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My new to me 912UL has individual KN filters.  Plane is Apollofox and I have logged 80 hours in 9 weeks, which is much more than usual for me and maybe the reason for an urge to fine tune and tweak. Engine was running less than perfectly smooth I thought, and coming from motorcycles I have seen how going from airbox to KN filter pods can throw off fuelling especially with CV carbs.   Generally larger main jets may be required and/or moving the needle clip to one position richer.

So thats what I tried on my 912 too and what I found was my engine runs much smoother in the midrange after moving the needle clips from the Rotax Manual specified 3rd notch (next richest), to 4th position (richest).   With the richer mixture it is now more responsive when manipulating throttle on short field landings and stronger too. This is the biggest change.  But it appears to run stronger also on top of the RPM range and I had to re-pitch the prop as WOT RPM went from 5800 to 6000. Spark plugs looks great, and not sooty, but exhaust tip colour is dark brown almost black.  I have not noticed any change in fuel consumption, if anything it is better now as engine runs more correct now.

Further plan now is to weld in a bung for wide band lambda (oxygen sensor) in the exhaust and see how fuelling look on the AFR gauge. Could be the 185 main jet is too small possibly, but I will not play with it without AFR gauge

Any comments or advice ?

  • Re: Carb settings without an airbox

    by » 2 weeks ago


    I would be interested to see what data you get from your lambda sensor at different engine speeds. Please let us know what you find.


  • Re: Carb settings without an airbox

    by » 2 weeks ago


    Did you do a manual and pneumatic carb synch before moving needle circlips? 


  • Re: Carb settings without an airbox

    by » 2 weeks ago


    If you install an Oxygen sensor, you will need to avoid 100LL gas and only use Unleaded MoGas or the newer UL Avgas.
    This is why Rotax engines do not have O2 sensors in the first place.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


  • Re: Carb settings without an airbox

    by » one week ago


    yes I did sync the carburettors, both before and after moving clip. Several times.   And only using unleaded mogas,

    I will need to remove muffler for welding in the bung but hesitate to cut the nicely installed securing wires inside the springs that also have a bead of silicone.   I assume for damping vibration.  Or I could take out the oil tank to get space on hoses replacement. 

    Motivated on improving after experiencing another fellow in the hangar who just purchased a plane with a 912ULS equipped with Edge fuel injection. Never knew a 912 could run that sweet. 


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