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suction piston carb problem


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MY suction pistons are not rising or falling at the same rate. I bought this car recently to replace my Z that was rear ended. I got the car all running well except the carbs and I never had this much trouble tuning them since I learned how to do it. What is going on is the rear suction piston is moving much slower than the front one (this is with the engine off or on). I tried to adjust the oil levels, I ended up with almost no oil in the rear and the front being as full as it can be, but the rear still moving slower, but not by as much. Has any one out their fixed this problem on their car. I was thinking of adding weight to the front piston; swapping the pistons (thinking that the wheir would be equal between the stack and piston); replanting the tops and having the front dipped twice, or removing a very small amount of material from the rear stack and piston. WHat did you do? and how well did it work?

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They are called dashpots and should work ok if you remove the top of both of them and clean them with a heavy duty metal polish inside the tops and the whole of the internal piston. When you reassemble move the tops loosely around until the piston rises and falls freely using the button underneath the carb. This all assumes the damper piston in the centre of the top is in position. They don't need much oil, only a teaspoon or so, to work correctly.

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First, is the problem a difference in how quickly the pistons move to new positions? ....or are they rising/falling through different distances as RPM's change? And... do you have round top or flat top SU's?

If it's a difference in how far the pistons move the first check needs to be for equal/unequal air flow using a meter like a UniSyn. Unequal air flow could be caused by a linkage adjustment or internal engine problem, or by altered (by a P. O.) & different piston weights or a missing or too strong downforce spring. In any case you need the original piston weight and spring downforce to get the correct venturi vacuum and mixture.

If it's a piston velocity during movement problem swapping the pistons is a good place to start. Pistons and domes were assembled as matched sets to get the right clearance (for correct air leakage rate). If your front and rear diameters are a little different the P. O. may have caused the problem by swapping them accidentally.

Other ideas --

1. Try moving the pistons in the domes by hand with both off the car. If there's more drag in the rear set you have a clearance/dirt/mechanical damage issue there you'll have to fix. This could cause either a steady state height or velocity of movement difference.

for a velocity of movement problem only --

2. Make sure the dome to air cleaner vent holes in the rear carb body aren't clogged. You need them to vent the space under the top (larger diameter of the piston. If they're blocked movement will be slow in both directions.

3. This doesn't fit your symptoms but is a worthwhile check on a newly purchased car. Are the damper valves -- the brass parts at the ends of the "dipsticks" -- both intact and clean? They work like shock absorber valves to resist fast piston movements. You can check for unequal valving by swapping dipsticks with the oil levels normal & checking for differences in piston movement.

Good luck

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OK I have done everything listed so far, I didn't metal polish but I cleaned them to where the metal looks new. What is happening is that the pistons are dropping at different rates with the car on and off, and when accelerating the one drops the slowest is the slowest to rise. I have rebuilt carbs on another 240Z and tuned them my self before.

I have tightened and re-tighten so many times I have ruled out miss alinement. I've ruled out bad needles or needle rub by removing the needle and having the same problem.

What I think has caused this problem is that when I went to reset the carbs, one carb the float was adjusted to where little to no fuel was in the chamber, the balance scews were adjust in a way that kept one of the cabs from working and delivering to the engine. IN other words the carb wasn't in use. Who knows how long it was like this. I'm thinking that the carbs now have different level of ware from this creating the unbalance, between the carbs. Thus allowing one carb to move more than the other.

I was thinking to day about trying different oil weights for each carb, any thoughts

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The four causes for uneven movement of the piston are. Dirt or buildup on the piston causing it to rub in the cylinder. Uneven air flow between carbs due to unbalanced setting . misaligned needle to nozzle setting, causing the needle to contact the nozzle. The dampener springs are no longer matched, the large spring that contacts the piston inside the dome. If you have cleaned off the vacuum piston and its cylinder, and the needle alignment is correct and there is no rubbing , the balance if the air flow is set correctly. Providing the engine it's self is in good order. It only leaves the springs and of course he oil in the dampeners which should always be the same viscosity. 20w preferred. I would be vary surprised to learn the unbalance is due to uneven wear as you mentioned. I would think that the engine would suffer a lean condition if running on one carb for such a long time, and I would defiantly do a compression check. Which might be a good thing anyway. Gary

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You obviously have different wear in the domes and on the dash pots. The amount of oil as well as the weight of the oil (I used 5/20 weight in mine) will determine the speed of rise and fall on each dash pot.

remove the domes, and the dash pots, inspect both springs, clean them both (dome and dash pot) with carborator cleaner (spray) then reinsert the dash pots. place the domes back on top and slowly insert one screw at a time, while lifting and letting the dash pots drop as you snug each screw down.

If at any time, the dashpot slows or scrapes, the dome needs to be re-adjusted by turning it back or forth (clockwise or counter clockwise) Also try snugging one screw, testing, then do the opposite corner, then test again, then another screw until all four are down and the dash pot stil moves freely in the dome.

After both carbs are done, slowly add oil to each cylinder, a few drops at a time. The key is to add oil till it rest inbetween the 2 lines on the dipstick. Test the rise and fall again, a few times, they should be the same at this point. If one takes longer to fall, add 2 drops to the other (faster moving) dashpot. The oil creates a smooth operation of up and down movement. each carb wears differently to you need to adjust accordingly.

Remember that the domes can become warped from heat and overtightening over a period of time so you need to check them and re-adjust them every once in a while.

Hope that helps,


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  • 4 months later...

it seems that I do have the same problem. When no oil is present you can move them freely and when filled up, they won't move at the same time, the other carb being slow than the other. it is like the oil is dampening too much even though I put the right amount.

I use 10wt oil.

Hoping for your further inputs.


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The dampeners work to slow the rise of the vacuum piston. This retarding of the rise, causes a RICHER mixture NOT leaner , as some think. The thinner the oil the faster the rise and the leaner off idle, the mixture is. This can cause a stumble that feels like ignition faltering. This is only effected at low rpm and off idle. After the engine is up to speed the rise is no longer an issue. You can run them with no oil if you are racing and keeping the engine speed up in the power band. I use 20w oil winter and summer , although our winters here in Oregon are not severe. To see if your dampener pistons are rising at the same rate you need to view them with the engine running, a dyno is a great help with this. Do not ever swap the parts from one carb to another. Especially on 3 screw SUs . The parts have been matched at Nissan at assembly. Since 3 screw carbs have nonadjustable nozzles , the position of the vacuum piston/needle is critical . The 4 screw carbs have an adjustable nozzle holder. Gary

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