Hello all, having a issue with my 440 running way to rich at idle in particular. The build is fresh 440 with six Pac pistons, 82 cc sidewinder heads, comp xs282 cam, Holly street dominator and 870 street avenger carb. I have a msd pro distributor set at 18 initial. I'm running NGK bcpr6es plugs. The carb is new with a 5.5 power valve. Currently I have the mixture screws set at 3/4 on all 4 corners.
At idle around 850 I get small amounts of black smoke puffing. My plugs are black soot and foul often. Looking for a direction to take to lean it out. I have a jet kit but I'm not sure if smaller will affect the idle. Any help would be greatly appreciated.
Others will be able to fill in with more experience than me, but I'll toss in my :Twocents:
I have set the idle mixture using a vacuum gauge (set to max vacuum and then open up by 1/8 turn). I also foun that the advance at idle helps let my engine idle without being super rich. You metnion 18 degrees advanced and this is about where I am set. I believe the black plugs can happen with lots of idling even when the mixture is not crazy rich. I do not know about the black puffs of smoke. I know if it is crazy rich, it can have the approaching burning eyes sensation.
I have been using an air fuel gauge and when I set the way I escribe above, it starts out a little rich and then when warmed up, works up closer to stochimetric, but sill in the gauge reading in the 13.5/14.5 A/F range. I think mine will run leaner, but it will stumble.
From my understanding of carburetors. The idle screws set the mixture at low RPM (idle) and this can extend up higher. The other aspects of the carburetor play a role at higher RPM. Assuming all ignition and vacuum leaks are absent, I would look into the mixture screws first.
Good luck with this.
if its puffing black smoke I would say you have to high of a float level OR blown powervalve OR to much fuel pressure
Take the carb off and look to make sure you don't have the primary throttle blades into the transfer slot because that can make your idle rich and the idle mixture screws become kind of lazy.
My similar build 440 has a 750 and I went up a couple sizes on jets. You may need to try going a few smaller. Don't be afraid to mess with it just know what you changed.
New carb or something that's been sitting around for some time. When I fired my Bee up it would burn your eyes out sitting idling but otherwise ran pretty good. Power Valve was absolutely toast when I tore it apart.
The fuel level is about 3/4 of the way showing in the sight glass. Currently my pump puts out about 6 psi. At idle. I put the 5.5 power valve in there replacing the 6.5 that came in the carb thinking it was a issue at 12ish vacuum. Im stumped. I may pick up a 750 to see if it changes up. The 870 is brand new so it's not been sitting. I have a jet kit, I was just under impression the primaries didn't affect idle mix.
Quote from: Rayzor on January 18, 2023, 05:14:43 PM
The fuel level is about 3/4 of the way showing in the sight glass. Currently my pump puts out about 6 psi. At idle. I put the 5.5 power valve in there replacing the 6.5 that came in the carb thinking it was a issue at 12ish vacuum. Im stumped. I may pick up a 750 to see if it changes up. The 870 is brand new so it's not been sitting. I have a jet kit, I was just under impression the primaries didn't affect idle mix.
Do you have a fuel pressure gauge ? if the power valve is good it wont change the AFR at idle with a different vacuum rating
Yes I have a fuel pressure gauge inline at the carb. I'm not getting a lot of changes from the idle mix screws. I have tried lowering my idle as much as possible to try and adjust the mixture but it's not much better then where it's at. I do not have and miss or bogs though. I ordered a autometer AF gauge so hopefully it can give me more info. Might just be the nature of the beast to load up on idle.
Quote from: Rayzor on January 19, 2023, 06:36:08 AM
Yes I have a fuel pressure gauge inline at the carb. I'm not getting a lot of changes from the idle mix screws. I have tried lowering my idle as much as possible to try and adjust the mixture but it's not much better then where it's at. I do not have and miss or bogs though. I ordered a autometer AF gauge so hopefully it can give me more info. Might just be the nature of the beast to load up on idle.
Look down the carb while its running & look at the boosters & see if gas is dripping out of them. BOOSTERS not the accelerator squirters
Just scanning through these and thinking some more. all good suggestions. I hadn't thought about looking at the boosters. There was one a ways up about the transfer slots. Did you check those.
I agree but that 870 is kinda on the big side to start with. :scratchchin:
Look around here something may help: https://www.mopar1.us/engine2.html
Another: https://www.ramchargercentral.com/threads/the-best-way-to-tune-a-holley-carburetor.326226/
One more: https://www.motortrend.com/how-to/how-to-custom-tune-holley-vacuum-secondary-carb/