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Thread: Power valve selection

  1. #1
    Join Date
    Mar 2010
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    27

    Default Power valve selection

    Why does your literature refer to idle vacuum when selecting a power valve? I select them by driving around with a vacuum gage and noting where the motor needs the enrichment. I seldom end up at "1/2 the idle vacuum" or " idle vacuum minus 2". I see lots of people on web forums treating "1/2 the idle vacuum" like an absolute rule 'because Holley says so'. Can you describe in detail the correct way to accurately choose a power valve? And what does idle vacuum have to do with it?

  2. #2
    Join Date
    Nov 2009
    Location
    Kentucky
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    Quote Originally Posted by markw View Post
    Why does your literature refer to idle vacuum when selecting a power valve? I select them by driving around with a vacuum gage and noting where the motor needs the enrichment. I seldom end up at "1/2 the idle vacuum" or " idle vacuum minus 2". I see lots of people on web forums treating "1/2 the idle vacuum" like an absolute rule 'because Holley says so'. Can you describe in detail the correct way to accurately choose a power valve? And what does idle vacuum have to do with it?
    The purpose of a power valve is for it to add the needed extra fuel when you are under wide open throttle. It is not intended to be an enrichment circuit during normal throttle cruising.

    If you are lean or rich during normal driving conditions, you need to tune the jets, not the power valve.

    The reason you check the vacuum at idle and take that reading and divide it in half, is so you do not have a power valve that will open at idle as this will cause an excessively rich mixture at idle. . Under WOT the vacuum of the engine will drop to 0 or near 0 and the power valve will then open adding the extra fuel needed during WOT.

    1972 Rally Nova
    1972 C-10 Short Step
    1970 Rally Sport Camaro
    1986 Chevy Silverado K10
    2009 F150 Lariat

  3. #3
    Join Date
    Mar 2010
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    Default

    Why do you think an open power valve affects the idle? How would the fuel get past the IFRs? I've taken the main jets out completely and the motor still idled fine. The power valve allows a leaner mixture for cruise and it must open under heavy throttle to richen the mixture. Not just wide open throttle. Since it only acts with heavy throttle on the main circuit, and a normal car doesn't idle on the main circuit, doesn't it make sense to take your vacuum reading while driving on the main circuit with heavy throttle? Are you suggesting that a 350 small block motorhome and a big block Vega would need the same power valve if they both had the same idle vacuum? Cause my experience says otherwise. I think the "1/2 the idle vacuum" rule is wrong and Holley should do it's customers a favor and rewrite their instructions. There are a lot of people with wide band meters now and our results don't agree with that theory. At the most it's a "ballpark guess" but it certainly shouldn't be called a "rule". Can someone from Holley comment on this?

  4. #4
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    Nov 2009
    Location
    Kentucky
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    Mark, I am a long time Holley employee.

    You are absolutely correct that you can choose the PV size by driving under a moderate to heavy load while reading a vacuum gauge and a WBO2. This is most accurate way to choose a PV.

    One thing we have to keep in mind is that most people do not have a WB02 meter and have no way to identify while driving when the engine needs the extra fuel. Although there are many people that may have them now, including yourself, this is not the norm for most people.

    The process Holley recommends of checking the idle vacuum, is easily completed by most people and will get the power valve in the proper operating range and prevent it from coming open to early. You have chosen to take power valve selection to the next level by utilizing new technology.

    Thank you for your comments and suggestions.

    1972 Rally Nova
    1972 C-10 Short Step
    1970 Rally Sport Camaro
    1986 Chevy Silverado K10
    2009 F150 Lariat

  5. #5
    Join Date
    Mar 2010
    Posts
    27

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    Thanks for the response. I think it would the your customers a real service if Holley would reword the instructions. People take the present wording as 'the gospel according to Holley' and don't realize there's a better, more accurate way. Look at some of the car forums and see how people treat this 'rule'. People are buying parts they don't need trying to satisfy the 'rule' and then complaining they now have heavy throttle lean spots. Try to tell them how to fix it by changing the power valve correctly and they quote the Holley 'rule' and refuse to change it. That idle vacuum thing should be called a ballpark setting and not a rule. And the correct way should be stated, even if the majority of people don't have WB meters. A large enough lean spot can be felt without a meter. A slight change in wording would go a long way to lower the confusion about power valves.

  6. #6

    Default

    Hey guys, I am new here and could use some help on this subject.

    I just replaced a 15 year old 750 "street" dominator with a new hp 750 "street" dominator. Unfortunately I sold the old one, so I can't put it back on!!!!!

    I have been to the track with my car many times with the old one, and just a few times with this new one. I am having some trouble with the new one as it compares to the old one.

    This carb is a little small for my car and I always have a slight amount of vaccuum at WOT ( I can tell because the cowl induction door on my hood opens some, but not all the way when I am at wot)....

    Anyways the old carb had 70's squared and 6.5 power valves with no jet extensions.

    The new carb came with 74's squared and with jet extensions, so I put them in. The new carb did come with 6.5 powervalves, but I have 8" of vaccuum at idle and I did just what the instructions said and switched them to 4.5 power valves.

    The car leaves very hard and then falls on its face some, not bogging, but i can feel it pulling a little less, then it seams to pick up again (kind of feels like i need jet extensions but I already have them), then as soon as I make the 1/2 shift, I can feel a power loss again, then it slowly seems to pick up some. All in all, I am off about 3-4 mph and 3-4 tenths.

    I have played with the float levels going from half way up the sight glass to 3/4, with no change.

    Could the power valves (both carbs have two, primary and secondary) be closing immediately after take off and after the shift?

    Also, I thought maybe it was running too rich, so I jetted it to 71's squared (the reason I thought it was too rich was because of the loss of power after shifting). While it seems a little snappier and cleaner on the street, no real change on the track can be noticed because of this other problem.

    What are my next steps? do you think the 4.5 's are closing during my pass?

    Another major problem with this carb is boil over after shut down with pump gas. I can see fuel dribbling out of the acc pump squirter after I shut the engine off, I can also see the fuel bubbling in the sight glass on occasion, again this is after the engine has been shut off and heat soaks....

    Any help would be greatly appreciated.

    Sean

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