i have a new 750 proform carb, i am going to get a vacuum reading here in a bit, but i was wondering, what's the rule on which power valve to put in? something like 2 numbers over the lowest vacuum reading? or ?

thanks,
glenn

I think Holley recommends 2 numbers under your vacuum reading at idle, but I like the one using half what your idle is at vacuum, ex: idles at 9" vacuum use a 4.5 powervalve. With a cruiser use Holleys recommendation, in a street/strip ride I'd go half. Just MHO ;)

thanks bob, appreciate the help. i have 8" at idle. i'll check out a 5-6, if it doesn't already have that in it.

Holley recommends running a power valve thats 1/2 the reading at idle.If you go just 2 numbers lower than idle your fuel mileage will suffer.

okay, i checked the vacuum the first time at the carb and got about 8" and it was very steady. at the manifold, it fluctuated a lot more, but was closer to 7" on average. i pulled out my primary power valve and the only #'s on it is a "45". am i to assume this is a 4.5? or ? if it is a 4.5, is it close enough or is it worth changing out to a 3.5? at the manifold vacuum was between 6 and 8. maybe a 3 PV?
is there a seconday PV? i just worked on a 650 holley DP and it only had a primary valve.

I'd use the 3.5 and dont know if they make a 3.Some carbs have a secondary valve but it depends on the model.Either use the same valves front/rear or plug the secondary one and jet it richer.Just what I'd do myself.