Power Valve
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Junior User
Joined: Sep 2003
Posts: 68
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From: Chandler, AZ
Power Valve
Does anyone know what a baseline powervalve should be for a 1985 F150 5.8 HO, Holley would be? The one that came with my rebuild kit runs so rich the truck won't idle and the old one is so lean that no matter how much gas squirts in from the acc. pump it stumbles. I tried a single stage 6.5 and it is still too rich and opens at idle. I have a 5.5 would that make much of a difference? I guess I'll have to break down and buy a vacuum guage!
Laughing Gas
Joined: Feb 2010
Posts: 781
Likes: 8
From: Sanford, NC
Try the 5.5 power valve. That's what I ending up putting in my 4180 that is on a
modified 351W in my 1981 F100 and it runs fine. The baseline power valves in these
H.O. 351Ws is a 2 stage valve and can give problems and poor performance. It would
be wise to use a vacuum guage to adjust the carb idle mixture.
modified 351W in my 1981 F100 and it runs fine. The baseline power valves in these
H.O. 351Ws is a 2 stage valve and can give problems and poor performance. It would
be wise to use a vacuum guage to adjust the carb idle mixture.
Senior User
Joined: Nov 2000
Posts: 285
Likes: 1
From: sunny san diego
The problems you describe point to another carb problem other than the power valves. A 6.5 doesn't open and allow more fuel unless your really getting on it. I would check float levels and MAYBE see if the P/V isn't leaking or blown. I hope this helps.
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Junior User
Joined: Sep 2003
Posts: 68
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From: Chandler, AZ
Well, I took the carb off and went through it again. I found that I had not centered the gasket on the new PV. Now it runs like before- stumbling on acceleration. Like you guys said, the PV shouldn't affect idle or untill the main metering kicks in and now it is working ok. I'm leaning toward a vacuum leak although I have plugged all the hoses one by one to see if the situation changes.
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Posting Guru
Joined: Oct 2010
Posts: 1,199
Likes: 1
From: Missouri
Holley 4180's are known to have an off idle stumble.
There are things that you can do to make minor improvements (larger accelerator pump discharge nozzle), but I think the real issue is nearly 30 years of carburator warp. A professional rebuilder would likely mill the surfaces.
There are things that you can do to make minor improvements (larger accelerator pump discharge nozzle), but I think the real issue is nearly 30 years of carburator warp. A professional rebuilder would likely mill the surfaces.
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Senior User
Joined: Nov 2000
Posts: 285
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From: sunny san diego
Try hooking a vacuum gauge to a source on the manifold. A reading of less than 18 could possibly indicate a leak. Spray wd40 around any suspected areas to see if idle/readings change. Evaluating a problem on-line is difficult at best but we're trying!
Posting Guru
Joined: Oct 2005
Posts: 2,282
Likes: 4
From: Oregon
Spring for a vacuum gauge, or you'll just be guessing and fighting an uphill battle. Just rebuilt my cousin's carb on the same motor a couple months back, he bought a generic kit with a junk 2 stage power valve. Adjusted over and over with my gauge hooked up, but couldn't get rid of the off idle stumble. Replaced it with a Holley brand single stage from the carb I had sitting around and runs perfect now.
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