About a year and a half ago I could tell the engine had a vacuum leak so I sprayed carb cleaner and did the spray soapy water and blow air up the PCV hose test. Both showed that there was a leak at the throttle shaft bushings on the Carter BBD.

I had them replaced by a local carb shop and all was good. I tuned the carb and didn't have to touch it again. I probably only put about 150 miles on the car in the last year and a half but I've gone out there and started it up about once a week with no problem.

Last week I finally get it on the road after an alignment and all the sudden there's an obvious vacuum leak. I run the same tests again and there's a leak at the throttle shaft bushings. At this point I'm thinking there's no way this is happening again.

So yesterday I removed the bottom off a spare Carter BBD and put it on my carb and I couldn't even get the car to start. So I put my original carb back together and it idled poorly after I installed it but I turned the mixture screws in about a turn and a half a piece and it was purring like a kitten with 20hg of vacuum. So I'm thinking ok the mixture screws were turned out too far because they were compensating for a previous vacuum leak. I sprayed carb cleaner at the throttle shaft bushings when it was idling and the engine never revved up. Leak magically fixed itself.

Now I go to drive it today and I'm back to square one at 15hg of vacuum and it wants to die any time it's at curb idle. I ran my vacuum leak tests again and sure as s***....it's showing a leak at the throttle shaft bushings. I'm dumbfounded at this point.

There has to be some sort of leak at the throttle shaft bushings in order for the throttle shaft to turn right? I mean it can't be sealed up 100% but it should still be sealed enough to not rev up if carb cleaner is sprayed on it correct? I just don't know how the problem magically fixed itself and came back because there are no detectable leaks anywhere else other than each side of the carb at the bushings.

Have you considered a vacuum leak at the carburetor base? The base is really close to the throttle shaft ends. Have you tried a new gasket.
Also, do you have information on exactly how the shop that you hired rebuilt the throttle shaft ends?
They may have rebushed them properly or maybe not.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Could be the shaft has a position that mostly blocks the leak along the shaft, and other positions where a leak is opened up. If the shaft or bushings are ovaled, when it parks at idle you could have variable position of the shaft in the bushings. Can you wiggle the shaft while it is idling (without twisting it - opening he throttle)?

Gasket could be an issue too, as DadTruck said, esp if the carb to manifold bolts are not tight enough or the gasket is damaged.

Lou

_________________
Home of Slant6-powered fun machines since 1988

Does the shaft leak in order for the whole thing to turn? Maybe.

I guess theoretically it must leak 'some'. But....I think a good mechanical engineer would tell you that if you have your clearances 'properly established' you approach the point where there is no leakage at normal engine vacuum levels. In other words, a seal is created due to the tight tolerances - the air would rather stay outside the carb (where the shaft is) than be sucked in. There would be 20 inches of vacuum measured on the inside of the carb where the shaft is - but 0 inches measured on the outside. Crank up the vacuum to 200 inches and it'd leak - but that'll never happen.

You can be sure that on the average 2019 model car driving down the road there is no shaft leakage. No way would they tolerate that. So your goal is to get to that point as well. I'd study a modern throttle body to see how they seal it - tight tolerances, brass bushings, or even Teflon O- Rings.

Can you visually see the leakage? When you spray a liquid on the shaft can you see it getting sucked in? If you just hear the engine speed up, that tells you it's getting in but not where.

At first my thought was that I guess the act of removing and replacing the carb fixed the vacuum leak because maybe the leak was just at the base gasket and not at the throttle shaft. The first go around I specifically shot the carb cleaner below the throttle shaft at the gasket and nothing happened. Shot right at the bushing and it revved up. The bubbling also came right out of the bushing when I sprayed soapy water. This go round, I've performed the same tests with the same results.

I have 4 spare Carter BBDs but this is a NOS one I bought years ago so I run this one. I checked the throttle shaft play by hand on all of them and the one I'm using has by far the least amount of play in every direction. I took the carb to this same shop about 15 years ago and had him replace the throttle shaft bushing and I've probably put roughly 100k miles on the car since then. It's just I've only put a couple hundred on it since the 2nd time he replaced the bushing.

What are you guys using as far as hardware to hold your carbs down? Any type of lock washers or nylon nuts or anything to keep them from backing out?