ROTAX 912 low fuel pressure at take off

Jeff Blakeslee wrote:

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.  

I am on two other Forums , where Rotax (& other engines) powered aircraft are discussed. One of the contributors kindly measured his Rotax fuel distribution manifold. (he also provided a photo showing the part & restricted outlets) It has two restricted outlets, both 0.35mm. One for fuel return, the other for fuel pressure sensor attachment (see photo).

As for the Rotax supplied restrictor jet (0.53mm) - I measured the one in my aircraft, that came with the partially completed project. The origional builder told me he acquired it from Rotax (even naming the sales agent). I ordered  a new jet, in the hope it might have a lower flow rate. The jet supplied, again measured by me, has exactly the same ID.

Yes! Rotax are supplied with components from many manufacturers and the #35 jet looks very much like a Mikuni VMK22/210 type (see photo). The thread diameter may be a point of difference - Rotax supplied one being 4.8mm x .8 pitch.

Update;

Installed the smaller Mikuni jet 15 (perfect fit) in place of the origional Rotax supplied one. 

New jet aperture  measure - approximately 0.075mm, compared with Rotax 0.52mm

Flew today - definite improvement in TO/Climb Out fuel pressure (minimum 3psi with boost pump) and in high speed Cruise 5000ft 3psi (no boost).

Still not as high as I would expect, so may be those that theorised, a restricted fuel supply (going through boost pump) may be correct.

Flow rate for the new fuel return jet (Mikuni 15 replacing the Mikuni 35 @ 7.06L/hr) is about 3.75 L/hr. As time & further measurements are made, I may revise this flow rate a little.

Update on Mikuni 15, (VMK 22/210 slow idle jet) restrictor,  return fuel rate.

Over several tests/hrs the flow rate would now seem to be approximately 5.25L/hr (Hobbs).

Flow rate, not a low as I had hoped (target 3-4L/hr) but will suffice, for now.