On my last flight last week, I noticed a vibration at all RPM's. My engine was installed in 2013 but has very low hours. I bought the plane with 60 hours on it and since the annual last fall, I've put about 30 hours on it. During the annual the carbs were synced by a Rotax tech. It's been running good. But I didn't put new plugs in it during the annual so today I replaced them. I was puzzled to see that the cylinder #3 plugs appeared much differently than the other 6. Any thoughts? 912ULS


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  • Re: Engine vibration and plug colors

    by » 3 years ago

    Black dry carbon is fairly normal after it has been idleing. It burns off during ghigh rpm use.This is affected by the air imbalance between front and back cyl. in the intake at low rpms.

    Something to check:

    This can also be affected by the mixtures screw on the bottom of the carb not adjusted right. (1.5 turns out from all the way in). Plug gaps may contribute to this if they are too wide. Normal is between .023 - .027 for the NGK plugs. Many set them at .025 since it's in the middle or .023 during cold weather months. 

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

  • Re: Engine vibration and plug colors

    by » 3 years ago

    Thanks Roger. Most of that last flight was pattern work and a little 4500 rpm cruise around the area. Short taxi back to hangar. No significant idle time other than low rpm during final approach.

    I remember seeing the tech adjust the screws like that during the carb sync. I can recheck. I adjusted the gap on all the new plugs to .024. Didn't check the old ones but I will. That might reveal something. I guess I'll give it a shot tomorrow. Nice weather coming. Hopefully vibration will be gone.


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