My Mighty Demon eats power valves
Thread Starter
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
My Mighty Demon eats power valves
I can't seem to get more than a few hundred miles out of a power valve. The Demons are supposed to have blow proof PVs but every time the car sneezes when I start it I seem to blow the valve. Yes it has no choke and a big cam so starting when cold can be a small adventure and I do remember starting it one day when it was quite cold. I got a big sneeze out of it and I think that is when it went. I have driven it a few times since and it runs like crap at low speeds,runs great under throttle. I buy Pvs in bulk now. 
Does anyone else go through PVs quickly?
Does anyone else go through PVs quickly?
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Give Barry Grant a call. Something is a bit odd. I've got one on my BB with rectangle port heads, a whole bunch of cam. Cold starts are also a challenge. I get a big sneeze out of it probably 75% of the time. And I have yet to replace a power valve,.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
If you look at our Demon or newer Holleys the tiny air passage is so small that an instant pressure wave from a backfire doesn't effect the PV.
I've got as big a single plane and cam as most anybody. .055 gapped Autolite plat plugs, @18 initial advance, 33 total mech, all in at 3200 rpm.
Turn it over for 5 seconds with the ignition off to bring up the oil pressure, pump twice and hold it on the floor, hit the ignition on lever. Keep the motor at 2500 rpm until I get over 100 degrees of water temp.
The only time that I have changed a power valve was when I was trying different things. It is still a 6.5.
To the poster that uses a 2.5........................... Drive with a vacuum gauge taped to the window in front of your face. Then tell me how often and for how long do you achieve less than 2.5 vacuum?
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
we use a socket designed for a power valve to install the power valves, if you use a open end wrench you can distort the power valve at the point where the diaphram is sealed to its body.
Also the new non stick metering block gaskets are more prone to leakage because they do not seal as well as the old stick gaskets. the bowl screws should be tightened to 50 inch pounds of torque every few months so the gaskets will not allow fuel to pass between the various circuits that are sealed by the metering block gaskets. We often see warped metering blocks on older holleys, when we find a warped block we replace it with the newer/stronger metering blocks that holley has with re-enforcing webing on the jet side of the metering block.
I hope this helps Henry @ olescarb
Also the new non stick metering block gaskets are more prone to leakage because they do not seal as well as the old stick gaskets. the bowl screws should be tightened to 50 inch pounds of torque every few months so the gaskets will not allow fuel to pass between the various circuits that are sealed by the metering block gaskets. We often see warped metering blocks on older holleys, when we find a warped block we replace it with the newer/stronger metering blocks that holley has with re-enforcing webing on the jet side of the metering block.
I hope this helps Henry @ olescarb
Melting Slicks
Joined: Jun 2002
Posts: 2,844
Likes: 99
From: Orrtanna Pa.
I don't know if this is related, but I used to get carb barbeques quite often with mine.Single plane, big cam, no choke. I have noticed that advancing my timing has all but stopped this. I am currently running 36 degrees locked,as an experiment I tried while trying to fix my stall at idle problem.
I finally fixed my idle with carb tuning, and may eventually go back into the distributer advance.
All said, I've yet to blow a power valve.
I finally fixed my idle with carb tuning, and may eventually go back into the distributer advance.
All said, I've yet to blow a power valve.
Racer
Joined: Apr 2008
Posts: 418
Likes: 0
From: destin fl
answer 3/4 throttle in 3 gears i live in florida ?????????? at idle with a new pv no black smoke out tail pipes. i use good fuel, 8" vac at idle tried 3.5 pv after carb pop black smoke returns at idle & strong fuel smell. roller cam holley 770cfm. need some advise please. thanks lynn.
ps have down sized jets several times.
ps have down sized jets several times.
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