Just manage to follow all the steps to tune my 1972 240z su carburetors including setting the idle mixture knobs 2 turns clockwise and balancing both carbs with fast idle screw to 3,000 RPM. I took the car for a spin and noticed that she hesitates when reaching 4,000 RPM in 1st gear. Could it be running lean? Should i turn the idle mixture knob to 2.5 turns?

 

Please advise what should i do to eliminate this hesitation.

2.5 Turns is the starting point for getting the proper mixture. How far down or up depends on the carbs and your car 

2.5 Turns is the starting point for getting the proper mixture. How far down or up depends on the carbs and your car 

Just found the culprit! The spark plugs were worn. I installed a set of NGK BP6ES, tuned both carbs setting the idle mixture to 2.75 turns clockwise and now the car does not hesitate or causes any backfire.

Just found the culprit! The spark plugs were worn. I installed a set of NGK BP6ES, tuned both carbs setting the idle mixture to 2.75 turns clockwise and now the car does not hesitate or causes any backfire.

Problem came back even with new NGK BP6ES spark plugs and 4 turns clockwise on both of the SU Carbs idle mixture knobs. The Factory gap of the BP6ES is .028 mm.

Edited April 10, 20159 yr by jalexquijano

Did you ever plug all the vacuum lines and idle the car. Don't spray the brake booster with starter fluid. Plug the vacuum lines to everything and eliminate possible vacuum leaks. Check the ignition system thoroughly...

C

Sorry but what do you mean by plugging all the vacuum lines? Plug them with what?

Problem came back even with new NGK BP6ES spark plugs and 4 turns clockwise on both of the SU Carbs idle mixture knobs. The Factory gap of the BP6ES is .028 mm.

I hope that's a Typo. Factory gap on those plugs is .8mm or .032"

Edited April 10, 20159 yr by Chickenman

Either unplug them from the manifold and put a rubber cap on the manifold port and a hose clamp; on a hose put a bolt in the hose with a hose clamp, golf tees work good in little hoses. Try to remove as many of the possible sources of leaks as possible. If that fixes the weak and erratic idle then add them back one at a time to isolate which one is the problem. Another trick for when the idle slows down after a while when you just let it sit and idle. Add a battery in parallel with the one in the car. Just use jumper cables to a known good battery. Not a battery in another car! See if the extra capacity helps the idle stay where it should..

C