The carb on my smal block engine has begun running too rich. It was rebuilt about a year ago and worked fine. Recently I noticed soot on the transom and the smell of excess fuel when ideling. Tried adjusting the mixture screws this weekend. They do not cause any change at all...from screwed all the way in to 3 turns out. Any ideas as to whats wrong?

probably a leaky power valve.

Forgot to mention...the idle quality isn't too bad...someone not paying close attention wouldn't notice the slight increase in roughness. Still think its the power valve?

Usualy when adjusting the mixture has no effect, it's a vacuum leak or dirty needle and seat. Does it stall or idle higher than it did?

q-jet or holley? If its a q-jet i suspect the well plugs are leaking,if its a holley i would pull it off and go thru it,even if its only been a year ago,look for bad power valves or loose jet plate screws,if you rebuild it yourself you can change everything for 50$ worth of parts,Smitty

Forgot to mention, its a Holly 4 barrel. Engine doesn't stall or idle any faster than norman, just slightly rougher. Plugs, transom soot, and smell tell me theres too much fuel going through it. Again, the mixture screws are ineffective. Any other ideas?

Also check the level of your fuel bowls.

Its not the float level....they're set at the right height. Guess I'll replace the base gasket first and if thats not it, attack the power valve.

Look down the carburetor while at idle and see if fuel is dripping out of the venturis. If it is then your float level is set a little high.

Put your finger over the primary vent tube. If it stalls the power valve is probably OK. If it does not stall then the bowl is being vented by a blown power valve.

If you had some crud on the needle & seat I believe that it would load up and stall instead of just running rough

Roby