[ttrip]

Need help with my 70 Chevelle, I have a 350 that was done 60 over with a Holley 600 (model 4160) and a Comp Cam (12-238-2) the duration @ .50 is 218 in. and 224 ex.

The problem I'm having is a black cloud coming from the tailpipe during aceleration and a running rich smell.

However, when I pulled a plug it shows perfect firing (brown ashey color).

Can anyone tell me what's going on and what I should check. Thanks Tom

 

[chevelletransamlegends]

a running rich smell ? never herd that one before ! hehe, i wouldnt worry about black smoke to much could be that the primarys on the carb are perfect but when you floor it and the secondarys kick in there running rich - should be able to change the jets on the carb to fix this and lean em out a bit, give more power as well when all done.

 

[Billy177]

I second that try leaning out the secondaries

 

[Pro68Camaro]

I wouldn't be surprised if you were rich in the idle circuit if it smells that way. Does it run well? Does it stumble at cruise or when whacking the throttle?

What power valve does it have? You'll want to make sure it's a couple of points below the vacuum signal at idle. That's a pretty mild cam and it probably idles pretty easily, but I'd err conservative on the PV and go with a 6.5 or something to factor that out as an issue.

You may smell it at idle and it's not an issue once you're driving it (hence the clean plugs). Unfortunately you can't really read the plugs after accelerating unless you kill the motor immediately. Any add'l driving will change the plug read.

Anyway, if it continuously blows black smoke when accelerating, I'd look at it being jetted too rich. How's it jetted?

 

[Tom Mobley]

you might want to check the accelerator pump squirter, somebody may have changed it. Did you buy the carb new?

It's extremely unlikely that it is rich on the secondarys, it uses a jet plate which are normally a size or two lean.

the only way the PV can make it rich enough to make black smoke is if it's bad, has a hole in the diaphragm allowing fuel to leak directly into the manifold.

if it has a bad smell at the tailpipes at idle it's likely missing, a cylinder balance test will tell the story.

 

[ttrip]

Great information, I will check on the secondarys and the accelerator pump. The carb is not new. I did however, have it rebuilt last year. I have trust in the guy that rebuilt it for me. The only thing he did say was that the wrong power valve was in it (too low) and he replaced it with I think a 6.5. He didn't mention anything about the jets. Tom

 

[ttrip]

The engine idles fine. I have noticed surging at half throttle. When you get on it, it is doggy. Tom