Pilot Jet 35

My "by-pass" is delivering over 7 L/hr (Hobbs) back to the fuel tank - is this a normal flow rate?

I believe the target flow rate for the return line on the 912 ULS is 2-3 L/hour.  If you calculate the flow rate using a #35 (.35mm ID) jet at 3.5 PSI you come up with a number very close to that.  You say you have a return flow rate of 7 L/hour.  The mechanical fuel pump is rated at 35 L/hour, so if you burn 28 L/hour on climb, plus the 7 L/hour through the return line, your are right there.  You may be on to something. 

It’s my understanding that when Rotax refers to a #35 jet as a restrictor, that means the jet is .35mm ID.  In the motorcycle world, Mikuni created some confusion by rating their jets differently. The jet ID number was based on the flow rate - a 35 jet would flow 35 cc/minute at full throttle.  This lead to an estimation that a Mikuni Jet ID was roughly twice that of other jets with the same number.  In other words a Mikuni #35 jet had an approximate ID of .7 mm.  I have read on aircraft builders forums that some folks were drilling their home-built restrictor at .7mm based on this information.

What is interesting is that if you calculate the return flow with a .7mm ID restrictor, you get about 7.5 L/hour, or about what you are observing.

Wow! 

Thanks for that Jeff.

I will be working on my aircraft later today -  will add measuring the ID of return line restrictor, to the list of jobs.

For reference, Idle Jet #35 is referring to  Rotax PN. 963143

Thanks again Bill - Idle/Slow Jet # 35 seems to be available from a number of sources.

Does Rotax give dimensions/performance for their version of #35?

Its quite a hot day here (35C) so I will be working on my aircraft in the cool of late afternoon/early evening and will measure the ID of the jet installed. Any change will depend on that measurement.