just finished my motor swap which some of you may know but im looking at any ideas of wher ei should start with jets and powervalves on a holley 750 carb. right now its got 72's in the front and not sure about the rear but its running really rich all the time.
if you have a 383 what jets and whatnot are u using.

 

I have a 383 sbc single plane intake last year i ran a 750 vac holley with 72 pri. 76 sec ran great.

 

If its running rich all the time you could have a blown power valve. They blow very easy. Just one little backfire will blow them unless you have the kit to stop them from blowing. To get the best power valve for your engine you first need to get a gauge and test the vaccum of your engine. You can test it off the intake somewhere. Then whatever reading you get you need a power valve thats half of that reading. Say you get a reading of 15 then you would need a 7.5 power valve.

 

If it is a newer holley carb it should be pretty resiliant as far as power valves go. It would depend on how new of a carb it is.... The newer ones usually incorporate power valve blowout protection. I guess the only way to tell for sure would be to take the carb apart and inspect the power valve.

 

you need to check how much vacuum you have at idle(manifold vacuum) this can be done with a cheap vacuum guage from any parts store. Say foe example you have 8 inches of vacuum at idle you would want to run a 6.5 power valve. if you had 6 inches of vacuum at idle you would want to run a 4.5 power valve . Stock 750 Holley card usually come with a 6.5 power valve. If you have less than 6.5 inches of vacuum at Idle you are allready working the power valve which would cause it to be very rich.

 

A blown power valve (ruptured diaphragm) will definitely make the idle fat because manifold vacuum will be pulling liquid fuel directly from the float bowl, and in large amounts.

A power valve of too high a number (vacuum) will NOT affect your idle mixture. The idle circuits are completely separate from the main circuits, which is what the power valve feeds into. Too high a power valve and you'll run rich, but only above idle speeds when you're running on the main circuits.

 

i thought u were supossed to take half of the vacuum and if its an even number go down the the next available one.

 

yeah, holley says to use half of the vacuum at idle to determine powervalve size.

and all holleys from 1996 and newer have the power valve protection.

 

i thought so

 

Do what you want just trying to help.

 

yea I normally end up running a higher # than half the idle but you could use that as a starting point. If you get a stumble as you get on the gas you can try changing the valve up a number. I run a 6.5 in my 383 (about 10" at idle) but my 355 liked a 10.5 much better (about 14" at idle)