|Adventures Under the Hood|
Rough, unsteady idle problems are all too common with the Carter BBD on the 258 CID engine. This can be caused by several different things, but the most common is a too-rich or too-lean fuel mixture.
You can get a clue to which it might be by removing the lid of the air cleaner and looking down at the metering pins. If they are all the way forward, the mixture is too lean. If they are all the way back, it's too rich. This assumes of course that all the other systems are working.
If it's too lean, check for air leaks around the carburetor base and the intake manifold gasket. Check all vacuum hoses for cracks, breaks, and leaks. A bunch of little leaks add up to one big one. If the hoses appear cracked and aged on an older engine, it would be worth your while to go to the auto parts store and buy new hose and just replace them all. It's inexpensive and not hard to do.
Check the Evaporation Canister for correct operation. It should not be purging at idle.
common cause of poor idling with the Carter BBD is clogging of the idle
tubes. If you look straight down the carburetor you will see the venturi
cluster. Removing it will reveal two small brass tubes, the "idle tubes". These
can become clogged, causing fuel to drip and poor idling.
Clean them with piano wire or some other suitable tool. Better yet, drill them out to .032" and reinstall. This can be done without removing the BBD, but take special care not to drop any small screws, gaskets, or other parts down the carburetor.
There is nothing to be gained by adjusting the idle mixture screws if the rest of the system is operating properly and the metering pins are more-or-less centered at idle. If they are, but you still have a rough or unsteady idle, then the cause of your problem is elsewhere.
Consider this: The computer controls the mixture at all times, which means at cruising speed and at idle. If you try to tweak the idle mixture, the computer will simply compensate for what you just did by moving the metering pins in the opposite direction.
If you are unable to center the pins as they should be by adjusting the mixture screws, and you are certain that the rest of the system is in good shape, then perhaps your carburetor is clogged up inside and needs rebuilding or replacing.