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If a cylinder misfires then wouldn't the wasted spark system mean that the unignited mixture might be ignited by the "wasted" spark on the exhaust stroke when the exhaust valve is open? That can't be good for the valves or the exhaust system. Is this a problem and can misfiles cause damage this way?

  • Re: Misfire then ignition on the exhaust stroke?

    by » one month ago


    A Misfire is not a good thing any way you look at it.

    However it is not as bad as you might think.

    Assume the engine has been running more or less normally and every once in a while it misfires.

    Gasoline vapor has an auto ignition temperature of a bit under 600°F.

    The exhaust manifold is almost always well over 1000°F as witnessed by the EGT gauge.

    As the the fuel/air charge is dumped into the exhaust manifold it is going to be ignited by the exhaust manifold components well before the piston ever reaches the top of the stroke and the waisted spark fires.

    The good news is that the charge is being ignited from the top down and not the bottom up.

    There will be a healthy "bang" in the exhaust pipe that will definitly get your attention, but it is mostly harmless as far as actual energy and pressures are concerned due to there being virtually no compression of the fuel/air charge.

    - - -

    I used to agressively down shift a VW Engine in a dunebuggy just to get it to misfire and shoot flames out the glasspacks and make a lot of noise for the pedestrians.  After 40 years, it was still shooting flames just fine.

    The real concern is the misfire forces the Propeller to drive the engine for a 1/2 rev instead of the engine driving the Prop.

    It is rough on the drive train. Not the exhaust system.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


    Thank you said by: Pilot Joe

  • Re: Misfire then ignition on the exhaust stroke?

    by » one month ago


    Thank you! How can I tell how often a 912 engine misfires ie the ignition probability at various RPMs and mixtures? Does the ECU in the iS model store any of that information?


  • Re: Misfire then ignition on the exhaust stroke?

    by » 4 weeks ago


    Faulty spark plugs cable , current leakage , spark plugs connector resistance value  out of range may cause misfire problem.


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