91 454's rochester 4 quadrahejets. Gill silent choice exhaust, premium fuel. Engines were rebuilt before I had them about 60hours ago.. Its more of grey film underneath than black but is definitely exhaust stain. Carbs are being rebuilt as we speak.

boat is a fountain fever.

do you run the silent choice a lot ? that restricts the xhaust quite substantially and will make it have a rich idle...

probably 25% of the time.

I must add the exhaust pipes themselves arent black with soot. Could the oil pump be pumping to much? I did lost about 1/2 to 3/4 qt of oil after most of the summer of boating, but I dont think thats to off base

Mine used to get black, idle was wayyyy rich, leaned it out and no problems now.

When I read the title of thi thread, I thought for sure this was a 496........

which screw is the lean one?

On a Quadrajunk...I mean Quadrajet there are two screws in the base on the front of the carb that are the idle mixture screws. Back in the 70's/80's when I messed with them we would use a baseline of running them all the way in and then backing them out one and a half turns. We would then adjust in or out from there.

If they are stock engines with stock compression, then stop running premium fuel. It does not burn as completely and causes the black soot.

i can't imagine 1/2 qt over 20 or 30 hours or whatever your summer i is way out of line... there are so many things that could do this to you... first of all... that carb is a 40 year old design from the era when nobody cared much about anything ... how old is your actual example ? a lot of them had problems with the floats saturating w/ fuel and getting heavy or sinking which would make them do rich things in weird ways depending on load... that screw is just an air bleed if memory serves and won't change the actual idle fuel total... but i can't remember what drives the idle circuit on that carb... never the less...
its the same advice i always start with... start with the simple stuff... is the timing correct at idle ? are the temps too cold, have the plugs ever been changed ? all the simple stuff first. has the carb ver been looked at and jetted by someone that actually knows what they are doing and not just watched 3 episodes of hot rod TV ? how about cleaning it virgin and doing a complete trip and never use the silent choice and see if that answers your question ... i suppose it just comes down to how much it bothers you vs how wrong it really is that will determine how much time and effort you spend chasing it... but if it IS legitimately rich, then you are wearing out the motor prematurely for sure...