I have a 1971 240z, I think the fuel mixture is leaning out. During checking for vacuum leaks, I sprayed carb cleaner between the intake manifold and the head and the RPM shot up. I assumed the gasket was leaky and naturally replaced it, but even with a brand new one it's still doing the same thing. Mixture nuts are turned all the way up and float levels checked, fuel lines and filters double checked, all is well. It's truly a mystery. The leaner I set it, the easier it starts up, but it also backfires much more and consequently dies right after. The richer I set it the less it backfires, but the idle feels far more rough and much lower. Raising the rear suction piston does nothing, raising the front one kills the engine right away.

Also, in order to keep the car idling I have to advance the timing all the way (distributor literally cannot be turned any further.) I know this can be a reason for the backfire but I can't get it to idle any other way. Help is greatly appreciated!

Edited August 20, 201311 yr by Donairs

try normal timing and jets at normal enrichment

Do what Blue said. It sounds like you still have a vacuum leak. Are all of the ports plugged/sealed? Any hoses cracked? Intake manifold cracked? Let's see some pictures.

Chuck