Classic/Antique Car Repair: Autolite 4100 fast idle sticking, accelerator pedal, carburetor body

Question
Hi Dick,

I've got a '66 mustang with the original 289 engine and its accompanying 4100 carb.  I had the carb rebuilt a few years ago but now the fast idle doesn't like to kick down.  If I pop the accelerator, it will throttle down, but as soon as I press the gas again, it speeds back up to fast idle.  Any suggestions on how to correct this?

Thanks!

Answer
Yes, I think your choke flap is sticking, probably due to a build up of crud in the linkage or else in the bore hole that the choke butterfly rod turns in.  

The way to clear this up is to get a can of spray type carburetor cleaner - GUNK is a good brand, from your auto parts store, then remove the air cleaner assembly, and with the engine off, have a helper hold the accelerator pedal to the floor.  

Now, with one hand, move the choke butterfly flap back and forth while you spray everything that moves with it, inside and outside the carburetor so that all the linkage is freed up and clean.  Be sure to hit the hole through the side of the carburetor that the flap axle turns in, and the links to the fast idle cam.  This should cure the problem.   

Let the engine sit for a while before you start it up, as this stuff is highly flammable and you don't want to spark a fire.  

Now start the engine (air cleaner still off) and see if your problem is gone.  If it is still sticking, you'll just have to do some carelful observation to see what is holding the idle stop in the "cold" position - without looking and watching, I can't guess at what it might be.

Sometimes this only happens with the air cleaner on - if this is the case, someone has probably overtightened the fastener that holds the air cleaner lid on - warping the carburetor body.  If this has happened, you may have to replace the carburetor - they don't take kindly to being bent twice, but it's worth a try.  Be gentle - the metal is very brittle!

Good luck!

Dick