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Discussion Starter · #1 ·
Holley 750dp. The former owner just told me that he had installed a power valve "block". What would this be for, and what else should I know. The car definitely seems to be running rich. Would this have anything to do with it?


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Bob Von Kaenel
Lincoln, NE
70 SS 402ci+.030 TH400 3.73 posi Cranberry Red

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With a power valve "block" it is always going to run a little fatter at idle, basically becuase you have to upjet about 10 sizes, in order to keep up with upper RPM fueling needs.

I'm no engineer, but here's my take on it: The powervalve is a vacuum operated valve, in most cases just in the front bowl. It begins to open as vacuum decreases(normally at the halfway mark of full engine vac), and allows more fuel to enter the metering block, equaling out the jetting front to back, but allowing for a cleaner idle due to the use of a smaller front jet.

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Discussion Starter · #3 ·
So...for a more streetable car would you put a power valve back in and back off the jets a couple of sizes?

Also, there's no vacuum advance on this Mallory Comp 9000 distributor. Does that play into this in any way?
 

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Bob, yes the lack of vacuum advance will cause poor economy in cruise mode.You should run it for any street car, particularly with your cam.

I assume the owner blocked off the rear power valve. Blocking the front is an ignorant move for a street car. The back should be jetted at 78 if there is a block. Having the rear ( secondary) power valve blocked off will not cause a rich condition. The front will, yes.

With your cam you should be pulling decent idle vacuum ( 15" or so). Your starting point should be 71/72 jets up front with a 095 power valve. #78 jets in back with no power valve, or #71/72 with a 055 power valve. I know becuase I ran a 750 on a 430 ci with your cam. Your manifold might be a bit different though, so use my specs as a guide and check your spark plugs for optimum color.

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Gene Chaas
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Discussion Starter · #5 ·
Thanks Gene- I appreciate your help a lot. I'll try the set up you describe. Just FYI,
It's got a Performer RPM manifold, stock oval port heads with the intake valves enlarged to 2.19, and 2" primary tube Hooker headers, 9 1/2 to 1 compression. It runs pretty damn good, but very rich. After I get the power valve in and #72 jets in, I'm going to look for a distributor with vacuum advance.

I changed plugs last week and they were very sooty black. There's blue white smoke when you start it and sometimes at idle. I'd always heard that you could smell a car running rich, now I know what it's like.
 

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Just to cover all the bases here, is it possible that by saying he "installed a power valve block" he is really saying that he took a carb initially delivered with a secondary metering plate (and hence no power valve) and installed a secondary metering block, with provisions for a power valve? My 780cfm #3310, for example, came with a secondary metering plate. I removed the plate and installed Holley's metering block kit with provisions for a power valve and changeable jets.

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Mike Newby

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A power valve may also be called an enrichment valve and it does just that. It enrichens the fuel mixture at throttle settings that are near or at wide open throttle.

[This message has been edited by Randy Mosier (edited 06-04-2001).]
 
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