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RPM drop on ignition check

in General Tech Discussion 6 years ago
Hello everyone

I have a 912ULS in my plane, about twelve years old and 750 hours. My problem is RPM drop on "mag" check. At 4000RPM Ihave about 50-75RPM drop each side with smooth running each side. At 3500RPM the same.

However at 3000RPM and below one side is good, 50-75 RPM drop and smooth on one side but the other side is 200-300RPM with very rough running.

If I'm running on the "bad" side only as the throttle is advanced through 3000RPM the rough running will even out and the engine will "accelerate" and run smoothly with no difference compared to the "good" side.

Is this likely to be a trigger coil setting issue? We disconnected the coils several months ago when the sprag clutch was done to avoid possible physical damage to them when pulling the flywheel off. Iran the engine for about five minutes on the "bad" side at about 2500RPM and all cylinders seemed to be equally hot (to feel, I don't have EGT sensors) but it is pretty rough down there.

The modules were replaced about a 100 hours ago.

Thanks Peter

  • Re: RPM drop on ignition check

    by » 6 years ago

    It is a little odd that it smooths out and rough at only one spot. I seriously doubt it's the modules or the coils. Typically a 200-300 drop is a spark plug or a poor connection in the spark plug boot. Try changing the spark plugs whether you think they are good or not. Then you can unscrew each plug boot and trim each wire end back 1/4" - 3/8" and then screw the boot back on ALL THE WAY. Then try your check. Most issues with a Rotax are not exotic problems, but simple and common. Always try cheap, easy and most common causes first. Once you change the plugs try the mag check first before you trim the wire ends.

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

      
    Report

  • Re: RPM drop on ignition check

    by » 6 years ago

    Hi thanks Roger Ill change out the plugs when I get back to the plane tomorrow.

    Peter
      
    Report

  • Re: RPM drop on ignition check

    by » 6 years ago

    Peter, if you suspect the ignition timing (trigger coils) then remove the plastic cover from the flywheel end of the engine, set the engine to top dead centre and paint a white mark on the flywheel so that it aligns with some easily visible datum at the top of the crankcase. Then use an inductive pick-up timing light on one of the plugs driven by the problem circuit and run the engine. Run it only on the bad ignition circuit and see if the position of the white mark stays in the same place at 3000rpm and 4000rpm. If it doesn't, then you know that you have a timing issue.
      
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  • Re: RPM drop on ignition check

    by » 6 years ago

    Hello everyone. Got time to work on the plane today. Eliminated plugs as a cause, trimmed leads as suggested. No change, symptoms as above. My very knowledgeable friend suggested we test leads with a timing light, did so and found a dead circuit on front R/H top plug at 2000RPM.

    The drop in RPM more like 400 rather than the 2-300 I mentioned above. So thinking that I could have a bad lead that works with sufficient RPM to overcome the fault. Would this be more likely than the whole coil going sour?

    Thanks Peter.
      
    Report

  • Re: RPM drop on ignition check

    by » 6 years ago

    Did you replace the plugs? That’s the only way to rule them out. If it is only 1 plug then it shouldn’t be the coil because each coil fires two plugs and not just one and all the plugs fire on each stroke. Not like a standard 4 stroke. Test the plug wire from the coil to that plug if it has a new plug in place.

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

      
    Report
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