Holley carb help
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Holley carb help

  1. #1
    Senior Member Mohavekid's Avatar
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    Default Holley carb help

    Ok, long story short, the carb ran perfect for years.

    Last season at random times it would stumble and die like it had water in the fuel.

    This winter I drained the tanks, found no water. I replaced the fuel lines, the fuel filters, the tank selector switch and fuel pump. Leaving the carb as the issue.

    This year it does the same thing. It is very hard to start hot and cold, when it does start there is lots of black smoke. It seems to be running very rich.

    I took the float bowls and metering blocks off and cleaned them with spray carb cleaner. No change except now it surges at idle. To reduce, but not eliminate the surge I had to close the idle mixture screws all the way. At higher RPM's it runs great until the stumble occurs, always at part throttle. Full throttle is fine.

    I want my reliable running boat back. Any ideas??

    The carb is a Holley 750 VC on a Harman Marine 454.

    Thanks
    Last edited by Mohavekid; 06-25-2012 at 10:01 AM.

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  3. #2
    Senior Member N2GLOCK's Avatar
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    I'm having similiar issues with my Holley except mine starts fine both hot and cold. Quite annoying actually. I won't take my wife and kid out until I get the issue resolved.

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    i would replace the p/v ...usually black smoke is the result of a blown p/v...sure cant hurt! ...just my .02 cents

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    Senior Member SoldHondaBoughtHondo's Avatar
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    X2 power valve. If you can (carefully) look down the carb with the motor at idle and the throttle plates are dry there are few other places that excess fuel can get into the motor.
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  7. #5
    Some guy obnoxious001's Avatar
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    Quote Originally Posted by Mohavekid View Post
    Ok, long story short, the carb ran perfect for years.

    Last season at random times it would stumble and die like it had water in the fuel.

    This winter I drained the tanks, found no water. I replaced the fuel lines, the fuel filters, the tank selector switch and fuel pump. Leaving the carb as the issue.

    This year it does the same thing. It is very hard to start hot and cold, when it does start there is lots of black smoke. It seems to be running very rich.

    I took the float bowls and metering blocks off and cleaned them with spray carb cleaner. No change except now it surges at idle. To reduce, but not eliminate the surge I had to close the idle mixture screws all the way. At higher RPM's it runs great until the stumble occurs, always at part throttle. Full throttle is fine.

    I want my reliable running boat back. Any ideas??

    The carb is a Holley 750 VC on a Harman Marine 454.

    Thanks
    How about buying a kit and rebuilding the carb, it's not that difficult, and it does sound like you have a problem if you have the idle air bleeds screwed all the way in. Did you check float level,, and make sure the needle and seats are working>

  8. #6
    Senior Member Mohavekid's Avatar
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    I have checked the float levels and reset them to just a slight trickle out of the sight hole.

    I removed the front needle and found the O ring disintegrating.

    I guess a rebuild is in my future.

    I checked the 'list" number which is 8572. Research indicates this is a 715 CFM carb, not a 750.

    Thanks

  9. #7
    Senior Member Bubbletop409's Avatar
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    I just went through the same ordeal with the rear carb on my 565. Changed power valves, gaskets, checked float level, lowered my fuel press. etc., all to no avail. Finally I unbolted the carb, put a couple pieces of small wood between carb and manifold and watched a steady drip from where the PV dumps the fuel when open, called and spoke to a Holley tech rep, he suggested I check the main body surface where the metering body mounts, I put a straight edge diagonally from opposite corners and was surprised to find such an irregular surface, one pass with a med. file told me just how bad that surface was. I carefully made a few cuts with the file and brought the surface back to true, cleaned and blew everything with air, new gasket, reassembled, hooked up fuel lines with carb still sitting on my wood spacers, filled the bowl and watched for a very long time before I was convinced my problem was solved. Tech told me with a lot of assembly/disassembly those surfaces can become distorted and leak internally.
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    Senior Member Ronboats's Avatar
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    Replace the power valve and clear the idle air bleeds in the main body to fix the idle surge,but
    A "random " problem that causes it to die at part throttle sounds more like an ignition problem to me.
    Or possibly a fuel delivery issue.
    Last edited by Ronboats; 06-25-2012 at 01:10 PM.

  11. #9
    Senior Member Mohavekid's Avatar
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    I ordered a rebuild kit from summit. Should be here this weekend.

    As for ignition, I checked for spark after one of the random episodes. It had spark at the number one plug.

    Once the fuel issue is solved I was planning to replace the old mallory dual points system to a pertronix set up. I don't want to start chasing two problems at once.

  12. #10
    Senior Member Mohavekid's Avatar
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    The rebuild kit came and I completed the process cleaning everything completely with carb cleaner and a toothbrush.

    I used the carb cleaner staw to clean out all the ports in the metering bliock and the air bleeds. i dried everything with compressed air and made sure none of the passeges were blocked.

    I reassembled carefully with all the new parts and gaskets, and reinstalled.

    The problem is worse than ever. It takes forever to start, it runs very rich with lots if stumbling. It will not idle at all.

    Just to verify the carb is the issue I put an old Holley 650 DP on it and it runs fine. Idles smooth and restarts instantly.

    Any thoughts on this issue? The old Holley is useable but a little small for this motor and it is not a marine carb. I really need to get this figured out.

  13. #11
    gn7
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    Quote Originally Posted by Bubbletop409 View Post
    I just went through the same ordeal with the rear carb on my 565. Changed power valves, gaskets, checked float level, lowered my fuel press. etc., all to no avail. Finally I unbolted the carb, put a couple pieces of small wood between carb and manifold and watched a steady drip from where the PV dumps the fuel when open, called and spoke to a Holley tech rep, he suggested I check the main body surface where the metering body mounts, I put a straight edge diagonally from opposite corners and was surprised to find such an irregular surface, one pass with a med. file told me just how bad that surface was. I carefully made a few cuts with the file and brought the surface back to true, cleaned and blew everything with air, new gasket, reassembled, hooked up fuel lines with carb still sitting on my wood spacers, filled the bowl and watched for a very long time before I was convinced my problem was solved. Tech told me with a lot of assembly/disassembly those surfaces can become distorted and leak internally.
    First, I qouted this because it worth reposting.
    Second, if you bought a universal kit that had multiple gaskets that you did not end up using, you need to DOUBLE, then TRIPLE check the base gasket you used between the carb main body and the base throttle plate. The 850 throttle plate gasket is different than all the others. The idle circuit does not line up with any other carb's main body. Tear the throttle plate off the carb and check the gasket you use to see that the idle ports in the throttle plate line up with the holes in the gasket.
    Its a very very common mistake.



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  14. #12
    Senior Member Mohavekid's Avatar
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    OK

    I took it apart to verify the gasket matches correctly with the idle ports. All appear to match correctly.

    Reinstalled. Still runs like crap, very rich and won't idle below 2500 RPM.

  15. #13
    Or Seth, either one Budweiser's Avatar
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    I'm going off of memory here, so give me a break if I'm not spot on (or even close).

    It sounds like your primary butterflies may be open too much, which will bypass the idle circuit altogether. I seem to remember having to measure the transfer slot, or the distance from the butterfly to the base plate (or something like that) in order to make sure the "idle" is actually running on the idle circuit. One of the fixes (again, from memory) if it won't allow enough air to idle and remain on the idle circuit was to drill a small hole in the down side of the primary butterflies. Not sure what size hole to drill... Double check everything and do a little research about this. It does sound like you might be idling on the "cruise" cicruit.

    Does adjusting your idle mixture screws change anything?

    Some of the others around here can confirm or debunk this, but pretty sure it's an accurate and common problem/fix.

    Good luck!
    -Seth-
    Last edited by Budweiser; 06-30-2012 at 04:04 PM.

  16. #14
    gn7
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    A dead giveaway that your idle circuit isn't working is nothing chances when you turn the idle mixture screw in and out.

    Its almost always what Budweiser posted. The throttle blades are open to far and the idle circuit no longer functions. The easiest way to know if you are where you need to be with the throttle blades is that the exposed portion of the transfer slot appears square. That means you have about as much transfer slot exposed as the slot is wide. It it appears to have length, you have the throttle blades open too far.
    Too much open blade opening will also lead to stumbles off idle on top of a shitting pig fat idle.



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