how do you get those SU's running perfectly?

2

240zDailyDriver

Forum User
I have been attempting to get the carbs running perfectly for the past week (I own a 1972 240z with the three screw carbs)... but it is always off somewhere. I have the jets adjusted right, it is just the balance screw that I can't seem to get into the right place. So if anyone knows a little trick on how to get the dual carbs running correctly, that would be awesome. Thanks!
 
In theory, as you probably know, setting the balance screw should just be adjusting it so that the slack between the 2 carbs is just removed, so that when you open the throttle the carbs move from idle to fast idle at the same time. What is happening that makes you think that the balance screw is not in the correct place? Does it 'stumble' or not rev cleanly when you open the throttle? Are you sure that there is nothing else amiss with the engine? ( ie intake vacuum leaks, carb air leaks, sticky carb pistons, tappets not set, ignition problems,old air filter) I know from experience that balancing SU carbs when some other part of the engine is not quite right is almost impossible. I can't tell how bad your car runs from the info you have posted, but I would say that if it runs well with good colour spark plugs and revs cleanly from idle you are 95% of the way to having the best possible settings.

Let us know how thing go
 
how are you balancing the carbs the best way i found is adjust the balance screw to take out the slack then with the air filter removed and a piece of plastic pipe about 5-7mm internal diameter put in one carb through the air intake and listen to the noise the noise will be air going into the carb then put the pipe into the other carb listen again the noise should be the same if not adjust the carbs to equal noise with practice you will get the engine bang on have you also checked valve gaps on your head there also easy to do and only need to be out a small amount to alter performance
 
Hmm... well the car is running alright.. it is just when I accelerate it kind of putts... but at idle it runs fine... almost no vibration. That is what made me think about the balance screw, because I know that the carbs themselves are running fine. I use a flow meter to check the amount of flow through each carb and adjust it that way. I could check the valves, thanks for suggesting that. Any another suggestions will be welcome. Thanks guys!
 
you say when you accelerate it strugles does it do it when you just rev it by hand with the bonnet up using the throttle linkage or only when under load when driving if it does it all the time id check your points and gap i had a car once that ticked over fine but try and rev it and it struggled and ran rough i put new points in and it was fine also what is the distrubutor cap like id check everything in that area regards 240zchevyv8
 
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