The Stovebolt Site Forums The Shop Area Engine Shop Possible vacuum leak?

My truck has been running rich for some time. Today I tried spraying some brake cleaner around the base of the carburator looking for a vacuum leak. The only place I got any reaction was within the circle drawn on the picture below. I was looking for an increase if there was a leak but what I got was a slow down. It was idling smoothly until I sprayed. The motor quickly slowed down to almost dying. Then quickly returned to idle. Any suggestions?

1956 3100 235 , professionally rebuilt Carter YK

Maybe the ported vacuum advance fitting is not tight. That would be a significant leak, and the brake cleaner may have essentially choked out the engine.
But you say it's been running rich. A vacuum leak at the base of the carb should make it run lean.

I sprayed all around the base with special attention to both ends of the vacuum line. No change. I sprayed the plugged port on the side of the intake manifold. No change . Then I went around to the the center of the circle and that's where the engine immediately slowed down and then quickly went back to a good idle. I tried it several times with the same result.

One other thing, the engine running rich was so bad that the plugs would foul bad enough that the motor would quit running after about 60 miles. After the Carter carb. that stopped but the motor will still bog down if I floor board it or just increase to the floor on a steep hill.

Who was the professional that rebuilt your carb? These days that doesn't mean a whole lot.

Fix the vacuum leak first and see what happens. It's possible to have a small piece of debris inside the carburetor keeping the accelerator pump from sealing or any other number of things.

Clean the daylights out of the carb with lots of carb cleaner inside and out and in every orifice. Sometimes that'll dissolve whatever might be causing trouble.

I purchased it from Jegs but I'm not looking to blame someone and I'm not going to tear it down until I know as best I can what the problem is. I don't understand why spraying a very flammable liquid into a vacuum source would cause the motor to almost die instead of speeding up. I did find out that a big reason for the running rich was the vaccum holes that didn't match up between the carb. heat insulator and the carb. Matter of fact they still don't match up but I created a new hole for the insulator with my dremel tool that got rid of most of the carboned up spark plugs and got me back on the road again.
P.S. Right now I don't yet know where the vaccum leak is, if it is a vaccum leak. . I spray the vaccum line fitting at the carb. ,and nothing happens.

You don't have to tear the carb apart. Just spray down the throat, vent, etc while running. Work the throttle by hand. You might free something up that's clogged with a small piece of debris.

You said you got the carb from Jegs. Is it an actual Carter YF or is it a newly made copy of a YF?

Use a propane torch to test for vacuum leaks. Just don't light it.
To verify what it's supposed to sound like when you find a leak, put the end of the UNLIT torch above the throat of the carb.

Thanks for the suggestions.

Otto it is an original Carter YF. I'll try your suggestions.


Carl I'll give that a try as well.

Carb cleaner isn't that good of a fuel, could be why it slowed down. If you have the over rich mixture problem solved, get it out on the highway and run hard for a while to really clean up the plugs and cylinders. If it doesn't idle better, change out the plugs. Fouled plugs don't always clean them self up.

As far as the vacuum leak, pull the carb off so you can see it up close and make sure the vacuum line fitting is sealed where it screws into the body. While it's off, look it over, tighten the screws, and make sure you have plenty of room at the spacer where you opened up the air holes. Also, make sure the intake manifold is clean where the spacer and carb bolt on, no rust or gasket bits. Use a straight edge across the base of the carburetor, after this many years, it's not uncommon to have bent mounting flanges. A solid flat surface with some 320 paper will flatten it back out if needed, but go slow. Over time gaskets compress and need tightened, and rebuilders are not known for using quality materials or the correct parts.

Those heat plates or spacer only use one gasket on the bottom, the carburetor sets right on it. Rotate it, flip it, what ever it takes for the slots and hole to align, or as you have done, make a new hole. They never will run right if the alignment isn't right, and while you are checking, make sure the spacer is flat.

I'm betting the vacuum passage to power valve is blocked, not getting manifold vacuum keeping power circuit open all the time.