Pilot Jet 35

The #35 jet orifice being .5 mm in diameter is surprising, I had not seen that in the manual. Everything I've read about Bing jets is that the jet number indicates the orifice diameter in 100ths of a millimeter. Now I want to measure one!  

Surprise!  It's not a Bing jet.  :)  From experience I can tell you 100% that changing the restrictor size will change fuel pressure on an 893115 Rotax fuel pump.  When they were first released, we did a bunch of testing on a test rig running the engine with and without the restrictor port active.  We tested about 15 pumps this way.  Leave the orifice size alone. 

Do an experiment on the ground.  Run the engine at 5000 rpm with the restrictor flowing - measure fuel pressure.  Block off the return line and run it again at 5000rpm.  That will tell you.  I would NOT modify the return orifice size in any way.  Just my .02.   

Lots of good information has already been shared in this forum post by others, however here is an excellent article that you should also read.

https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/fuel-systems/basic-fuel-pump-system?fbclid=IwAR0VhCfjjubp87R_VBu7k0GTR6EObQk9G56o9EfFZF6TaN3jAE8Nk9pVKU0

Here is a link to a write-up we released regarding the fuel return line and vapor lock https://www.rotax-owner.com/en/flightsafety/476-ai476

Rotaxtech;

"Surprise!  It's not a Bing jet.  :)"

The brand/maker of my fuel return restrictor/jet is of little import. What matters here is the orifice size & resultant flow rate.

"we did a bunch of testing" 

It would have been helpful if you gave the range of fuel return rates.I have yet to determine what size jet is in my fuel supply. I have however determined the amount of fuel being returned is well over 7L/ hr(Hobbs)  .

"Block off the return line ............"

No intention of blocking/removing the restrictor - I understand its function and would not wish to unnecessarily increase the chances of fuel vaporization.

"I would NOT modify the return orifice size......."

Why not?

If, as has been suggested, my known fuel return flow rate (7+L/hr) may be compromising my full power fuel demand (around 28-30L/hr). The combination (35-37L/hr demand on pump) may render the fuel pump supply of 35L/hr in question. A reduction in return line jet size seems to be the logical solution.

I am well aware that a reduced return line flow rate, has the potential (due to slower fuel flow) to raise the fuel temperature, thereby increase the chance of fuel vaporisation. This must be weighed against the potential for the pump to deliver adequate fuel (at full power) to the engine.

This also raises the question; - is there any reduction in pump efficiency over its 5 year service life? (my pump/engine is new) thereby making, what is now, a minor problem a lot more concerning.

I think what some folks are missing here is that this thread is really a continuation of Sean’s earlier thread, where he states he has low fuel pressure in his Sonex, but only during climb out. He does not know what restrictor orifice size is in the aircraft (or if it is the correct one), but he has determined the return flow is >7 L/hour.  He is trying to determine if the return flow is too high, thus compromising fuel pressure at full engine load.  So the questions are:  

1. What is the recommended return flow ( I believe it’s 2-3 L/hour)?

 2. What size orifice will accomplish that (.35mm or .5mm)?

Here is his previous thread:

https://www.rotax-owner.com/en/912-914-technical-questions/9609-fuel-pressure-mystery