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  • Re: Fuel Orifice

    by » 6 years ago


    Many shorten the bottom end off the jet. Then solder the jet in a brass hose fitting.

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


  • Re: Fuel Orifice

    by » 6 years ago


    Thanks for the tips. I have a #35 jet coming, I already have the 5mmx0.8 Tap. I found a good picture on Skytek Aircraft's site will use it as a reference.


  • Re: Fuel Orifice

    by » 4 years ago


    Hi Bill,

    The manufacturer of my SLSA (with the 912is engine) did not provide this vapor bypass circuit but has provided me with an LOA to add it. I have read the Rotax 912is install manual and they intentionally don’t define the restrictor orifice size or optimum flow rate. Using an online flow rate calculator I determined I should get about 3-GPH at 45 PSI through a .35mm jet, but I was wondering how you came up with the recommended flow rate of 3-GPH? That’s about 10% of the total return flow so it makes sense, but I’m interested where this number comes from. 


  • Re: Fuel Orifice

    by » 4 years ago


    The size was likely determined Empirically. ( A Fancy term for Trial and Error.)

    My DIY Orifice delivers 3-5 GPH which looks like a healthy dribble to the eye.  Nothing exciting.

    More significant is that any fuel vapor (Mostly Butane Gas) reaching the same orifice will pass through it at ~1 Gallon per Minute, restoring the prime to the pumps in a matter of seconds.


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


    Thank you said by: Drew Broussard

  • Re: Fuel Orifice

    by » 4 years ago


    Thanks for the information Bill, I used the Andair Nozzle with a .35mm jet and expect good results.  Just for fun I may do an actual test of the bypass flow rate to see how it compares to the calculated value.  


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