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Help with holley tuning...off idle backfire?

Discussion in 'The Hokey Ass Message Board' started by Bugman, Mar 27, 2004.

  1. Bugman
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    Bugman Member

    We're working on a 350 SBC, we just finished putting a Holley street dominator manifold, lunati cam, and Holley 4160 750 CFM carb.

    We're trying to tune it and set the idle. We get a backfire through the carb if you open the throttle quickly. It also fas a tad bit of hesitation off idle. Any suggestions? Thanks is advance.

    -Jeff
  2. Joes50
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    Joes50 Member

    I just went through that same thing on my small block. I eliminated the hesitation by increasing the accelerator pump squirt by changing the plastic cam that bolts on the throttle. I also advanced the timing a few degree's more and it's been running fine ever since.
  3. 286merc
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    286merc Member

    Holley backfire = blown power valve

    Get the new improved version, only took them about 63 years to come up with it.
  4. I couldn't agree more with Joe's50. If that accelerator pump lever isn't tight against the plastic cam, it'll do the same thing. Look down the carby to see if you're getting a fuel shot as soon as the throttle blades start opening. Adjust as needed.
    Also, is the engine warmed up jet? If its' dead cold, you may need just a scosh of choke until you build a little heat in the engine to stop the backfire..
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  5. Glen
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    Glen Member

    If your pump action is set up right, you can cheat just a little by turning your idle mixture screw 1/8 of a turn fatter...assuming you are at "perfect" idle mixture.
  6. Donzie
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    Donzie Member

    Is there a rule of thumb for perfect idle mixture?
  7. Hot Rod To Hell
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    Hot Rod To Hell Member

  8. Flat Ernie
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    Flat Ernie Tech Editor

    [ QUOTE ]
    Is there a rule of thumb for perfect idle mixture?

    [/ QUOTE ]

    If everything is set up correctly & the carb is sized right, 1-1/2 to 2 turns out is a good starting point. Set your idle & timing, then adjust idle screws for highest manifold vacuum.

    You can get an off-idle stumble/hesitation/back-fire from both too much squirter as well as not enough. For most cars, #25-28 squirters are sufficient. If you need a larger squirter, you need to consider a hollow squirter screw too.

    If you don't have a late-model Holley w/backfire check-valve, install one. Summit sells a kit (from Spectre I think) for less than $5 - cheap insurance against blown power valves. Not every backfire blows powervalves though.

    Your accelerator pump arm needs to move the pump lever immediately, however, at full throttle, you need a little movement left in the spring bolt that goes from the throttle to the pump lever. Watch this if you start playing with pump cams - don't let it coil bind or you'll bend the linkage lever. If you don't like the pump timing, most all have 2 (some have 3) holes. Putting the screw in the #2 hole will generally bring the pump in quicker. (when looking at the carb/throttle shaft from the drivers seat, the #2 hole generally moves the cam clockwise compared to teh #1 hole).

    For most street cars that need a little extra squirt, I like the orange cam on #2 hole w/#28 squirter.

    Also, don't rule out ignition timing for off-idle backfire...

    [​IMG]
  9. Bugman
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    Bugman Member

    It's a brand new out of the box carb. I thought it was to big, being a 750 on a 350 motor, but that's what he wanted. We got it mostly gone, but every now and then it pops through the carb when you snap the throttle open.

    It almost seems like the acc pump isn't working right. I know how it should work, and this one seems funny. When you let off the gas after holding it open, it still spits a little bit of fuel out the pump nozzle. What could cause this?
    Thanks.
    -Jeff
  10. Flat Ernie
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    Flat Ernie Tech Editor

    There's a needle under the accelerator pump nozzle - it may be sticking open & the fuel is being siphoned out. Does the squirter have two small brass tubes pointing at the venturis? I prefer the squirters w/o the tubes to help prevent siphoning.

    Another potential is the spring bolt is adjusted too long, so the pump arm isn't returning all the way. Tighten it up a bit until it's just loose against the pump arm & then back it off a 1/4 turn or so to just take out the slack.

    Tough to diagnose w/words alone...

    [​IMG]
  11. Deyomatic
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    Deyomatic Member

    We went through the same shit with Old Car Pilots setup back in December. Fucked with the carb for a week and never figured it out. we looked for vacuum leaks, changed power valves and jets and nothing worked. He even changed the intake gasket. It turned out that his distributor was just a couple of teeth off.
    You may want to try that.
  12. warbird
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    Bellingham, the other Washington

    warbird Member

    Be sure the secondary throttle is closed against the stop screw and that it's properly adjusted. Also be sure the check ball in the passage to the secondary diaphragm isn't stuck. Had that happen once, and it would retain just enough vacuum to hold the secondarys open slightly. Took me a lot of screwing around to find the cause of that off-idle stumble.

    And every new Holley I've messed with had a lot of loose screws right out of the box. Everywhere, including the throttle body to main housing. If those are loose, you might have a vacuum leak there.

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