Holley 750DP Bog

Discussion in 'Troubleshooting & Diagnosis' started by Pilot53, Jul 23, 2009.

  1. Bikefixr

    Bikefixr Veteran Member Lifetime Gold Member

    1,643
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    Mar 13, 2006
    Pilot, you'd have to get so lean to burn a piston that I doubt you could live with the driveability issues. I have tuned primaries several ways. I've used the old emissions machines garages used to use....I'd run up the RPM and watch the gauge. I've used an O2 sensor hooked up to a voltmeter to read the Oxygen percentage on the exhaust and that's super sweet . You can see what is happening to your curve as you drive. Or, you can trial and error. Just lean the jets down ( stay out of the secondaries!) The idea is to just use light throttle, steady-state driving on a highway. If the car surges slightly, you're too lean. Go up a size and retest. If the surge is almost gone, go up 1 more size. You are now as lean as the engine will tolerate and still drive well. Use the secondaries to give you max HP, the primaries for max Mileage.
     
  2. Pilot53

    Pilot53 Veteran Member

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    Nov 30, 2008
    New City, NY
    Ok so leave the secondaries rich, could the mix be thrown off if the secondaries are too different from the primaries? In other words, will it become "unbalanced" if the primaries are jetted lean and the secondaries are rich once the secondaries open up? And do the secondary metering blocks have ifr's?
     
    Last edited: Jul 26, 2009
  3. Pilot53

    Pilot53 Veteran Member

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    Nov 30, 2008
    New City, NY
    Ok, so Ive put wire into the primary ifr's and am running #80 secondary jets and #65 primary jets. The carb still bogs a tiny bit from a stop but not nearly as bad. The problem I am having now is that if I open the throttles very slowly from a stop I get an off idle stumble, if I open them quick it doesnt happen. And I have only altered the primary ifr's, do I need to do anything with the secondary metering block?
     
  4. markw

    markw Veteran Member

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    Jun 6, 2009
    Illinois
    You may need to go to a smaller size wire in the IFRs. That orifice is responsible for the part throttle driveability before the main jets start working. Did you readjust the idle mixture screws after installing the wires? Opening them up some may help. How does it run at a steady cruise at around 3000 rpm? That would be right in the range of the main jet. Have you tried more initial advance? Once the primaries are tuned for mileage you can drill out the PVCRs to bring the WOT mixture back to a safe level. Double pumpers usually have large jets and small PVCRs. Vacuum secondary street carbs usually have small jets and large PVCRs. (Jets + PVCRs = WOT mixture.) This jetting difference is why double pumpers suck so much gas on the street until you re-tune them.
     
  5. Pilot53

    Pilot53 Veteran Member

    262
    0
    Nov 30, 2008
    New City, NY
    Yeah I probably need a smaller wire. So what do i do with the secondary ifr's?
     
  6. markw

    markw Veteran Member

    816
    0
    Jun 6, 2009
    Illinois
    Most of the carbs I've seen have the secondary IFRs in the vertical channel under the cup plugs you see on the top of the metering block. If you have them in the standard locations then by all means put a wire in them. Otherwise you get into drilling out the plugs, removing the original IFRs, and tapping the metering block for new IFRs in the standard location. This site has a lot of info on this kind of modifications... http://www.innovatemotorsports.com/forums/showthread.php?t=7576&highlight=brass+set+screws Do a search there and you will find lots of pictures and info. Don't try it unless you're confident in your abilities, once you drill you're committed. (That's why there are so many used carbs on Ebay?) You need a number drill set cause the holes are really small and .001-.002 change makes a big difference. All of this is the reason Holley makes the HP series carbs. They already have the passages tapped for easy changes, but I'm cheap so I just convert my own. Measure everything before you drill and keep a log book of everything you do. If you're not comfortable with drilling then just work with the primaries.
     

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