427 Carb Jet Sizing

[blykins]

How much vacuum are you making?

Bill, I would agree that a lot of guys typically put carbs on that are too big. In this case, I think Jim is on the fence.

Usually, when the carb is too much for the engine, it shows lean as the engine isn't efficient enough and can't suck hard enough on the carb to produce a good signal.

In this case, he's possibly running a little rich, which could be just a jet change, air bleed change, or if he has lower vacuum, it could be opening the power valve prematurely.

[jhv48]

I seem to remember I'm running about 12 on the idle vacuum.

[mrmustang]

Originally Posted by blykins

How much vacuum are you making?

Bill, I would agree that a lot of guys typically put carbs on that are too big. In this case, I think Jim is on the fence.

Usually, when the carb is too much for the engine, it shows lean as the engine isn't efficient enough and can't suck hard enough on the carb to produce a good signal.

In this case, he's possibly running a little rich, which could be just a jet change, air bleed change, or if he has lower vacuum, it could be opening the power valve prematurely.

Yes, he can eventually tune this carb, after spending more than a 750DP would cost, yet he will still end up with drivability issues. Where if he dumps the 850 now and drops to a 750DP, he will spend less time and money in the long run, and get better, more efficient power out of his current engine.

Bill S.

[Mdman352]

Buy an LM-1 by Innovative Motorsports and tune the carb yourself. Most carbs I have seen used on Cobras are running WAY too rich. That being said, an 850 cfm should work. Re-jetting will help immensely.
David A.

here is a interactive selector for you.....

Holley Interactive Carburetor Selector