My 455 idles/cruises very rich so I decided to roll in and get it sorted. Holley 4150 vacuum secondaries running e85, 11.25:1 compression, mild+ cam (about 10 inches of vacuum), 2800 stall th400, MSD Ready-To-Run distributor. Engine was tuned with actual e85 I got at the local track although I run the pump stuff in town which varies from e50 to e70 which of course is making it run even richer. Mid to WOT my Air to Fuel gauge looks good, around 10. Idle and light cruise are 5-6, which is pretty fat. Carb has 80 primary jets so I jetted down to 76 with very little improvement. I have the idle air mixture screws pretty much closed now so and it is still way rich.
I want to dial in timing before moving back to the carburetor adjustment. When I had motor tuned initially it was set at total timing of 35 degrees and no vacuum advance. With total timing at 35 degrees, initial is at 18 degrees. Wondering if I should circle back and look at hooking up vacuum advance. I don’t know what the trade offs will be but I am doing more cruising lately and that is when the rich mixture is most noticeable. Appreciate any input.

 

A blown power valve diaphragm will cause a rich idle. Has it backfired on you since the carb was installed, Thats usually when they blow.

 

My distributor is set up with 18º initial and I have a 10º vacuum canister (original 24º unit with a home made limiter) connected to full manifold vacuum. The added timing from the vacuum canister connected to manifold vacuum really helped the idle quality.

 

Will check the power valve. I’m at 18 initial as well but only because I want to be at 35/36 total and then checked initial once total was set. I have figured out how to adjust the mechanical advance if need be. Mechanical advance is all in at 4000 (factory setting by MSD) but looks like a lot of folks run their mechanical all in at 3000 so that adjustment might help with cruise mixture.

 

You can put 60° of timing in it, it won’t matter. Until you start playing with the air bleeds and idle circuits in that carburetor, your cruise air/fuel will stay fat.