Fuel Orifice

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. 

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.

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.