would the idle passages and jets have normally been cleaned when it was rebuilt?
Should have been cleaned, but mistakes happen. But as others have suggested, first make sure there are no vacuum leaks, and that the gaskets are sealed (spray them as described in that web site above).
@yadmas:
Here’s something you can look into.
Do a search for:
Carter 4bbl AFB carburetor tightening the air cleaner warps the carburetor air horn + stalling.
Back in the 70s, many Chrysler engines used the Carter 2bbl carburetor. We encountered warped airhorns on hundreds of carbs. It occured from overtightening the wingnut on the top of the air cleaner.
The warranty fix was to remove the airhorn, place a piece of wet sandpaper on a flat glass surface, and then rub the airhorn on it (in a circular or back-n-forth pattern) till you could see the warpage was gone.
I just did a search and it came back saying that same problem existed on the 4bbl Carter AFB carbs as well.
You need to make the ignition adjustments before you can set idle mixture adjustments.
The Carter AVS on my Dodge has the single air bleed idle adjustment screw. If there are and deficiencies in the ignition system or vacuum leaks, the air bleed screw won’t have any effect on idle mixture tuning.
I don’t think returning the carburetor to the rebuilder is the solution.
A new distributor on a car that old is always a good idea, especially if the screws for the vacuum advance are stuck. The mechanical advance is sometimes worn out. So good work there. If points are new, run the file through them back and forth twice to insure contact. Sometimes the car won’t start without it or will fail the day after if it rains.
UNRELATED: Make sure the #5 and #7 plug wires are on the right plugs. Usually a small block Chevy problem not Pontiac.
A google search suggests a couple more ideas:
Common Causes and Fixes for Carter AFB Idle Instability
- Vacuum Leaks: Unmetered air entering through a cracked vacuum line, a worn throttle shaft, or a leaking carburetor base gasket causes a lean, unstable idle. Spray carb cleaner around the base and linkages while the engine runs; a change in RPM means a leak is present.
- Exposed Transfer Slots: If the idle speed screw is turned in too far, the throttle plates open past the idle transfer slots. This bypasses the idle circuit entirely, making the idle mixture screws unresponsive. Back off the idle speed screw to close the throttle plates correctly.
- Clogged Idle Passages: Modern ethanol fuels leave varnish deposits that easily block tiny idle channels and jets. Remove the idle mixture screws, spray cleaner, and use a fine wire or compressed air to completely clear the passages.
- Incorrect Fuel Pressure: Carter AFB carburetors are sensitive and generally flood or run rich if fuel pressure exceeds 5.5 PSI. Use a pressure regulator if your fuel pump pushes too hard.
- Improper Adjustment Procedure: Lightly seat the idle mixture screws, then back them out 1.5 to 2 turns as a baseline. Adjust them with the engine warm to reach the highest, steadiest vacuum reading.
You get the idle speed correct first, using the steps i detailed in my post of a few days ago. Then with the vacuum gauge connected, gently and slowly turn the idle mixture screws clockwise and then counter clockwise until you get the highest vacuum reading. Please don’t take offense, but you’re jumping around too much. Take this step by step in the order I already gave you.
It could be the dwell IF the distributor shaft bushings are worn out. It"s a simple test. If you have a dwell meter and the points are set correctly, the dwell should be 30* and rock steady at all RPM’s. If it is not steady, then the new distributor was a good choice. The other way to check it is with the engine off and the cap removed, turn the engine until the points just open. Then physically move the shaft back and forth (not turning). If the points are opening, the shaft is worn.
As for the mixture screws, the initial setting is all the way in until contact, DO NOT TIGHTEN, than back 1.5 turns. With the engine running, adjust for highest vacuum, around 20". Turn both the same amount, it should not exceed 1.75 turns total, in other words, not more than a quarter turn out from base, nor quarter turn in from base setting.