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  1. #1

    Default Possible fuel issue

    I have a 1974 omc 140hp gm 4 cylinder, I rebuilt top end new head gasket, thermostat etc..... I took it for a spin ran awesome no issues hit a big wake since then it hasn't ran right. It has only about 1/2 throttle before it pings like it is starving for fuel. Previous owner installed electric fuel pump and external filter. I checked pump wasn't happy with fuel pressure I put new pump and filter in, no filter in top of fuel pump, after all was changed I blew backwards thru fuel line and heard no obstacles in fuel line or vent. I pulled filter out of carb it was gummed up waiting for new one now. I am curious if I am on the right track chasing a fuel issue or is there something else I am missing
    Last edited by mevans; 05-10-2020 at 10:30 PM. Reason: Trying to add pics

  2. #2
    Join Date
    May 2007
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    Kawarthas
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    3,240

    Default Re: Possible fuel issue

    Quote Originally Posted by mevans View Post
    I have a 1974 omc 140hp gm 4 cylinder, I rebuilt top end new head gasket, thermostat etc..... I took it for a spin ran awesome no issues hit a big wake since then it hasn't ran right. It has only about 1/2 throttle before it pings like it is starving for fuel. Previous owner installed electric fuel pump and external filter. I checked pump wasn't happy with fuel pressure I put new pump and filter in, no filter in top of fuel pump, after all was changed I blew backwards thru fuel line and heard no obstacles in fuel line or vent. I pulled filter out of carb it was gummed up waiting for new one now. I am curious if I am on the right track chasing a fuel issue or is there something else I am missing
    starving for fuel doesn't make an engine ping.

    Starving for fuel will cause a lean sneeze or backfire out the carb under heavy load.

    Start with a compression test, look at the plugs while they're out.

    Do you still have the original points distributor? the timing advance weights love to gum up and stick. That'll make it ping. Distributor can be disassembled and cleaned with brake clean and lubed with new oil.
    Factory Manual Best, Seloc 2nd best, Clymer if you're desperate.

  3. #3
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    12,039

    Default Re: Possible fuel issue

    ..........
    Quote Originally Posted by mevans View Post
    I have a 1974 omc 140hp gm 4 cylinder, I rebuilt top end new head gasket, thermostat etc..... I took it for a spin ran awesome no issues hit a big wake since then it hasn't ran right. It has only about 1/2 throttle before it pings like it is starving for fuel.
    Pinging is an indication of Pre-Ignition. Old degraded gasoline may also cause pinging.

    Check the progressive and total advance as per your OEM specs.

    A 1974 GM 4 banger will use a mechanically advancing ignition distributor.
    See if the distributor flyweight return springs are in tact and that the flyweights themselves are free to move.
    Make sure that the little springs are not rust compromised.
    The rotor should move the flyweights (in the advancing direction) freely and with little force, and the flyweight return springs should return the rotor completely.



    Previous owner installed electric fuel pump and external filter.
    Keep in mind that the electric fuel pump requires a LOPS (low oil pressure switch) to be installed so that in the absence of engine oil pressure, the power to the fuel pump is cut. This is a USCG regulation.

    I checked pump wasn't happy with fuel pressure I put new pump and filter in, no filter in top of fuel pump, after all was changed I blew backwards thru fuel line and heard no obstacles in fuel line or vent.
    You should NOT have been able to blow backwards through the fuel line.
    If you are able to, the Anti-Siphon valve is either messed up, or has been removed.
    The Anti-Siphon valve is also a USCG requirement (for gasoline engines).

    I pulled filter out of carb it was gummed up waiting for new one now. I am curious if I am on the right track chasing a fuel issue or is there something else I am missing
    What you are doing may not solve your issue, but you are certainly on the right track by cleaning up the fuel delivery system.

    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

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