I have a 57 Carter AFB on my Ford 312. It wants to idle at 3000 rpm. i noticed the secondary venturi are flowing fuel even though all the ****erflies are closed. Any ideas on what all to check? i noticed there are gaskets under the venturi.
This is one of the very few AFB's that is a vacuum secondary. Check the secondary diaphragm. These go bad with time, and will cause problems such as you are experiencing. The diaphragms are VERY expensive, and many opt simply to plug off the entire circuit rather than spend the money for the diaphragm. Carter didn't like this style carb even when they were new, and refused to make them after a few years; going to the "demand" secondary used on the rest of their AFB's. The vacuum secondary was used only on Ford products and a few Chrysler products. Jon.
i also made sure the plates ****erflies are closing fully which appears that they are. i re-read the post. i'll look at the diaphram tonight and see if its ok.
Was it okay before? It sounds like a big vacuum leak. If you just installed the carb on the car look for a port that should be plugged but isn't. (And if the car ran fine with that carb until now, ignore this post!)
it seemed to have run fine on a tired 292 but the fuel mileage was off i thought. I checked the secondary diaphram and it seems ok. not visiably torn. Should i plug the vacuum port to verfiy it isn't the culprit?
If this specific carb ran well on a different engine; then probably the carb is not the issue. As others have stated, probably vacuum leak. Possibly incorrect carb to intake gasket? If bakelite spacer under the carb, possibly cracked? Jon.
i'm going to look at the spacer and the carb base to make sure there are no cracks. i'll let you know what i find.
Solved the issue, problem went away when i replaced the gasket between the manifold and carb spacer. I'm trying to figure how it would cause this issue and not one like i'm having now where i'm only getting 10" vacuum plus it doesn't want to idle steady. i know i do need to do a hot lash so i'll start there and look for vacuum leaks. Good news is no more idle of 3000 rpm!
The lazy mans' method of checking vacuum leaks is with a can of starting fluid. Spray around base of carb and all vacuum lines. A leak will reveal itself by raising the idle about 2,000rpm. Also for the rough idle, if that carb has set for a long time, I pull the mixture screws out and run a welding tip cleaner through them. Can't tell you how many times I've had crud blocking one mixture port.
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