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Dear All,
I have a Rotax 912 in a flexwing trike airframe. Since new the engine has run rough between 3500 - 4500rpm. Before going back to the aircraft manufacturer I checked carb balance, carb heaters, changed the prop to a different type, tried different needle positions and checked that all in genaral was OK - with no change to the vibration. It then went back to the aircraft manufacturer who rechecked everything, took cylinder compresion readings (130/135 on one side, 120/140 on the other). No improvement.

Above 4500rpm and nelow 3500rpm the engine is sweet. Between those values it has a high frequency vibration felt through the airframe and if you look at the engine the carbs are bouncing around.

On previous 912 engines I have always lowered the needles 1 notch lower than standard (3 from bottom) - which is how this engine is now tuned - but I have also tried both standard and 1 notch richer settings and these make no difference to the vibration.

Carb balancing gauges show no irregularities in the vibratory rev range.

Before returning it to Mr Rotax does anybody have any further ideas please?

Many thanks
Andy Buchan
  • Re: Rough running, 3500 - 4500rpm, Rotax 912

    by » 11 years ago


    Hi Andy,

    I have found two things that make the engine vibrate in this rpm range. First is a poor carb balance. You said you did that, but I start my balance at 3500 and check it up through 4000 rpm. If you set it at 2800 like some then the carbs can easily be out of balance at the higher rpm and you don't fly at just above idle at 2800 rpm.
    The second is a blade on the prop is not perfectly pitched like the other two if you have a three blade. If you have a 2 blade then just one of them. It doesn't hurt to double check and only takes a few minutes.
    Some engines do seem to have a small rpm range that may feel slightly rougher than the other rpm settings.

    The last thing to double check would be to do a static balance on the prop (fairly easy) and then a dynamic balance. (needing a special monitor) ($250-$300 for the procedure).

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Rough running, 3500 - 4500rpm, Rotax 912

    by » 11 years ago


    Thanks Roger.
    I take your point about carb balancing - we balance at lower revs than 4000 but then check how the gauges read higher up the rev range. Too excisting making adjustents at 4000rpm!

    Unfortunately we do not have a dynamic prop balancer. We checked one prop static and it was fine, as was tracking and pitch when fitted. Also tried an alternative prop type (brand new) and there was no perceptable difference to the vibration. Also, it is a higher frequency vibration and we are tending to think it is engine induced rather than prop induced.

    Our next try is changing exhaust silencers (the engine has a twin silencer system.

    Thanks again for your response

    Andy

    Thank you said by: YEN NIEN YU

  • Re: Rough running, 3500 - 4500rpm, Rotax 912

    by » 11 years ago


    Andy one of the guys at Ince with a 912 Quantum was having similar probs,

    In the end he had his prop dynamicly balanced and that cured it...yet his static balance was fine.

    Nick.

  • Re: Rough running, 3500 - 4500rpm, Rotax 912

    by » 11 years ago


    Hi Andy,

    A Dynamic balance can help. I sure someone around you can do one. The other thing that will cause this is the gearbox friction torque. You may need to re-shim the gearbox to increase the pressure on the bellville washers. If you are already in the average 450-480 in/lb range then probably not an issue, but if you are down in the 300 range I would re-shim.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


    Thank you said by: YEN NIEN YU

  • Re: Rough running, 3500 - 4500rpm, Rotax 912

    by » 11 years ago


    Another little problem confirmed today is that it is drinking coolant - with no external leaks - so the problem may be deeper than adjustments.....

    Nick - thanks for the input. I think you are refering to Quantum 912 GOAMF now owned by CP? (I owned it a long time ago then sold it to a student, so flew it for quite a few hours). Dynamic balancing is certainly a good thing to do - but my problem is with a new machine. I can get dynamic balancing done at Netherthorpe which is close-by and it is certainly worth considering in the future.

    Andy

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