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Thread: Rotax 912 runs rough

  1. #11
    Super Moderator Av8r3400's Avatar
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    Default Re: Rotax 912 runs rough

    Jim, What are you running for spark plug gap?
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  2. #12
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    Default Re: Rotax 912 runs rough

    Quote Originally Posted by Av8r3400 View Post
    Jim, What are you running for spark plug gap?
    Tried for .022, that is what the minimum in the manual is I believe. Just to double check myself, I tried .025 on each and it wouldn't fit. My feeler gauge has a .010, then .012, then .015 in a row, so that's why I did it the way I did. Hope that made sense. JImChuk

  3. #13
    Super Moderator Av8r3400's Avatar
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    Default Re: Rotax 912 runs rough

    I start with .020-.024" (.5-.6mm). I believe that Rotax is more than a little ambitious with their gap recommendation (.6-.7mm). My engine runs and starts much better with a tighter gap.

    The other thing is to check the CDI harness grounds.
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  4. #14
    Senior Member Dave S's Avatar
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    Default Re: Rotax 912 runs rough

    I believe Rotax has also recommended tighter plug gaps where cold weather starting may be an issue.

    Roger that on the CDI grounds for sure. Internal breaks on a wire or near a wire crimp can cause intermittent continuity and may be affected by vibration at one time/rpm or another. Don't know that in this case but not beyond possibilities. The earlier kitfoxes with reversed manifolds also creates a situation of modified CDI ground wiring that may be subject to vibrations and stress in a way the original rotax ground connections don't.
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  5. #15
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    Default Re: Rotax 912 runs rough

    Not sure if I found the problem or not, but decided to see if I could check the trigger coil gaps. I was just barely able to remove the oil tank from behind the engine and that gives me some access to the trigger coils. But the expression "10 lbs of crap in a 5 lb bag sure applies". I sure didn't want to have to pull the engine, especially if it wasn't the problem. Anyway, here is what I found. The little round pickups on the the trigger coils were all rusty. I'm guessing that would have some effect. I don't know if they rusted since I installed the engine, I kind of don't think that is the case. Engine had sat for quite a while. But at any rate, I'll clean them up with some 400 grit sand paper, and try to check the gap. Hope this was the problem. Getting tired of wrenching instead of flying. JImChuk

  6. #16
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    Default Re: Rotax 912 runs rough

    Bit of an update, although the jury is not quite in yet. I did try to check the trigger coil gaps, pretty near impossible in that small space and my big hands. I did find out that .015" feeler gauge wouldn't go through on the ones I was able to check, so they don't seem to be real wide. I did sand the small round pickup points, and that was quite a chore as well, but they are cleaner then they were. Got it back together today, and fired it up. I didn't have the cowl on, so it didn't get as hot as it probably will, but at 3850 rpm, (which my manual says it the rpm for a mag check) I saw about 150 rpm drop on the side that was dropping close to 500 before. The other side seemed to be about 160-170 drop., It was the better side before. Tomorrow when I get a chance, I'll reinstall the cowl, and get it warmed up more, it seemed like it ran rougher once it was fully warm. Another change was when I put the choke on at fast idle, it didn't speed up like it did last time, and after a bit it started to stumble like it was choking out. Sure hope I got it, I'm kind of feeling like I found the problem (with a bunch of help from the guys on this forum) So thanks again for the comments and advice. Stay tuned.... :-) JImChuk

  7. #17
    Senior Member Dave S's Avatar
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    Default Re: Rotax 912 runs rough

    Jim,

    Thank you so much for sharing what you have found. Information like this is likely to be important to others down the road.

    Keep us informed on what the final analysis shows.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

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