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 carburetor off and back the idle screw out until it doesn’t hit 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.
Doesn’t Idle Well
Surges, or simply runs rough. This can be caused by any of several reasons, not in any order:
- Put a vacuum gauge on one of the vacuum ports (carburetor, or manifold). Adjust idle speed to manufactures specification, engine warmed up. Adjust idle until vacuum stays steady. If the idle doesn’t change, then you have other problems.
- You may have a vacuum leak. Check around carburetor mount, intake and any vacuum line attached. Use something like spray carb cleaner to spray around gasket. If engine speeds up, or smooths out, you found the problem.
- Check the idle passage from the idle mixture up to the top of the float bowl. Blow air in the hole and make sure it’s all the way through.
- The venturies may not be sitting flat, or has the wrong gasket. Are any holes covered up that shouldn’t be. Sometimes the gasket needs trimming to fit flat.
- There are idle air vents that lead up through the carburetor float bowl and through the top of the carburetor. Make sure all small passages are clear.
- Does the engine smell gassy? Perhaps it’s running rich, or flooding. This would be a another list of possibilities.
- There is a transition hole above the idle mixture hole that also leads to the top of the carburetor. Be sure this passage is clear.
Off Idle CircuitTake a look at this video that illustrates the off idle.