When, for example, the smoke escapes from an electrical component (i.e., say, a Lucas voltage regulator), it will be observed that the component stops working. The function of the wire harness is to carry the smoke from one device to another; when the wire harness "springs a leak", and lets all the smoke out of the system, nothing works afterwards. Starter motors were frowned upon in British Automobiles for some time, largely because they consume large quantities of smoke, requiring very large wires.

It has been noted that Lucas components are possibly more prone to electrical leakage than Bosch or generic Japanese electrics. Experts point out that this is because Lucas is British and all things British leak. British engines leak oil, shock absorbers, hydraulic forks and disk brakes leak fluid, British tyres (tires to you Yanks) leak air and the British defense establishment leaks, naturally, British electrics leak smoke.


                                                                In Closing

The result is you end up with a stock-looking 240Z with a ZX motor, superior electrical system and ignition, 5-speed, upgraded drivetrain AND a high-ratio rear end. A good old fashioned hot-rod using factory parts! Thank you Nissan for making all the parts interchangeable!


  The Z Car to play with....... a few thoughts on the subject...

If you want to find a base model to use for your fantasy racer, try to find one of the first 12,000 or so 240Zs made from 1970 to early 1971. These are identified by a "240Z" badge behind the side window and two grills on the rear hatch. These '70-71s were the lightest of all ZCARs, at about 2300 lbs, making them the best for performance buildups. Later models went to a round Z badge behind the side window and removed the hatch grills. Unfortunately the weight went up a few hundred pounds due to chassis strengthening.

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