I am trying to sync and adjust mixture on a 240z with round top SU carbs on an l28. The problem is this. I am doing the piston lift technique to getting the mixture right and the engine drops in RPM when i lift the rear carb. Just like it should. But when i lift the front carburetor the engine dies. It doesn't seem to matter how rich i adjust it the engine just dies.

Another intresting symptom is when i put the unisyn on the front carb the engine drops in RPM significantly. When i put it on the rear nothing simular happens. The airflow going in each is the same.

I am thinking that the rear carb is messed up. I think the previous owner of the carbs took it apart because it looks much cleaner. Let me know what you think. Thanks.

I am adjusting the mixture on the correct carburetor(the one i am not lifting the piston on).

are you getting fuel to both carbies?

Lot's of possible answers here. Take the dashpot out and check the needle is in the right place, ie flush with the face of the dashpot. Check also the needle is not bent and that the jet is central in the housing. It would also be worth checking that both carbs have the same needles and the same springs in the dashpot. It might be worth changing your needles to SM's for the L28 and also the jets.

Lot's of possible answers here. Take the dashpot out and check the needle is in the right place, ie flush with the face of the dashpot. Check also the needle is not bent and that the jet is central in the housing. It would also be worth checking that both carbs have the same needles and the same springs in the dashpot. It might be worth changing your needles to SM's for the L28 and also the jets.

BY ALL MEANS DO NOT INSTALL S M NEEDLES. PERIOD. If you want a set of SM needles I have a new set I will give you. this mith has been passed on so many times , I fell for it too . don't do it. Your engine will run so rich it will wash the oil off the cylinder walls and you will have a premature engine damage. 54 needles for a 280 with SUs. I have pages of documentation from engine builders that race Zs . Your mileage will go in the tank also. :pirate: Gary

I am trying to sync and adjust mixture on a 240z with round top SU carbs on an l28. The problem is this. I am doing the piston lift technique to getting the mixture right and the engine drops in RPM when i lift the rear carb. Just like it should. But when i lift the front carburetor the engine dies. It doesn't seem to matter how rich i adjust it the engine just dies.

Another intresting symptom is when i put the unisyn on the front carb the engine drops in RPM significantly. When i put it on the rear nothing simular happens. The airflow going in each is the same.

I am thinking that the rear carb is messed up. I think the previous owner of the carbs took it apart because it looks much cleaner. Let me know what you think. Thanks.

It sound like the air flow is not right . I know you said that you did set it with a uni-syn . Like Nigel43 said it could be any of several things if some one has had it apart. Make sure the fuel lines under the carbs is not stiff and or brittle. They can cause the jet to stick down some and this will mess up the mixture and you wont be able to set the carbs. These small hoses must be fuel rated lines and the small lines at the parts store is too stiff . Ztherapy or Nissan have the proper fuel line for this. Check for a vacuume leak also .

Maybe I should have made myself clearer on the subject of SM needles. They are only recommended if they are used with the related jets [nozzles] preferably in 70/71 carbs.