fbpx

 

I have a engine stumble that I intermittently experience at WOT both on the ground and in the air.

The engine is a 912ULS in an RV12, I'm in phase I on this plane, meaning everything is brand new and I'm flight testing, I saw this issue in the first flight and every flight since bar one.

I have taken the carbs off, taken the top off them and cleaned every orifice with both carb cleaner and then compressed air. I have completely redone the carb and sync setup every single step, the carbs show good balance from idle through WOT.

One other strange issue is that when I start my rollout my RPM is 5170 and by the time I complete my climbout it is at 5050 (yes it is decreasing even though the prop should be unloading as airspeed increases), I promise my throttle setting is hard against the firewall no change. Here is that in savvy




when the stumble occurs I'm getting a spread in my EGT of about 150F higher on the left and I have confirmed that is indicating the correct sensor and side



I have captured the stumble on the ground in a video, I swap between instruments and my carb sync tool is still attached, doesn't seem to go out of sync, the EGTs separate and RPM wanders

you tube video

I did talk to Roger Lee about this, he had suggested cleaning out the carbs, which I did and re-do on the mechanical and pneumatic sync.
  • Re: engine stumble at WOT

    by » 3 years ago


    So cleaning the carbs had no effect?

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-574-1080 Home (TRY HOME FIRST)
    520-349-7056 Cell

  • Re: engine stumble at WOT

    by » 3 years ago


    Do you have an Auxillary Electric Fuel Pump?
    If so, Pull the fuel line off each carb.
    Point the open hose into a clean container.
    The pump should be able to deliver a gallon of fuel in 2 minutes, and no more than 3.

    I would look at the fuel supply next.
    Check your fuel filters.
    Brand new tanks and hoses are notorious for construction debris.
    Don't forget the filter screen in the mechanical fuel pump.

    Do not overlook the fuel tank vents.
    It is hard to get fuel out unless you can get air in.
    This condition will get worse the longer the engine runs.

    Keep an eye on your Oil Pressure.
    72psi at 214°F is pushing the limit.

    P.S. Next time, Turn the camera.
    Always record Video in Landscape format.
    Unless you play it back on a handheld device, it doesn't fit the screen.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

  • Re: engine stumble at WOT

    by » 3 years ago


    i do have an aux pump its alway on. I did already try that test and passed....

    I checked my gascolator screen and cleaned that (it was clear).

    Is there a screen in the mechanical rotax pump ?

    I checked the vents and confirmed they are good and I checked the return line

    ha I will sorry on the camera !

    why is the oil pressure so high ? I'm running aeroshell

  • Re: engine stumble at WOT

    by » 3 years ago


    Did you pump the full gallon? The problem may not reveal itself unless you keep the flow going for a good period of time.
    10 seconds of, "It looks good to me!" is not a valid test. 3-5 gallons is even better.
    If you truly can deliver a gallon right at the carbs in under 2-3 minutes, The fuel lines are clear.

    What is the BluishGrey line on the RPM Chart that starts to fade after a few minutes?

    Fuels + Air + Spark + Compression = Power
    What is lacking???

    Is the Air Filter clear?
    Is should be light Pink.
    Dark is dirty.
    Bright RED is over Oiled.

    You might try flattening the Prop Pitch 5100rpm is overloading the engine at WOT.

    What Weight is the OIL? AeroShell WHAT?

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

  • Re: engine stumble at WOT

    by » 3 years ago


    Next to check would be the spark plugs.
    Check for cracked insulators, unusual coloring, and the gap size.

    Room Pressure Air will spark at ~1000 volts per Millimeter.
    The voltage required to spark is roughly proportional to the air pressure.
    A 10:1 compression engine is going to need a minimum of ~10,000 volts per mm of spark plug gap to fire.
    That is why spark coils typically deliver 25,000 volts or more to jump a relatively small gap.
    The compressed air/fuel mixture makes a pretty good insulator.
    More is NOT better is this situation.
    A larger gap does not necessarily produce a larger spark, it produces a weaker spark if any at all.

    The lower pressures at lower throttle settings are easier to spark.
    WOT is where the spark is most likely to misfire.
    Given the opportunity to bypass the spark plug's spark gap, the spark will take the path of least resistance.
    Are the Plug Connectors well attached?

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

You do not have permissions to reply to this topic.