My Holley has 2 Power Valves are they both needed? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-02-2008, 05:23 PM
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My Holley has 2 Power Valves are they both needed?

I have a Holley #4781 it has a front and rear power valves. Are both needed? I had a Mighty Demon and it had just the front PV!

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Old 08-02-2008, 06:06 PM
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It is what we all called a double pumper. Actually they call it a double pump.
It is a 850 cfm carb, which makes it a bigger carb. You didn't say anything about what it might go on, but I guess you didn't ask.

It will squirt out both front and rear jets. Better for high performance, but it would suck for good gas mileage. I have never tried to run with out the back one hooked up. I guess it would work, but that is not how it is designed. I would trade it for something else if you don't want the dual pump.
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Old 08-02-2008, 08:00 PM
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you can remove and install a plug (from Holley). when you remove you need to increase the jet size by about 6 sizes to get to the same performance. The secondary pump is independent of the power valve.
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Old 08-02-2008, 08:07 PM
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dbug...

Do you have to increase the jet sizes even if you still have the primary side PV in?
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Old 08-02-2008, 08:13 PM
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Quote:
Originally Posted by 406 bug
you can remove and install a plug (from Holley). when you remove you need to increase the jet size by about 6 sizes to get to the same performance. The secondary pump is independent of the power valve.
Other carb manufacturers call the PV and Economizer Valve. That is because it lets the engine run a little lean when the engine is not under load. When the engine goes under load, the PV opens, allows more fuel into the cruise circuit, which richens it up. Like 406 bug says, you will have to go up on your secondary jet sizes if you put the plug in instead of the valve. Unless you are using it for race only, you will waste fuel, your part throttle low or no load performance will go down, and you may load up the plugs. Race only - you may want to do it. Street - keep the valve, make sure it is in good shape and the correct vacuum rating.
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Old 08-02-2008, 08:25 PM
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yes, just the jets in the secondary. This would be in an effort to maintain performance. quite honestly, if it runs fine I would not mess with it.
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Old 08-02-2008, 09:11 PM
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Quote:
Originally Posted by 406 bug
you can remove and install a plug (from Holley). when you remove you need to increase the jet size by about 6 sizes to get to the same performance. The secondary pump is independent of the power valve.
Good info, I miss under stood, but didn't remember that.
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Old 08-04-2008, 12:51 PM
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Quote:
Originally Posted by camaro freak
I have a Holley #4781 it has a front and rear power valves. Are both needed? I had a Mighty Demon and it had just the front PV!
That would be correct in that most of our carburetors only use a front power valve unless it is an oval track or road race version.The only real big concern with a rear power valve would be on a real hard leaving drag car. As some of the others have mentioned if you are going to plug it be sure and go up on the jetting 6-8 numbers .
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