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  1. #1
    Silver Member
    Join Date
    Oct 2006

    Default Two cylinder Onan engine running with one cylinder on Vacation

    I've got a Gravely 8163T with the B43M engine. It has a single barrel carburetor. One cylinder is working lousy, runs wheezy and makes a fair amount of black smoke. The other cylinder runs real good, burns clean.
    It is a single barrel carburetor with coil. I have tried two carburetors and two coils, same result.
    Is there some sort of vacuum related problem here? Not enough air to complete combustion in one of the cylinders, it appears. Compression in both cylinders is around 120 pounds.

    Is it a spark plug problem or lead wire problem?

    This has baffled some of my engine buddies and we have not come up with a solution.

    Waddya think?

  2. #2
    Platinum Member
    Join Date
    Jan 2012
    Ford 7710, Ford 550, Ford 1910, Ford 4430, KD Forklift, Ford LGT17H, New Holland L255 Skidsteer

    Default Re: Two cylinder Onan engine running with one cylinder on Vacation

    Have you adjusted the valves?

  3. #3
    Elite Member
    Join Date
    Sep 2010


    Did you try changing and ajusting the gap on the plug. Black smoke indicates incomplete combustion. A week spark would do this.

  4. #4
    Silver Member
    Join Date
    Oct 2010
    Dallas, TX

    Default Re: Two cylinder Onan engine running with one cylinder on Vacation

    Since one cylinder is too rich and the other is OK, I suspect that someone removed the intake manifold, did not replace the gaskets and now one side has a vacuum leak. To compensate for the vacuum leak they adjusted the carb. Now instead of one cylinder too lean and the other OK, now one is too rich and the other one is OK.

    I would order two new gaskets, remove the intake manifold and check for any leaks. Some of the Onan intake manifolds were a two piece design. If the PO split the manifold, then I would just buy a used one and replace it as resealing that two piece manifold is a real challenge.

    Do not use a simple paper gasket on the intake manifold. The portion of the intake port closest to the exhaust port of the block gets pretty hot. Hot enough to deteriorate a common paper gasket in short order.
    Last edited by RichardTX; 10-28-2012 at 01:12 PM.

  5. #5
    Gold Member
    Join Date
    Jul 2007
    Cleveland Tenn

    Default Re: Two cylinder Onan engine running with one cylinder on Vacation

    Try a new plug
    Ck to see if you have fire at through the plug
    I would try to switch sides with spark plug wires also.
    Check to see if you are geting fire through both wires
    I have seen a lot of twin cylinder engines run on 1 cylinder and it was fire each time

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