My carburetor knowledge is extremely limited, and trying to help a good friend of mine here. He has an 830 Holley HP 80509-1 on his stroker small block. This carb has been on the engine for a good 4 yrs, and has always ran real well. He just started having an issue with it. Out of the clear blue, it all of a sudden doesn't want to idle.

Started to investigate, so he took the air cleaner off, and also took off the stud that goes in the center of the carb which holds the air cleaner on. Fired it up with these two items removed, and now the engine now idles fine, just like it used to before it started acting up. Put the air cleaner stud back in the hole, and the idle goes crappy...to where the engine dies.

Took the air cleaner stud back out, fired the engine up. Idles fine. Placed finger over the hole where the air cleaner stud goes, and there is strong vacuum there at idle and also at any engine rpm. Revving the engine to any rpm and placing your finger over the hole makes an immediate difference in how the engine runs.

What's going on with this carb, that there is vacuum where the air cleaner stud screws in, and that this is effecting how the engine runs and idles?

Thanks in advance!
TJ2