I bought a proform 850 main body to try and get a little better performance. It does feel like it runs better, havn't gone to the track yet. The part that i dont understand is how lean that it wants to be jetted. The stock main body ended up at 74/84 jetting, secondary power valve plugged. This wasn't a perfectly tan plug but a preety good brown. With the new body I am running 69/80 jets and the plugs are pure black. I dont want to jet down any further in case somthing is causeing this and it goes lean on top. I have tried changing the primary power valve (3.5) but this did not help. These jets just seem to small, a friend has a 750 on a 350 and he runs like 74's in his and it is mild.

Here is the rest of the combo

11.5:1 468, afr 305cc, 264/267@.50 .630/.650, Vic jr 4500 stall 20*initial 38* total

Thanks

What power valve are you running?

With that 265* cam you are probably down around 5 or 6" vacuum at idle in gear and need about a 2.5 or 3.5" PV. If you are running a stock Holley 6.5" PV then it is probably open at cruise causing your engine to run rich.

That carb should want something in the neighborhood of the same jets a Holley 850 HP would want. I think that would be right about 80 square (or 80F/86R with the rear power valve plugged)

Are the plugs black when running at idle, that would be a idle mixture screw problem or ilde air bleed problem.
I have a Proform 950 body and it has #71 idle air bleeds, and on my mild 383 I only have the idle mixture screws out 1/2 turn each. So I got some larger idle air bleeds (73) for it - havent stuck them in yet.

I am running a 3.5 PV and it does have only 5" of vac at idle at 1050 rpm but as soon as it gets to about 19-2000 the vac goes to about 16" and it never cruises below this.
I have read to jet down like 1 jet size or somthing foe every 2000 foot of elevation and i am at about 6000ft but that still doesn't make up the difference

Thanks

Yeah, maybe its just the idle calibration that is too fat . . . that WILL make your plugs look rich.

The idol screws are set at about 1 1/4 turns out this is about were it makes the most vacum but it is hard to tell when it only makes 4-5" anyway so it don't change very much. If i go any leaner than 1 turn out it runs hot while in slow moving traffic with a lot of idoling.

Also i had to drill 1/16" holes in the throttle blades to get it to idol with out to much of the transfer slots exposed and to make the mixture screws do anything at all.

I will try leaning them out even more and see if that helps the plugs.

Thanks

sounds like Eric 68 is on the right track,you would need a 2.5" power valve @ 5" vaccum.
that does sound like a tricky combo to tune,maybe it's time to spring for a wideband a/f meter?
Eric

I have ran a 2.5 in it before the new main body and that didn't make any difference but i will try it again.

thanks

If its fat at idle - its the mixture screws, idle air bleeds. (idle feed restriction feeds fuel to the screws, so it may be too big also)
If its fat at part throttle - its the idle feed restriction in the jet plates.
If its fat at full throttle - its the jets and high sped air bleeds.
Thats a general discription of how it works. ;)