throttle shaft vacuum leaks

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.

I had a similar problem with the shaft leaking. This was on the "Ford" 2 BBL and it to drove me nuts in trying to find out what the problem was. Although the carb did not have that many miles on it (less than 80,000), it was more like the how many years it was on the car (32 years). I guess it plume wore out just sitting on the car. The way I discovered it was by using some WD 40 in and around the linkage. The second I sprayed the linkage, the car almost died. And yes, it was running a little bit lean. Just a thought and my remedy was to trashed the carb and find something a little more to my liking.