throttle shaft vacuum leaks

[heyjavelin]

If your car has an erratic idle speed/poor idle quality/can't adjust idle mixture screws, here is a good quick test. After ruling out all other vacuum leaks, take a can of Berryman B-12 or any other similar carburetor spray and spray around where primary throttle shaft goes into carburetor body with car warmed up and idling. If idle speed increases, the throttle shaft bore is worn and you have a vacuum leak. This is fairly common on old carburetor bodies.

You can check thru Hemmings to find someone who can 'bush' the throttle shaft by drilling out body and adding a bushing that will restore tight fit to throttle shaft OR look for another carburetor. If you have a throttle shaft vacuum leak no amount of adjustment can make the car run right until the vacuum leak is removed. Also this will lead to a lean condition and can contribute significantly to premature engine wear and valve failure.

These slight vacuum leaks will also vary in intensity. Car runs sorta ok one minute, then idle speed can pick up on it's own next, or can not even want to idle at all. This is from wear in throttle shaft bore e.g. one moment it can leak worse than the next particularly as throttle is moved back and forth.