Power valve not opening

[DrCharles]

My Autometer dash vacuum gauge agrees well with two underhood-type large gauges (and the idle vacuum hasn't changed, so I can rule that out as a source of error). During the 80 mile round trip to the car show, I had an opportunity to watch the gauges more carefully. I filled the fuel cell with alcohol-free pump premium.

Idles around 13.8-14.2 AFR (not a steady reading due to the lumpy idle from the overlap). The exhaust already smells bad enough, I don't want to richen it more if I don't have to...

Cruise 14.5 @ 15" Hg.

This PV starts to open around 7-7.5" (it's clearly stamped 8.5!)
Fully open at more like 6", where the AFR drops rapidly to 12.5 and stays there as I keep opening the throttle more to stay at 5-6". That does not last long even in 4th gear

I know it's not a true bistable switch - it has to open far enough so that my relatively large .066 PVCRs are not restricted by the PV seat. Now I'm wondering how exactly PV ratings, even if accurate, are measured - where the valve starts to open? Or fully open?

Either way the one I picked does not seem to be quite rich enough. Time to try the (alleged) 9.5" I guess!

[GTO Geoff]

Never been a Holley fan & the joke that is called the PV system is one of the reasons. Wonder why no other carb uses the Holley PV system.........

Using a delicate spring against a rubber diaphragm to balance the very low force of vacuum seemed like a definition of insanity to me...

Rubber. Rubber that can be affected by petrol; that goes hard with heat; that goes hard with age. So not surprised to hear that a X.Y power valve has changed it's value.

[rgalajda]

You can test power valves with a vacuum gauge before you install it. You will see at what point the valve begins to open and when it is fully open. Sometimes the after market valves are off a bit.
The power valve rating should be at full open.

[phill]

Its fiddly but you can adjust them if you need to.

[DrCharles]

How would you adjust them? Stretch or compress the spring?

[bigblockmark]

There is a round part on the back end that threads onto the base.
It is Locktited in place. If you turn that, it will adjust tension on the spring and change the operating range. You can re seal it with wicking Locktite.

[DrCharles]

Thanks! I learn something new every day
Do you have an (approximate) number of turns per in.Hg?

[bigblockmark]

Using the test tool, which is not that expensive, test a new 6.5 rated power valve. Turn it in 1 full turn and see the result. Go from there. Springs for different value power valves are not all the same.

[phill]

I don't know if its posted on here, the old forum or the old old old innovate forum where a few of us originated but basically.

1. drop the diaphragm end in some water (see next step for why)
2. heat the spring retainer gently with a propane torch to break the loctite seal
3. grab the centre of the valve with small needle nose pliers, larger pliers to wind off the spring retainer.
4. Adjust height of retainer, fiddle with springs and test, test, test until you find a vac rating that makes you happy.
5. reseal spring retainer with fuel resistant loctite, happy days.



*Unable to credit whoever originally posted it, it certainly wasn't me but I've done it a bunch of times over the years. It works really well.