Holley power valve?

[Buford]

I have a 351 with a performance cam and at idle (rough) there is low vacuum and I think the power valve is open letting too much gas in...I know there are different value power valves and would like some suggestions on which ones I should try to help clean up this "fat" mixture under low vacuum idle conditions?
This is a 600 CFM Holley four barrel carb.
Thanks for any help, Buford in New Mexico.

[Ole Don]

Holley power valves are usually stocked at any parts house. The are stamped on the package, and very small on the side of the part. One stamped 65 will open at 6.5 inches of vacuum. 35 at 3.5 inches, and so on. If you have not had a good enough idle yet to get a decent vacuum reading, start at 35 or 25, then fine tune up one number at a time until you like how it runs. An open end wrench will usually fit, but be very careful when tightening the part, it requires very little torque with that long wrench.

[Sid]

As a rule of thumb whatever your vacuum reading is divide by 2 for a power valve. 12 inches vacuum 5.5..... 14" about a 6.5 and so on. That will give you a good starting point. A good way to check for a leaky power valve is to turn mixture screws all the way in. If it keeps running you may have a bad power valve.

[JeffB2]

The original valve would have been a 65,since the valves are cheap I would get a 45,35 and a 25 you may want to get some jets one or two sizes smaller also.

[Buford]

Thanks guys, very helpful....

[cousinjd]

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You might also want to check for the rear throttle plates not completely closing at idle.

[miker98038]

Not to start a p---- match, because I'm no expert on Holley's. But I've seen a lot of discussion on the secondary blades for 4160 Holleys needing to be open a small amount. Try a search on that here or over at yblocksforever. I think Greenbird56 did the best explaination.

[TedEaton]

You didn't mention if you're dealing with an automatic transmission or a standard tranny. If it's an automatic tranny, then take note of the manifold vacuum at idle and in 'drive'. The power valve being used should then be rated for opening at least 2" Hg below that number. If the engine is idling at 12" Hg in gear, then you need a power valve rated at 10" or less for opening. In this case a 8.5" opening valve is readily available and will work just fine in your case. Ted Eaton.

[JeffB2]

Originally Posted by TedEaton

You didn't mention if you're dealing with an automatic transmission or a standard tranny. If it's an automatic tranny, then take note of the manifold vacuum at idle and in 'drive'. The power valve being used should then be rated for opening at least 2" Hg below that number. If the engine is idling at 12" Hg in gear, then you need a power valve rated at 10" or less for opening. In this case a 8.5" opening valve is readily available and will work just fine in your case. Ted Eaton.

It has been said to divide that by 2 many times over,shouldn't that come out to a 5.5 valve roughly ?

[Talkwrench]

I don't really now why Holley comes up with setting a power valve [economy valve] at idle when it has nothing to do with that circuit. I set up using cruise vacuum.

[Ford blue blood]

But it does open at it's specified setting and puts gas through the main metering circuits not the idle circuit.

[Ole Don]

I don't really now why Holley comes up with setting a power valve [economy valve] at idle when it has nothing to do with that circuit. I set up using cruise vacuum.

When an hot cam is involved, very low vacuum at idle will open the power valve and dump raw fuel in causing all sorts of questions. I like to use a vacuum gauge and drive the car the way it is intended to be used, and choose a power valve just below the lowest vacuum reached. At wide open throttle, it will open and work as intended.