When the Holley 1940 carburetor throttle shaft gets .010 or more of wear, it is time to add bushings to the throttle body. Without a run-out mike this might be difficult to measure, so estimate anything over 1/32″ in sideways action. When the throttle shaft is too loose it will cause a vacuum leak, which will make the engine idle rough. Besides measuring you can test this situation by spraying carburetor cleaner around the throttle shaft while the engine is running. If the rough idle smooths out, then you need to re-bush.
If you don’t think you have the expertise to tackle a job like this, then you should take it to a machine shop, otherwise here is one way you can accomplish this without having to have a lot of sophisticated machinery.
The bushings required can be found in our Holley 1940 carburetor parts catalog.You will need to order two bushings.
Carefully ream out the hole so that the new bushing will fit. Use an expandable reamer HS 6A. It needs to expand to at least .345″. You want to ream the hole until the bushing fits into it tightly. Not so tight that you have to beat it in, but it should take some pressure. If you get it too tight and have to beat it in place, the bushing will reduce down and the throttle shaft might not turn as freely as it should. You can add a bit of JB Weld around the outside of the bushing to make sure it stays put, but with the right fit you will not have to do this. Once you get the bushing in grind down anything that is sticking out so that the bushing is flush with the throttle body.
That is it. Replace the throttle shaft and you are on the way. Take your time, because it doesn’t take much of a mistake to ruin the throttle body.