I have a 1986 Dodge D150 pickup with a 225 slant six with a Holley 1945 1BBL carburetor and all OEM equipment. It normally starts and runs fine. However, if it has not been started for a few days it sometimes will not start and if it does start it stumbles badly and will not idle. The fix (as strange as it seems) is to remove the needle valve, inspect it and re-install it. The engine will then start correctly and run smoothly. There is never any trash in the needle valve. What could cause this problem?

Misadjusted choke or improperly installed air cleaner preventing the choke from closing. Removing the needle and seat requires removing the air cleaner which would permit the choke to shut fully.

Alternatively, it may be a problem with the lean burn system on the car.

Have you replaced the needle and seat? Checked the condition of the float and the float setting? Rebuilt the carb and verified all adjustments? Confirmed that all vacuum hoses are hooked up and properly routed for the lean burn system?