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912 miss under 2500 perfect over

in 912 / 914 Technical Questions 7 years ago
Well we are at a loss so far so turning to the experts. Have a 912ul with 160hrs #1 cylinder not running under 2500rpm, runs fine 2500rpm and up. Not running per shacking badly and able to hold exhaust pipe (laser thermo checked to just above ambient). If only ran below 2500 can pull plugs and they are wet and smell of fuel. Plugs pulled while running under 2500 have good spark. No change on either ignition above or under 2500. Head doesn't leak any fuel past valves, valves and seats look good. Cylinder and piston/rings look good. Rockers appear to be moving correctly and full travel. Cam lobes look new, push rods look good and are straight. Have tried some other things and tried thinking through possible scenarios, so far no luck. Haven't been able to find any other similar problems searching the internet. Any ideas or possibly similar problems?

Thanks, Ben

  • Re: 912 miss under 2500 perfect over

    by » 7 years ago

    How about a mag drop check at let's say 2300 rpm (in the bad rpm range) and then again at 3500 rpm.
    If your engine has soft start disconnect this from the solenoid and try running it without soft start. Just had an issue similar to this.
    How is the compression test on #1?
    If this doesn't work have you swapped ignition modules with someone and then run it?

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

      
    Report

  • Re: 912 miss under 2500 perfect over

    by » 7 years ago

    Have check ignitions at different rpm above and below 2500, no change. It doesn't have soft start. I have swapped the boxes that are there, will have to find someone willing to let me swap both boxes. Seems odd both boxes would have that issue. Was thinking timing was off, but since it fires twice and two cylinders, the opposite side is fine, not real sure. Got 140psi on #1 but am gonna do a leak down as well just to double check for leaks.
      
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  • Re: 912 miss under 2500 perfect over

    by » 7 years ago

    Swapped wires and coils/ignitions today. Checked all jets etc for carbs. Thinking it's in the carburetion. Believe the 1/3 carb is flooding up to the 2500. Odd but if I suck on the bowl vent tube it seems to smooth out nicely? Floats are floating and fuel height appears to be correct, what else will cause a rich situation up to 2500ish rpm? Enricher? Best check/test method? Thinking to stop up enricher draw straw from bowl.
      
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  • Re: 912 miss under 2500 perfect over

    by » 7 years ago

    One reason I don't have the carb very high on the list is you say it only affects the #1 cyl. The carb should be affecting #1 & 3.
    Have you replaced the #1 plugs? Run the engine up at 3500 for about 5 minutes and then immediately shut it down and see if the #1 plugs are still black and wet.
    You did a differential leak down compression test?

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

      
    Report

  • Re: 912 miss under 2500 perfect over

    by » 7 years ago

    I understand where your coming from, since #3 is good at all rpm on the same carb. But I've checked all over #1 mechanically and can find absolutely nothing. Tried all new plugs, and swapped #1 plugs with another. And pulling the plugs after running anything over 2500 (smooth range) give light brown plugs vs the usual wet black. Leak down gave good numbers on all cylinders #1 being second lowest none less than 80/100. It's possible my testing method is not correct giving inaccurate conclusions so feel free to question anything. Have had several mechanically inclined folks working with me, so far everyone is scratching the heads...
      
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