ROTAX 912 low fuel pressure at take off

Just ordered Mikuni VM22/210 type, slow idle jet #15.

Will let you know what effect it has on my fuel system.

FYI:

The attached chart purports to show jet ID dimensions:

https://www.allensperformance.co.uk/carb-kit/jet-size-chart/

Rotax supply a ready made fuel distribution manifold - the restrictor jet(s) on this manifold have been measured at approximately 0.35mm

My Rotax supplied restrictor jet 35 measures 0.53mm, delivering 7+L/hr 

Yes I know that jet diameter is but one factor in flow characteristics but in the absence of other data, this is all I have to go on.

So I am going with #15 (0.335mm) from the chart - translated into an order for Mikuni VM22/210 type, slow jet #15 in the hope it will deliver about 3L/hr

Hi Sean,

As I stated earlier, Mikuni sizes their jets by flow, while for most others the jet number equals 100ths of a millimeter.  It’s interesting you found out the Rotax factory built fuel restrictor is .35mm, I suspected that.  Calculations indicate this should give you around 3 L/hour.  I doubt that Rotax designates exactly what #35 jets distributors sell, and perhaps you are seeing what happens when a distributor selects a Mikuni #35 jet as their offering.  Most people probably think all #35 jets are the same.  It will be interesting to hear if this solves your problem, if so this may explain a long standing issues for others.  

Hi Jeff,

The jet I measured was supplied by my Rotax dealership and has a Rotax PN 963820 "PILOT JET 35". The jet in the aircraft was also supplied by the same Rotax dealership

It seems odd that the Rotax jet (0,53mm) has a very different ID to the Rotax fuel manifold (0.35mm)

If you are certain that the orifice in the Rotax fuel manifold is .35mm, then part number or not, the .53 jet seems suspect.  That being said, the length and the shape of the entry to the orifice also play a role in its flow rate.  It would be telling if you could compare the flow rates of the Rotax manifold to a restrictor using your #35 jet.  I am sure Rotax does not manufacture the #35 jet, so a Rotax part number may end up getting put on whatever part they buy out.