Classic/Antique Car Repair: ERRATIC IDLE SPEED, throttle return spring, holley 4160

Question
I HAVE A 1970 R/T CHARGER WITH A 440 4BBL. THE CARB IS A HOLLEY 750. WHEN THE IDLE IS SET AT 900 IT WILL FLOAT UP TO ABOUT 1100-1200 RPM NO MATTER HOW OFTEN THE IDLE IS SET IT WILL ALWAYS FLOAT UP.  IT DOES NOT MATTER IF THE MOTOR IS HOT OR COLD.  ANY HELP REGARDING THIS ERRATIC IDLE MATTER WOULD BE GREATLY APPRECIATED.

Answer
750 CFM seems like too big a carburetor for that car, but that would not cause the idle to creep up.   If you are sure that the throttle return spring is in the right place and strong engough, I suspect a vacuum leak.  Try this:

With the engine idling, slowly slide a piece of cardboard across the top of the carburetor (with the air cleaner off) so as to block the air flow into the primary bores.  If at ANY point, you hear the engine speed up, there is a vacuum leak somewhere.  Get yourself a propane torch (unlit) and wave it's snout around all the gaskets on the intake manifold and at the base of the carburetor - if that makes the engine rev- you've found where the air leak is - just do the detective work and find the bad gasket or whatever is causing it.

If you have the orignal Holley, make sure that the end caps are not warped letting extra gas dribble into the intake - the Holley 4160 is famous for that problem.   If that is the problem with yours, get yourself an Edlebrock replacement for the famous Carter AFB - the best, most reliable carburetor ever, and be done with all the Holley problems.   That's what I do with all my 440s - they run perfect with the AFB.   Around 450 CFM is a good carb for that engine, unless you want to go drag racing and have the cam and breathing mods to use it!

Dick