I have a 8-71 blown 555 with two 950 Holley carbs running 10 psi of boost. Problem is at idle the sucker burns your eyes out of your skull! (very rich mixture) I have changed power valves to meet the vacuum signal required with very little difference. Will changing the primary jetting down a couple sizes hurt performance, or clean up the idle? What about changing the low speed air bleeds to a bigger size? Or should I plug the power valve circuits? The fuel pressure stays at a solid 7psi. and the floats are set at the correct level. Timing is set at 35 deg. locked out. Could a weak ignition coil be a problem ? I am using a Mallory Hy fire VI box (685) with a Jegs canister type coil (its all I had at the time) . Other than the rich idle it runs great.

Just make certain is in fact rich at idle and not lean. Lean can misfire and stink at idle too.

Before you mess with air bleeds (jets and power valves will do nothing for your idle) how many turns out are your idle mixture screws when the idle is set for best vacuum?

A quick way to see if the idle air bleeds help, is to take out the 2 idle
air bleeds on the primary side of each carb. See if it will still run, in most
cases if its that fat it will still idle (it may have a flat spot), this is just a
temporary thing to do to see if it helps out the idle - dont drive it that way.
This will tell you if you need to open up the idle air bleeds. You can
always buy some more and put it back to where it was. Open them up
2 sizes at a time.