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Discussion Starter · #1 ·
I just swapped in a new larger cam and now the exhaust burns my eyes! I was told I need to swap power valves to 2" below manifold vaccum. I hooked up a mity-vac to the manifold, I jumps between zero and fifteen at idle. If I rev it up it smooths out to twelve. How do I determine what power valve is correct? I currently have a 6.5.
 

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The most accurate method is to hook up a vacuum gauge with some long tubing so you can read it while driving. Drive a bit at different steady speeds to see what cruising vacuum the motor pulls. Then go to a valve rated at about 1/2 that reading. For example, vacuum around 12-13 would call for a 6.5 vavlve. It's not 2" below the reading as is commonly stated ( even in the Holley website - a tech there told me it was in error ).

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Rick Dorion
69 RS Conv,355,M20,4:10
 

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Dude, here's what you have happening. The lumpy cam (lumpy as in "has big lift/duration" not lumpy as in "is no good") will cause your idle vaccuum to be a bit shifty. So, as a result, your power valve will always be open. This is not desired. What you will want to do to correct the problem is to get a power valve that won't open until absolutely necessary. I would go with a 3 or 2 in. power valve. This way, the low vaccuum caused by that wild cam will not cause the power valve to open under normal idle, but the power valve will stilll open when the engin calls for the extra fuel under heavy accelleration.

Another thing to consider is a posiible vaccuum leak. If you are getting funky vaccuum reading at idle, then you may have a leak somewhere; usually a small oriface on the carb. Check that out.

BW
 

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Discussion Starter · #4 ·
Thanks guys. I called a Holley tech yesterday, and it turns out I did have a vaccum leak. Once corrected I got a steady 8-9" vac. He told me to go with a 4.5" valve. I have not installed it yet.
 

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Yeah, that sounds about right. You may wish to play it safe and go with the 4.

Good luck, and keep us posted.
BW
 

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The power valve controls mixture of the power circut, which is fuel that goes through the main fuel booster nozzles.
Fuel that goes through the idle circut is governed by the idle feed restriction and idle air bleeds not the power valve.

A power valve with a hole in the diaphram can leak fuel directly into the carb and cause a rich idle mixture that way.

If the vaccum is too low, and you have to open the throttle blades too much to keep it running, the carb will uncover the "transition slot" just above the idle fuel hole in the primary throttle bores.
It can pull additional fuel through these holes, and may run rich from that.

The power valve govererns part throttle cruse mixture when closed, and at full throttle it opens for full rich mix.

The vaccum leak added air but no additional fuel to go with it.
The lower vaccum also causes the carb to mix fuel very poorly.
A good vaccum test guage has a snubber or restricting valve on the inlet to choke the vaccum signal and dampen out the pulsing, this will give you an average vaccum reading, but undampened will indicate vaccum leaks or bad cylinders more.
David

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[This message has been edited by davidpozzi (edited 05-04-2001).]
 

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davidpozzi I don't want to start a fight but the idle circuit draws off the main circuit. If the power valves are open it will see more fuel and draw more. Just like putting in bigger main jets will also richen the idle circuit.
 
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