I'm trying to narrow down a wandering idle in my '67 390 T-Bird. It doesn't wander so much as not stay on setting from one day to the next. I can't seem to find an idle stop screw or adjustment on the Autolite (4100 4BBL) carb. All of the choke system is working properly (I had the thing rebuilt already). There is a large thumbscrew on the very rear of the carb near the base, and that does change the idle, but it sounds like all it's doing is allowing air into the carb as opposed to holding the throttle. The car also runs with a pretty profound misfire lately.

If anyone knows of a printed procedure for adjusting the carb and idle, I'd sure appreciate it (with pics would be even better). Thanks.

Try contacting Pony Carburetors 505-526-4949. They should be able to help.

Take an unlit propane torch and work your way around the top of the engine will running. If you hit a vacuum leak, the engine will speed up. I had a similar problem on a different vehicle. Turned out it was the gasket between the intake and the spacer.

Thanks for the tips, guys. As it turns out I think I have the 4300, not the 4100, which apparently isn't the best carb Ford ever produced. No intake leaks (thanks Vince), but I did get the idle settled using that thumb screw on the rear carb body. At least for now...