PaulJ2 wrote:
My guess also is dirt in the idle circuit. One more thing I didn't see Wes cover. Make sure the idle-stop solenoid is working on the carb. The throttle linkage rests against this while idleing with the key on. Retracts with key off further closing the throttle to prevent dieseling.
This solenoid is often a culprit, thanks to PaulJ2 for mentioning it, cuz I plum forgot. Like he said, the solenoid holds the carb slightly open for correct idle speed as set by the idle speed adjustment screw. Since these smog motors idle so lean, they tend to diesel on after shut-down, so this "stop" solenoid is designed to rob all air from the intake by closing the butterflies off entirely when the engine ignition is shut down. If you observe and have someone cycle the ignition key on and off, the solenoid should click and visibly lift the throttle ever so slightly with ignition on. Or it's broke.
Since vacuum leaks are very common on engines with old, cracked hose lines, that is definitely a possibility. It would pay to eliminate the possibily by global replacement, or at least very careful inspection. Messy, but sometimes squirting oil on suspected gasket joint or hose leaks will momentarily plug the leak with thick oil, allowing it's detection.
The idle mix screws may be a bit hidden by having a plastic lock cap over them to prevent tampering, but these can sometimes be saved with care, or just break them open if the truck is not subject to smog checks.
One danger in removing idle mix screws to back-blow (squirt WD-40 etc) through them, is that the engine may idle better with them being falsely adjusted to make up for a slight vacuum leak, one of the reasons I recommend very carefully resetting them back to the exact original position. The right way to do it is to end up at the same factory setting when all leaks are fixed. Although many people abhor smog era motors and try to beat the factory settings, the OEM setting with all EPA equipment intact usually works the best overall.
Keep your chin up, your RV 454 will perform quite satisfactorily when all is again well. They are great motors and the Rochester Quadrajet was my favorite. They all flow at least 750 cubic feet when wide open, yet run quite economical on the two small front venturi throats when maximum power is not demanded. Good luck.
Wes
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