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Thread: Power Valve Questions

  1. #1

    Default Power Valve Questions

    I'm new to Holley and just got a new 0-80555C, and I'm working on getting it setup right. I need to understand what exactly does the power valve do? I understand when the vacuum goes low enough, it allows more fuel to be used, but where is it entering? Does it have its own built in, for lack of better terms, its own "jets"? If not, what determines how much extra fuel it allows?

    Also, I tend to either drive easy, hardly ever hit 1/4 throttle, or I go WOT through the gears. Would putting in a high flow PV that only opens under very low vacuum like a 2.5 work OK? Like I said, I really don't use most of the throttle range, so it wouldn't matter to me if I felt it "kick in", instead of a smooth acceleration from idle to WOT.

    Thanks, Tony

  2. #2

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    I'm new to Holley and just got a new 0-80555C, and I'm working on getting it setup right. I need to understand what exactly does the power valve do?
    Power Valve = Add more fuel under heavy load and low vacuum situations. Without the power valve for typical street driving, you create an rich or lean condition. If you were to plug the power valve and do nothing else, you create a lean condition under high load low vacuum situation. If you plug the the PV and then put in larger main jets to compensate for it, your going to have a rich cruising condition.

    I understand when the vacuum goes low enough, it allows more fuel to be used, but where is it entering?
    When the power valve open, it is adding extra fuel into the main wells on the metering block.

    Does it have its own built in, for lack of better terms, its own "jets".
    Yes the PV has it own jets. They are called PVCRs (power valve channel restrictor). On standard Holley metering blocks they are not adjustable. They can be drilled out, tapped for 6-32 to make adjustable. (Not recommended) After market metering blocks already have them tapped and made to be adjustable.

    Also, I tend to either drive easy, hardly ever hit 1/4 throttle, or I go WOT through the gears. Would putting in a high flow PV that only opens under very low vacuum like a 2.5 work OK? Like I said, I really don't use most of the throttle range, so it wouldn't matter to me if I felt it "kick in", instead of a smooth acceleration from idle to WOT.
    That's a question that can only be answered by you. You have to kinda tune to your driving tendencies. There is no "KICK IN" from a PV. It just enriches your fuel ratio at low vacuum high load situations.

  3. #3

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    Thanks! Onto my next question.

  4. #4

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    I'm going to hijack this thread for my question. I have Truck Avenger 670 cfm running on an AMC 304 in a Jeep. Fresh motor with RV cam, .060" over (313 cu/in now), 9.5:1, Performer intake. I just cannot get this thing tuned. I have tried squirters, cams, and a 4.5 power valve. While at around 1500 RPM under a load, it's backfiring through the carb, and I have to let up on throttle and ease back into it. It's an automatic transmission, and when it shifts up, it brings the RPM back to about 1500, and it backfires through the carb and has no power. A couple questions -

    1) Is the bottom line that the carb is way too big for the engine?

    2) Currently, it has the Blue Cam, 2.5 power valve, 37 squirter. It still does it, just not quite as bad. Any suggestions on what direction I should take?

  5. #5

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    Quote Originally Posted by SaifAuto View Post
    I'm going to hijack this thread for my question. I have Truck Avenger 670 cfm running on an AMC 304 in a Jeep. Fresh motor with RV cam, .060" over (313 cu/in now), 9.5:1, Performer intake. I just cannot get this thing tuned. I have tried squirters, cams, and a 4.5 power valve. While at around 1500 RPM under a load, it's backfiring through the carb, and I have to let up on throttle and ease back into it. It's an automatic transmission, and when it shifts up, it brings the RPM back to about 1500, and it backfires through the carb and has no power. A couple questions -

    1) Is the bottom line that the carb is way too big for the engine?

    2) Currently, it has the Blue Cam, 2.5 power valve, 37 squirter. It still does it, just not quite as bad. Any suggestions on what direction I should take?
    Well first off, we need to know what is causing the backfire. I suppose if the fuel ratio was super lean, it can cause to backfire and buck around. At 1500 RPM your fuel is coming mainly from two circuits of the carburetor. Idle circuit & transfer circuit. The mains have probably not kicked in just yet, and if they are it would be very minimal. That 670 is just fine for your application.

    Double check your timing & plug wiring. Just to be sure. Sometime the simple things are overlooked.

    When you say at around 1500 RPM under load, are you cruising steady or slightly accelerating?

    Does its backfire when you attempt to start the Jeep or only when under load low rpm?
    If you run the rpms up to 2500+ in a low gear does it run better or worse?
    Do you have your timing specs available?
    How much vacuum at idle are you generating? With an RV cam, you should be in the 14+ area.
    Have you tuned the idle mixture screws? If so, please tell us how you tuned them.
    Last edited by 69cam; 08-11-2015 at 09:46 AM.

  6. #6

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    Timing right on, plugs wires are right.

    The 1500 under load is accelerating. If I really ease into it small hesitation at 1500, then past that it is fine.

    Backfire happens under load low RPM, I can have it in neutral, and hit the throttle and revs just fine.

    Vacuum about 14 @ 1,000, 11 @ 700.

    I started with the screws out two turns, and have been doing trial & error leaner/richer from there.

    Thanks.

  7. #7

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    A little stumped by this. The two statement kinda contradict one another from a carb standpoint. If you can have it neutral and rev it and it does fine, but you can't have it in gear at a relatively really low rpm, and accelerate without bogging. From a carb standpoint, this is normally an indication of LEAN condition.

    You stated that by adding a 37 pump shooter, blue cam, and dropping to a 2.5 power valve it got a little better. These changes are kinda off setting one another. You shouldn't need that huge of pump shooter on a 670, and the blue cam is one of the most aggressive cams that Holley has. If it got a little better by these changes you are fighting a lean condition at low rpm. If you have 11" of vacuum at idle rpm, when you are accelerating normally you might be getting down to the 4-5" vacuum reading. By installing a 2.5 power valve you are never giving your power valve a chance to help enrich the fuel ratio. Then only time it is getting low enough to open a 2.5 is on HEAVY acceleration. Change that out to 6.5 or 7.5. Holley's rule of thumb is to be about 1/2 of your idle vacuum for power valve size.

    I'll go through a checklist of things that could cause a lean condition.
    Check the fuel height in the bowls - adjust accordingly. Barely dripping out if site plug or if site glass 1/2-3/4 of the way up in the site glass.
    What's your fuel pressure? Is it maintaining a steady pressure?
    Have the main jets been changed? If so put them back to default setting if default setting is bigger then what you have in there.
    Adjust the idle mixture screws to 3 turns out. 2 should have been pretty rich. Take if for a short ride around the block just to see if helped any.
    Make sure engine is warmed up pretty good. Double check you accelerator pump linkage. Make sure it is pumping fuel instantly:
    https://www.youtube.com/watch?v=k-Nx5HEzvlY
    Last but not least, check your timing curve. Timing at idle? Timing at 2500 rpm? If your timing is not advancing properly, it will hesitate as well. If you have to low of idle timing it will hesitate.
    Last edited by 69cam; 08-12-2015 at 05:05 AM.

  8. #8

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    I have almost exact same issues.

    304 cid with an RV cam
    456 gears
    36” tires
    At just about sea level.

    This is where I have it today, and it seems to be functioning without the back fire...finally.

    Holley 0-90670
    Jets: 60 main, 63 secondary
    Power valve: 6.5
    squirter: 40 with high flow screw
    Accel pump kit: 50cc
    Accel pump cam: Brown

    Although, 7 mpg, don't think I like that very well.

    If this TA 670 doesn't dial in with some better fuel mileage, I may be looking for another carb.

  9. #9

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    Thanks, I had to travel out of town, and will be back on it next week. I appreciate your help on this. CJ

  10. #10

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    Had a chance to do a couple small things yesterday. Went to a 6.5 power valve and 3 turns out on the idle mixture screws. It's better. It does seem that it is moving the right direction. I still have to ease into the throttle but not as much. Still, if under a load, say driving along in 3rd (automatic) an pulling along around the 1500 rpm mark will back fire through the carb. But the popping through the carb does not seem and extreme. If I let up and ease back in it will power through the 1500 RPM area and once I get to 2000 it is good. I notice more now that if when in neutral I blip the throttle there is a ever so slight hesitation, could be because of the big shooter and cam.

    Accel pump is good so is the float level.
    Fuel pressure is steady at 6.5 psi.
    Still need to get a timing light on it.

    Any suggestions on the next step? Thanks.


    UPDATE: I drove it around a bit yesterday after writing this post. The Jeep has a 20 gallon tank. I'm not exaggerating, I drove 15 -20 miles, and burned nearly a 1/4 tank of gas. Something is not right.
    Last edited by SaifAuto; 08-21-2015 at 02:04 PM.

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