I have been working on this engine off and on for the past year, attempting to rectify this problem.  I have been on the rotax owners website with this problem, without getting enough information to help.  I am using the Rotax 912 ULS Line and Heavy M/M’s to ensure the work is performed correctly.  My troubleshooting skills are fairly good, this is not my first rodeo with the Rotax engines, and I have been thru the Rotax 912/914 series classes thru heavy maintenance and renewal and have worked on several Rotax 912’s.  I purchased two complete carburetor overhaul kits from California Power Sports and installed them.  The engine runs very similar to how it ran before I did the overhauls.  I did install a new fuel pump before getting into the carbs as the removed fuel pump pressure was low at high output around 2 psi while at 6 psi at idle and midrange.  All fuel lines are new, the old fuel was drained, fuel tank cleaned, fuel shutoff valve overhauled etc.  I installed new carburetor choke cables from actuator to the L/R chokes.  The engine runs fine from idle up to about 4900 to 5100 rpm, with smooth acceleration.  At the 4900 to 5100 rpm point it begins to bog and runs slightly rough and will not accelerate smoothly again until around 5300 rpm.  During the bogged down time, the throttle control stick will move approximately 4 inches of travel (1/3rd) of it’s total travel before the rpm picks up and the engine runs smoothly again up to 5500rpm.  The engine throttle arms and air valve move smoothly throughout the range from idle to full open.  Any thoughts?  References to other resources? 

Art Granger AP2115976  (435) 272-9999 Hurricane, Ut.

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8204_1_Mike Goochs Powered Parachute Dream Machine Replace choke cables and rebuild carburetors 08-25-2020.docx (You do not have access to download this file.)
  • Re: 912 ULS rough running thru rpm range 4700 to 5100

    by » 6 weeks ago

    There are different types of "bogs."


    - Lean bog: too much air, not enough fuel. 

    - Rich bog: too much fuel. 


    After the engine is warmed up, try pulling the choke (enrichener) in the "bog" range and see if the problem gets worse or better. Those tests will tell you if it's a lean or rich condition that you're dealing with. Once you can identify the type of "BOG" you have, you can then go from there. 


    - If you have a "Lean" Bog, you might have an induction leak.


     - If you have a Rich Bog, then you might have a jetting issue. I think you said, you installed the new fuel pump? Do you have the metered orifice installed on the fuel return line?



  • Re: 912 ULS rough running thru rpm range 4700 to 5100

    by » 5 weeks ago


    clean,balance carbs,

    check plug caps shielding wire not entering cap as screwed in to wire!


  • Re: 912 ULS rough running thru rpm range 4700 to 5100

    by » 5 weeks ago

    Sounds lean. Try putting the needles on #3 position (or #4 if they are already on #3) and see if the richer mixture helps. Make sure the slides are aligned and move freely, once the caps have been tightened.

    (The choke acting like a mixture control trick was debunked many years ago by the #1 Conrad Beale. Try it for yourself, run the engine at some midrange RPM, slowly pull the choke until you get the max rise in RPM. Write down that RPM & EGT readings. Now push the choke in and advance the throttle to that same RPM you got with the choke partially pulled... are the EGTs the same as you wrote down? If so, the choke only increased the RPM and did not lean it out)


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