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post #1 of 17 (permalink) Old Jul 14th, 15, 03:07 PM Thread Starter
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power valve question

I have a 454 with a 290 510 cam. Holley 850 double pumper and performer manifold. I started to have a vacuum leak. I changed hoses and finally found it was coming from carb gasket. I started to also have back firing thru the carb. The holley had a 5.5 power valve and I accidently put in a 6.6 poweer valve.
1 would a higher power valve number increase idle speed? It went from 1000 to 2500 and then backfired when I hit the throttle.
2 Should I go to 5.5 valve again? My car idles at about 71/2 to 6 inches of vacuum in gear.
What do you all think?

Thanks

Jerry
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post #2 of 17 (permalink) Old Jul 14th, 15, 05:46 PM
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Re: power valve question

It will not change idle. Are you sure you used the correct gaskets?
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post #3 of 17 (permalink) Old Jul 14th, 15, 05:52 PM
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Re: power valve question

The power valve is based on cruise/ mashing on it vacuum, not 'idle' as holley states. A car with a lot of cam will have low idle vacuum but good cruise vacuum. Drive with a vacuum gauge in the car and see what your vac reads off idle/cruise. When your butterflies are closed and the idle ports are all that the car is running from, it's all the car is running on.

Only way power valve should affect idle is if your air bleeds are too small or you have the primaries cranked open to idle.

Too much pump shot or wrong timimg can cause the backfire from the carb
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post #4 of 17 (permalink) Old Jul 14th, 15, 05:56 PM
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Re: power valve question

With a 6.6 power valve and your vacuum drops to 6 it would activate your power valve. But that would make you go rich and kill the idle. If the idle went up you have another problem.

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post #5 of 17 (permalink) Old Jul 14th, 15, 07:26 PM
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Re: power valve question

Power valve does not affect idle, not on a holley, not on a qjet. As soon as that engine starts turning some RPMs his vacuum reading will go UP, when he mashes on it the vacuum will likely NOT reach 6", meaning the power valve wont open, the PV should be determined by cruise vacuum and WOT blasts, averaging the difference (i.e 27" at cruise, 15" at WOT, go somewhere between say 18-19)

http://www.corvetteforum.com/forums/...gine-idle.html

And FYI I run a 290H in a smallblock, I tried tuning by 'the book' for weeks and made it worse.
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post #6 of 17 (permalink) Old Jul 14th, 15, 09:06 PM
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Re: power valve question

Power valve has nothing to do with Idle vacuum. like ap68 said, power valve adds fuel to the main circuit, the main circuit is not open at idle, unless the power valve gasket is leaking or throttle blades are open to far, it will not effect idle.

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post #7 of 17 (permalink) Old Jul 14th, 15, 10:33 PM
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Re: power valve question

Double check timing (12-14 deg BTDC (or whatever builder suggested)) check for vacuum leaks, (i use a smoke machine, others use UNLIT propane torches, propane etc.) check your distributor cap and wires, then fiddle with the carb.

Actually I have that backwards, check plugs/wites/etc, then vac leaks, then timing.

Then play with carb settings
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post #8 of 17 (permalink) Old Jul 15th, 15, 12:34 PM Thread Starter
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Re: power valve question

will check again. the only thing i have changed is the power valve. I checked to see if the distributor moved and it is solid and tight. I will take off carb again and put another valve in. If the carb back fired twice I assume valve is bad.I will let you know.

Jerry
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post #9 of 17 (permalink) Old Jul 15th, 15, 01:56 PM
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Re: power valve question

The OLD holleys have no backfire protection for the power valve, if your holley is 20 years old or newer it has a way built into it to prevent backfire damage.

A 1500 RPM increase in idle means the butterflies are open more than they should be or that you have a pretty sizable vacuum leak somewhere (lean mixtures (too much air) burn faster than rich ones (IF your PV was busted and IF it had an affect on idle it'd be rich and slowing down the engine not speeding it up))


If you changed base gaskets did you put the right one back on? Are the butterflies hitting it and hangine open?
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post #10 of 17 (permalink) Old Jul 15th, 15, 03:12 PM Thread Starter
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Re: power valve question

Well I might as well confess. The run away idle was caused by throttle spring that wasnt connected to a anchor to close throttle. I figured that out when I saw the spring just laying there. So I took the 6.5 power valve out thinking it was bad since it back fired twice really bad. I put another 6.5 valve I had a extra and got car to idle. Here is the rub. In park the best I could make with tuning was about 11 vacuum in drive it makes about 5 inches vacuum. Should I try another power valve say 4.5 to 3.5? Any ideas? cam is 290 duration 510 lift. Its kinda big.

thanks

Jerry
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post #11 of 17 (permalink) Old Jul 15th, 15, 05:19 PM
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Re: power valve question

What are your idle RPMs in drive? How much drop between park/drive?

Hook a vacuum gauge up, a mityvac works, cruise around, take note of the cruise RPM, then do some dull theottle blast and see what the vac is when you do. My car (basically same cam (290 duration, .501" lift)) makes 6-8" at idle and 20" or so at cruise, only dropping to around 8 during a full theottle blast.

I bought every PV holley made trying to tune my carb in, ended up going with the one the carb came with
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post #12 of 17 (permalink) Old Jul 15th, 15, 06:33 PM Thread Starter
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Re: power valve question

Hi ap68. My car idles in park at about 900 rpm. It drops to 650 to 700 in gear. Vacuum is about 12 inches vacuum and then drops to about 5 inches vacuum. I am not even sure what power valve is factory for this carb. I got it from a friend. Do you know what power valve came on this carb. Maybe it will be listed on Holleys website.

Thanks

Jerry
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post #13 of 17 (permalink) Old Jul 15th, 15, 06:43 PM
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Re: power valve question

It came with a 6.5 pv. Set the idle mixture first, then get the idle to where you want it in gear, note the difference between neutral and drive rpm, the less it drops, the closer you are to the correct AF ratio

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post #14 of 17 (permalink) Old Jul 15th, 15, 07:36 PM
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Re: power valve question

Just a thought, timing may have a determining factor, but you may want to take a good look at the float levels. If you are loading up and when you push the throttle you dump excess fuel well had that similar issue years back on a firebird 74 formula. Worth a try. Good luck

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post #15 of 17 (permalink) Old Jul 15th, 15, 09:40 PM Thread Starter
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Wink Re: power valve question

will check air fuel mixture first. I have it at one and a half turns out from bottoming out. I didn't have time to hook up a vacuum gauge had to go to work. Also I will look at float levels. And will check timing again but dizzy didn't move. All these are great advice.

Thanks

Jerry
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