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Old 08-30-2005, 08:33 AM
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Modifying Holley baseplate so power valve references manifold on s/c motor

After searching the archives on referencing power valves for supercharged motor I'm a little confused (seems to happen more and more the older I get). Holley sent me info to modify my 600 vac secondary carb's baseplate by connecting the spark port with the existing power valve vacuum feed hole by cutting a shallow channel on the top of the baseplate and then plugging the power valve and spark port holes on the bottom of the baseplate.
I'm confused at to whether the power valve will then reference vacuum or boost, or both? Is this the correct way to do this? I read some not-so-kind comments about Holley's tech help.
Can someone explain exactly how the power valve works throughout the throttle's operating range? I know that it opens and closes according to vacuum, but I'm confused about how and when boost affects it's operation.
Thanks!
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Old 09-15-2005, 02:39 PM
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Heres the reason. The power valve is held closed by neg pressure (vacuum). As soon as the negative pressure goes to positive pressure(at the set rate of the power valve), the power valve then opens, dumping fuel. By moving the vacuum to the advance port, you then regain a neg pressure(vacuum) so the power valve doesn't dump fuel right off the start. So, with a charger, the best vacuum is above the charger, in the carb ports. I hope I said this correctly.
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Old 09-15-2005, 09:37 PM
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You got a leaky spark tube...
 
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Holley Question...

This post reminded me of something I am having some issues with. I bought a 750 DP from a friend of mine and from about there is a very bad stumble when I step on the gas at cruise rpm. I dont have any problems off idle to about 2000. It's at its worst at about 2500 rpm and gets better as rpm goes up. He told me he had tried to make the carb work on a 350 he had but it was too big. I think he said he installed 69 and 74 jets in it and kept the 6.5 power valve. He told me I should move to a 10.5 power valve. I was told it would probably be too big before I put it on the engine but it doesnt seem to run excessively rich and it really lets the engine rev too, I'd just like to fix the stumble at part throttle cruise. The carb is a 4779-2, I think he said he bought it about 15 years ago. Any suggestions?

P.S. I thought I opened a new thread when I made this post, I did not mean to piggy-back on another post.
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Old 09-15-2005, 11:44 PM
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It sounds like you might want to check the pump arm clearance, and make sure you are getting a pump shot of fuel as soon as you move the accelerator. Also, make sure your distributor advance is correct at that rpm's.
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Old 09-16-2005, 06:22 AM
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Boost Ref. PV

The reason you’ll boost reference the Power Valve on a Roots Supercharged engine is there are times at part throttle when you can still have vacuum between the supercharger, and the carburetor, but none below the supercharger. Referencing the PV to your intake manifold it will allow the PV system to operate as intended.

If you block off the body, so that the carburetor will not see vacuum from below it, then modify your channel so that your ported (or timed) vacuum fitting on the side of the carburetor now feeds over to the PV you’ll be OK. Just connect a vacuum hose from there to your intake boost/vacuum gauge.
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Old 09-05-2008, 10:21 PM
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I know this is a really old thread but I found this.. Boost Referenced Power Valve. Pictures make things easier to understand..
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