Rough running 912

Category: General Tech Discussion 3 months ago

:I have a rough-running 912. When doing a “mag” check, I get a 75 RPM drop on the right (smooth) and 150 or more on the left (rough). Both carbs have been overhauled, then balanced with CarbMate. No improvement. Spark plugs look great.  BTW – The engine seems to run smooth for the first 20 seconds from a cold start. Any ideas on what I can check next?

  • Re: Rough running 912

    by » 3 months ago

    We have had six major causes for rough running of our Rotax 912 engine. 


     They are: faulty ignition cable, improper grounding of ignition module, and broken exhaust baffle.


    The remaining three causes are carburetors not synchronized, imprecise propeller pitch, and worn Belleville washers in the gearbox.



  • Re: Rough running 912

    by » 3 months ago

    Major 🐱.

    Carburetor Problems

    Three things can go wrong. 

    First, fuel-air mixtures in the two manifolds during idle may not be identical. 

    Second, the mechanical operation of the throttles may have play so that throttle positions can change at random. 

    Third, the position of one throttle relative to the other may be wrong so that different manifold pressures are produced. The latter effect is most pronounced at low- to mid-range rpm.

    Fuel-Air Mixtures for Idle

    The first item is easily checked and corrected. For each carburetor, the idle mixture screw is gently turned until it reaches its stop, and then it is opened 1 1/2 turns. 

    This value may have to be changed later, but it suffices for investigation of items two and three.

    Google it



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