Q: My Carter AFB is not idling well and the center mixture screw doesn’t seem to make a difference. I’ve tried two different carbs with the same issue.
NOTE: Not all AFB carbs have the center adjusting screw
A: Well the fact that it does the same thing with two different carbs tells me it’s not likely the carbs. I would look at the electrical first. A compression test might reveal one or more low cylinders which will cause an idle problem.
Don’t discount a vacuum leak that hasn’t been found yet. The center screw not making a difference is worrisome, that smells of a vacuum leak to me. Double check the mounting gasket to be sure it isn’t leaving some port exposed. Spray carb cleaner around all vacuum sources and mounting gasket. A change in RPM tells you that you’ve found the problem.
Q: Why does my AFB idle too fast?
A: Idling too fast can be caused by a vacuum leak, or the carburetor itself.
1st disconnect the throttle linkage to make sure it isn’t keeping the throttle open. Take the caburetor off and back the idle screw out until it is free of the cam. Look at the throttle valves to make sure they close all the way. Very little light will shine through. If they have been put in the wrong way, then they may not be sealing. Make sure the fast idle screw is not touching the cam when the engine is warm and the choke valve is all the way open.
Most likely something is holding the throttle valves open.