I have a Holley 800 DP (HP 500 version) on a 489 stroker that seems to be running rich at idle lately. I don't have an AF gauge set up on it to really tell, but it sure doesn't pass the nose test. Oddly enough, I don't seem to be getting a massive amount of black transom disease to go along with it, but you can definitely tell that it smells rich, to the point that I am getting complaints from the wife and kids to "shut that thing off before it kills us all". I don't recall this always being a problem - seems to me that it started to manifest itself last year, but my recollection could be a little foggy due to, well, exhaust fumes.

I know that idle mixture screws can control the AF mixture at idle, but you can pretty much rule the main power jets out of the equation, right? Would the power valve be a possible culprit? My thinking is that anything that can affect vacuum level at idle could possibly cause a rich condition from the power valve, since it is vacuum sensitive. I guess I need to get a vacuum gauge on it for starters.

Any other ideas, carb gurus?

Check to insure butterflys are below the "off idle slots" in the throat of the carb and adjust idle metering nedels w/vacuum gauge. Holley tech u-tubes are your friend.
Good luck.

The only way a power valve will cause a rich idle is if its blown or it has become loose.

Get a can of carb cleaner and blow out the air bleeds.

I'll give that a shot. I'll pull out the idle screws and get some thin wire to rod them out too.

Ck vacuum like you mentioned as well as PV. I suspect you'll have this solved quickly.

pull the primary meetering block off and ck to see if the dia side of the power valve is wet, they love to blow, leak at idle..

If you close both idle mixture screws in, the engine should stall from lack of fuel, if it doesn't it's getting from somewhere else, like a blown power valve as mentioned above

Budman did you ever get to open that thing up yet to see what kind of power you picked up with the new engine ?