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I have posted about this before and I thought that I was past the issues, but they seem to have started again. At full throttle I occasionally get an engine stumble, this happened again today twice just after take off and led me to abort the flight. This is a brand new mid 2016 build 912 ULS (16 hours)

I have done the following

- taken both carbs completely apart, cleaned with air and carb cleaner
- checked all my fuel lines, at each connection I have run a volume of fuel over time test and I have run several gallons through the lines
- checked my fuel bowls for any contamination several times
- re-synchronized my carbs both mechanically and pneumatically.

The problem had all but disappeared until I experienced it at alititude (9500') on a trip to Payson recently. I confirmed my engine was on the float exchange service bulletin and I swapped all my floats out (one side was 7.7g). But after the float exchange problem seemed to get worse...

As you can see in the image below, when the issue happens its preceeded by the left EGT going 100F hotter than the right (this always happens when you look at the logs).

I also should point out that my static WOT RPM and my climb RPM are pretty much the same, and in climb I seem to lose 50 or so RPM as I climb out, that to me is wrong, but I cant seem to trace any cause and my CHTs and EGTs are cool...

At this stage the only thing I can think of doing is trying to swap out carbs, that it is some strange carb related issue... Can anyone help ? Please :) I have to say this has totally sapped my enjoyment of an otherwise perfect plane.

  • Re: Plagued by engine isses 912ULS

    by » 7 years ago


    ...
    As you can see in the image below, when the issue happens its preceded by the left EGT going 100F hotter than the right (this always happens when you look at the logs)...


    I see the RPM stumble FOLLOWED by the EGT going Hot.
    The EGT peaks at the END of the event.

    This indicates to me that the cylinder goes lean and starts to miss.
    The EGT climbs as a result of the Lean condition.
    Something is Stuck/Blocked and the rough running knocks it loose only to return.
    It may not necessarily be debris in the fuel circuits.
    If the Carb Piston/Diaphragm occasionally binds, The results might be similar.

    Does this cycle repeat every 40 seconds?
    It would have been better if you had not cycled the throttle after the second event.
    I would like to see 3-5 stumbles to see if there is a pattern.

    Just because you feel you have cleaned the carburetor does not mean that they are clean.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


  • Re: Plagued by engine isses 912ULS

    by » 7 years ago


    If there is something still in that carb it sure its stubborn !! I'll take the left side apart again and use a magnifying glass this week.

    It doesn't seem to cycle, here is another log of if happening, this is on the ground.

    how can I validate that there is no binding of the piston diaphragm ? I assume that even at a constant WOT setting the diaphragm is moving as small changes in load etc occur ?

    I'm at the point where I'm just thinking of replacing the carb with a new one, as this has consumed so much time.

    https://photos.google.com/photo/AF1QipNxmbgx1LBzZx20lLP_cUB9hLh3Mg9trbXGm8GQ

  • Re: Plagued by engine isses 912ULS

    by » 7 years ago


    At this point, you do not know what the problem is or what to repair or replace.
    Leaning could;d be caused a shortage of fuel or an excess of air.
    If you feel you have exhausted the fuel side of the equation, start looking at the air side.
    A small crack in the Carburetor Coupling could be the problem.
    You could replace the carburetor only to find the problem still exists.

    Find the problem rather than blindly replacing parts.
    "Shotgunning" is an expensive way to troubleshoot!

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


  • Re: Plagued by engine isses 912ULS

    by » 7 years ago


    I had a similar issue in 2016. I cleaned and balanced carbs, replaced fuel lines and cleaned the fuel distribution manifold, and replaced throttle cables and springs. I had a full tank of 91 octane no ethanol Conoco gas. As a last resort I dumped the gas (I put it in my car) and refilled with 100LL. Problem went away. That was spring 2016. The exact same thing happened spring of 2017. That time I just drained the tank and refilled with 100LL again. Problem went away again. I am speculating it was bad gas couple with Conoco changing from winter to summer blend. Since then I run half car gas and half 100LL. No more problems.

  • Re: Plagued by engine isses 912ULS

    by » 7 years ago


    It sure looks like the fuel (Mogas vs. 100LL) may be the issue. I run 92 octane clear Mogas all the time, but I always filter it as it goes into my tanks unless it is purchased on-airport. Airport tanks are generally much cleaner than service station tanks.

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