I have a SBC, 600 cfm single pumper with vac. secs., .474 lift cam. What size jets should I be using? I have a problem with a rich mixture and have tried everything. A guy at work said my jets may be the wrong size for my application. Any help is appreciated!

Sqaty, you always have to check it's not the power valve on these holleys'. the only way I know of is to remove it and suck on it ( after you;ve cleaned it of all the residual gas) and see if it holds. While you;ve got the carb apart, check the primary jet sizes and drop down 2 sizes.

See how the plugs look after that and go 2 more sizes if neccesary.

Just so we don;t assume anything: How do you know your mixture is rich? And what is the "everything" you;ve tried?

Talk to ya...

Gene Chas
Gold Member 62
67 SS396/427

Gene,
the reason I think the mixture is rich is because when I run the motor, the exhaust smells like raw fuel. The plugs, exhaust pipes and the valves ( I have the heads off right now) have black residue all over them. I just assumed this was from a rich mixture...please correct me if I am wrong.

I have tried to adjust the mixture screws, floats, and have checked for vacum leaks. When I adjust the mixture screws (i've tried turning both ways) accordng to protocol, the motor never really "bogs" down or the rpm's dont seem to change. I am thinking maybe the carb is dumping fuel. I did have short problem with some backfire through the carb, maybe that messed something up????

Backfire+Holly=bad power valve

SQ,
Usually, Holley carbs are pretty close for most applications the way they come from the factory. if the carb has been "messed" with, the first hting I would do is put it back to stock configuration. Go to your local speed shop and have them look the carb number up in the Holley cat and get the jets and power valves that came in it stock. This will be a good starting point.


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Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100