New 912 Won’t Start.

Installed new 912 in Zenith 750 in December 2021. Frist started engine in February 2022 to verify connections. Ran engine about 5 minutes and 120F. No Problems

Engine and plane set about 4 months in workshop in South Texas. Engine had residual fuel. South Texas summer temperatures exceeded 100F often.

Moved to a hanger in July. Tried to start engine a second time.  Engine wont start, not even an occasional POP.

Could it be vapor lock? Or evaporated fuel varnish in both carburetors? Or something else?

·         New Rotax 912 ULS – 2

·         Fuel: 91 Octane, mogas with 10% ethanol

·         Bing CD carburetors

·         Indications on EMS, while turning starter:

         Oil Pressure 65 psi

                Fuel flow 1.7 GPH

                  CHT  about 95F

                 EGT about 100F

  • Re: New 912 Won't Start

    by » 8 months ago

    Do you have fresh fuel in the tanks? First you may want to drop the carb bowls and clean the idle jet . There is a small jet in one corner of the bowl. The other one is a hole right next to the main jet that you see hanging down. Take a starlight tip screwdriver and stick it up in the hole and unscrew the idle jet. Then take about 8" of #18-20 ga. wire and strip back about 5" of the insulation. Then undo one single strand. Take this and run it down through the center of the idle jet. Slide it back and forth and turn it in circles as you do this. Then flush it out with some carb cleaner from a spray can then blow it out with some high pressure air. Put this jet back in place. Put the bows back on and make sure that the rubber bowl gasket seats in the carb bowl groove and you don't leave it out and pinch it. I don't know what you idle rpm is, but this time full choke and crack the throttle. If you have an electric pump turn it on. If it tries to start hold the key until it fully starts. Don't turn lose of the key too early. Don't worry as the starter will automatically disengage when it starts.


    Last thought. One that first start did you do a mechanical sync on the carbs? If it does start and it runs for a while don't forget to do a pneumatic carb sync.



    DO NOT use a drill bit or welding rod tip cleaner on the idle jet.

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

  • Re: New 912 Won't Start

    by » 8 months ago

    I find that I can only start my (cold) engine with the throttle fully closed and the choke pulled fully. If I have the throttle open even a small amount I get nothing. It's worth then just checking the basics:

    1. If you pull the prop around can you feel the compression?

    2. Pull the top spark plugs on each cylinder and see if there is evidence of any fuel getting to the cylinders.

    3. If there is then ground the plugs on the crankcase and crank the engine to see if you are getting a good spark.

    Let us know what you find.

  • Re: New 912 Won't Start

    by » 8 months ago


    "I find that I can only start my (cold) engine with the throttle fully closed and the choke pulled fully."

    Closing (activating) the choke, causing an over rich fuel to air mixture, is undermined by opening (activating) the throttle. This is normal.

    Some engines (fuel metering systems) will tolerate some throttle opening better than others.

    Air & engine temperature also play a big part in engine start performance and again engines will vary in their response to these conditions. 

    An extreme example of how the choke & throttle counteract is;

    If you "flood" the engine (so much fuel that there is insufficient air/oxygen to allow ignition) - the correcting action is; Open the choke fully - Open the throttle fully - Crank engine until ignition - Be quick to close throttle, as engine RPM will rise fast.

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