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Thread: Power Valve Question

  1. #1
    76miller
    If the carb has 6.5 p/v in the prim. and sec. and you change them to 4.5 what would that do ? Release fuel sooner or later depending on vacuum or the amount of fuel that is released is more or less ? My goal here is to consume fuel when cruising around at 3000-3500 rpm, but have enough at w.o.t . I know, make a pass and read your plug....but I'm trying to understand how it works. Thanks

  2. #2
    gbelt
    That valve is to allow extra fuel during high load operations. With a 6.5 valve, that extra fuel will come in when the manifold vacuum drops below 6.5 inches, if you change it to a 4.5 then it will open at that vacuum level, so it will release fule later. If you have low vacuum at idle and cruise then that may not be enough to keep the valve closed, but the result is usually fouled plugs. Put a vacuum gauge on your engine, check the reading while just cruising at the rpm you mentioned, if it's above 6.5 the valve will be closed anyhow so you wouldn't gain any economy by going to a 4.5. What kind of cam and what vacuum do you have at idle?

  3. #3
    76miller
    Ok, I wasn't to far off w/ the way these things work. I don't have a location for a vac gauge on my intake unless I take it off and drill and tap so thats not gonna happen till the winter, but to any your question I have a solid camshaft w/ .620 lift , and a old dyno sheet says I have 2lbs. of vac hg. at 3000, 4lbs at 3500rpm,and 9lbs at 5000 rpm. ( Doesn't sound right?)So what size power valves would I want ? I just talked to a guy that said 8.5 p/v in the primary and 6.5 p/v in the secondary is safe for a big block. Opinions wanted ??

  4. #4
    Infomaniac
    What you are wanting to do is exactly what the PV is designed for.
    Another term for that circuit is the power enrichment or economizer circuit.
    The number on the PV is how much vacuum it takes to keep it CLOSED. No fuel flowing from that circuit.
    To adjust it properly you need to cruise your boat at the throttle position you want it to be economical. Measure the intake vacuum and make sure you have a PV that will remain shut at that vacuum.
    Most text books have you measure idle vacuum and pick a PV that is a specific number away from that value. The reason for this is most people can only measure their intake vacuum with the engine idling and them not driving.
    Your dyno sheet tells you what vacuum you have at those RPM's but at wide open throttle.
    I don't care what anyone says the PV cannot affect idle mixture. Unless it is blown. Ruptured and leaking.
    The PV dumps into (supplements)the main circuit after the jets and the main circuit is not flowing any fuel at idle.
    The only fuel at idle is coming from the idle circuit and is independent from the main circuit.

  5. #5
    76miller
    Infomanic- Thank you for the reply. The vac. #'s @ rpm that I left in the last posts, does that seem right, and if so, what size p/v would you run ?

  6. #6
    Sleeper CP
    76 Miller,
    Run a 5.5 or 4.5 P/V (I would run a 4.5) in the primary side only. Plug the secondary P/V port and up jet 5-6 jet sizes. Under hard acceleration the secondary P/V gets unloaded with fuel. Buy jet extensions for the secondary side. There is a reason drag cars don't run secondary power valves.
    Does you carb have progressive or soft progressive linkage? I can't imagine your secondaries opening up before 3,500 unless you are running the soft progressive linkage that starts to open the secondaries up at approx. 1/4 throttle.
    As far as your dyno sheet is concerned the vacuum (read in inches) was not hooked up. I have a few sheets that read the same way. I don't recall that you are running a dominator carb so you might be able to go to a good auto parts store and buy a carb spacer with a vacuum port in it or you could easily drill and tap the spacer and leave your manifold alone for now.
    Just my .02 Good luck with it.
    Sleeper CP
    Big Inch Ford Lover
    Jon

  7. #7
    Infomaniac
    Infomanic- Thank you for the reply. The vac. #'s @ rpm that I left in the last posts, does that seem right, and if so, what size p/v would you run ?
    Honestly I diodnt pay any attention to the actual readings you posted. It was a dyno pull and pretty much useless so I didnt pay attention.
    Looks like someone else answered it for you.

  8. #8
    76miller
    76 Miller,
    Run a 5.5 or 4.5 P/V (I would run a 4.5) in the primary side only. Plug the secondary P/V port and up jet 5-6 jet sizes. Under hard acceleration the secondary P/V gets unloaded with fuel. Buy jet extensions for the secondary side. There is a reason drag cars don't run secondary power valves.
    Does you carb have progressive or soft progressive linkage? I can't imagine your secondaries opening up before 3,500 unless you are running the soft progressive linkage that starts to open the secondaries up at approx. 1/4 throttle.
    As far as your dyno sheet is concerned the vacuum (read in inches) was not hooked up. I have a few sheets that read the same way. I don't recall that you are running a dominator carb so you might be able to go to a good auto parts store and buy a carb spacer with a vacuum port in it or you could easily drill and tap the spacer and leave your manifold alone for now.
    Just my .02 Good luck with it.
    Sleeper CP
    Big Inch Ford Lover
    Jon
    Jon- Thats a good idea, I didn't even think about drilling and tapping the spacer. Thats what I will do, and then I can figure out what size p/v exactly. Thanks Jon

  9. #9
    Sleeper CP
    Jon- Thats a good idea, I didn't even think about drilling and tapping the spacer. Thats what I will do, and then I can figure out what size p/v exactly. Thanks Jon
    Wayne got your PM.
    I wouldn't run a 8.5 P/V in the primaries you'll be dumping fuel all the time. You stated that you want to conserve fuel under 3,500 rpm's IMO a 8.5 P/V is to high. And pull out the secondary P/V and buy a set of Moroso jet extensions (brass) and up jet 5-6 jet sizes on the secondaries. Just my .02
    Oh, What RPM does your engine rev to a wide open throttle?
    Sleeper CP
    Big Inch Ford Lover

  10. #10
    76miller
    Sleeper- W.O.T is 5300rpm w/ an A impeller, the reason I'm making changes is because last time out I had a exhaust gasket go bad and when I pulled it off it looked like I leaned out. I remember the engine stumbling a little bit when I went from 3500rpm- W.O.T, like it wasn't getting enough fuel. I also changed from a flame arrestor/Bassett scoop to a K&N/Fiberglass deal, so I was getting way better air flow and not enough fuel...IMO. Causing the gasket to fail at the header. I idled for a while before shutting off so reading the plug was kind of out the window. Now I bumped up the secondaries from a 86 to a 90 and was thinking that I needed to fix that stumble on the primary side w/ a larger p/v, currently it is a 4.5. ????? :idea:

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