Power Valve Tuning??? - Chevelle Tech
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post #1 of 15 (permalink) Old May 26th, 20, 1:08 PM Thread Starter
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Jason
 
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Power Valve Tuning???

I have a tuning issue with my Holley 750 HP carb and I'm looking for some advice as to how I can resolve the issue. If I'm driving down the road in 4th gear (car is a 4 speed) 45-50 mph, 2200 rpm and I floor it, I get a lean bog that I can't clear up. I've confirmed that this is a lean bog with my LM2 wideband, while I'm cruising*my AFR is at 13.7. The bog or lean spike only lasts for a split second then clears up and my AFR is back on track. I've tried changing to larger squirters, going from one step at a time from 31 to 37 and there is no difference. I've adjusted the accelerator pump arms correctly and have tried different profile cams but still no change. I have disconnected the secondary's to see if the issue is with the primary's or secondary's*and*I'm getting it in the primary's alone.*

I think the issue is with the power valve and would like to confirm before I just throw parts at it. I've read a lot online and found some good posts on here as to how to get the correct power valve for your application. When I went through the carb I installed a 6.5" power valve, it's what came in the kit and the car idles at 11"-12" of vacuum so I figured I was pretty close if you use the divide by 2 method. Further reading, I've seen it referenced that the power valve should be 2" less than your idle vacuum, which would be roughly 9" of vacuum. By changing from a 6.5 to an 8.5 or 9.5 will that help in my bog and allow it to richen up faster? Is there something else I'm missing and should look for?

Thanks
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post #2 of 15 (permalink) Old May 26th, 20, 1:27 PM
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Re: Power Valve Tuning???

What is the engine vacuum at the cruise point, just before you "floor it"?
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post #3 of 15 (permalink) Old May 26th, 20, 1:34 PM Thread Starter
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Jason
 
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Re: Power Valve Tuning???

Forgot to post that, 18"
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post #4 of 15 (permalink) Old May 26th, 20, 4:15 PM
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Bob
 
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Install the highest pv made and drive it.

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post #5 of 15 (permalink) Old May 26th, 20, 4:18 PM
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Re: Power Valve Tuning???

install a quick opener 105 if you have a lean stumble. ignore all those old wives tales about dividing numbers , all that.

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post #6 of 15 (permalink) Old May 26th, 20, 8:35 PM
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Kerry
 
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Re: Power Valve Tuning???

Install a 10.5. The idle vacuum divide by 2 is not correct no matter if you hear it from Holley or not. Idle vacuum in gear and 1-1.5 below that is closer. But half of vacuum at cruise rpm is usually the best.
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post #7 of 15 (permalink) Old May 27th, 20, 8:19 AM Thread Starter
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Jason
 
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Re: Power Valve Tuning???

Thanks for the reply. Is there such thing as a quick opening power valve or do you mean the 10.5 will open much sooner than the current 6.5?
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post #8 of 15 (permalink) Old May 27th, 20, 8:22 AM
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Re: Power Valve Tuning???

Sounds like you need more initial advance...
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post #9 of 15 (permalink) Old May 27th, 20, 9:13 AM
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Re: Power Valve Tuning???

Jason, the PV numbers equate to their opening vacuum, so a 105 opens at 10.5" Hg, a 095 at 9.5" etc. I run a 095 only because in a 4 gear a 105 MAY dump every time you shift. The 427 required a 085 for this reason, the 600 ft lbs 467 can use more.

i witnessed this as I left my vacuum gauge permanently mounted in the car. Helps for carb tuning. ( and told me when my valves were out of adjustment - much more reliable than my ears)
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post #10 of 15 (permalink) Old May 27th, 20, 10:05 AM
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When cruising at high vacuum your pbly on the idle / transition circuit a 750 dp usually has a rich circuit
so it may just need a little tuning
but with guage in car when cruising just slowly accelerate when you hit that lean stumble note vacuum go up one pv number til stumble goes away if you have 18 cruise vacuum then as others have said 6.5 might be too low but typically its fine for performance engine. do you a tall rear gear

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post #11 of 15 (permalink) Old Jun 6th, 20, 8:33 AM Thread Starter
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Jason
 
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Re: Power Valve Tuning???

Switching out the power valve did make a difference, my local speed shop only had an 8.5" in stock so I went from the 6.5" to the 8.5". The lean spike while at cruise 2500 rpm went from an afr of 18.3 to an afr of 15.9, so I'm heading in the right direction and I can actually feel the difference in the car. I think I will order a 9.5" or a 10.5" and see what difference it makes but I'm glad I'm getting somewhere. I also need to jet the car up a little as she's cruising a little leaner than I like, then I will reconnect the secondaries and tune from there.
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post #12 of 15 (permalink) Old Jun 6th, 20, 8:58 AM
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Re: Power Valve Tuning???

What does your timing set up look like - specifically?

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post #13 of 15 (permalink) Old Jun 6th, 20, 10:48 AM Thread Starter
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Jason
 
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Re: Power Valve Tuning???

Initial is set to 26-degrees, with a 10-degree bushing my total is 36. Vacuum canister is restricted 10 degrees, so at idle I'm at 36 degrees, initial plus vacuum. I'm all in at 2800-3000 rpm.
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post #14 of 15 (permalink) Old Jun 6th, 20, 2:10 PM
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Re: Power Valve Tuning???

Quote:
Originally Posted by malibujason79 View Post
Initial is set to 26-degrees, with a 10-degree bushing my total is 36. Vacuum canister is restricted 10 degrees, so at idle I'm at 36 degrees, initial plus vacuum. I'm all in at 2800-3000 rpm.
Sounds like you have Plenty of timing which is good. I would suggest bumping up the jetting two sizes front and rear and see what you got.

A note regarding those AFR meters. They are a good tools, but you just have give the engine what it wants and ignore the AFR readings for the time being. You will chase your tail to no end with those things. I should know, I ended up installing EFI because of it LOL!
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post #15 of 15 (permalink) Old Jun 6th, 20, 6:20 PM
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Re: Power Valve Tuning???

Stewart Warner Motor Minder

Like Gene said a vac gauge inside is a plus.
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