912 UL Low Fuel Pressure and Power Loss

Thank you for all your suggestions. 

The mechanical fuel pump was replaced, therefore I don't think that the problem lies there. 

@Rotax Wizard: Does the routing of the carburetor vent lines affect the fuel pressure? I thought it only affects the mixture ratio

@Sean Griffin: That indead sounds very similar to my problem. However, when I do nothing, the pressure stays low and after around 10 seconds, the engine starts sputtering.
I have very large cooling openings in the cowling, I don't think that it gets to hot in there...however, I have not meassured it.

I am thinking about reducin the return orifice to #30...

Sounds like you might have restricted flow before the pump or perhaps a vacuum leak before the pump. What size fuel lines do you have and are there many tight bends?

I would be inclined to run a fuel flow test to the pump.

Did you use teflon tape on any of the fittings? Being a new build, it is possible to have contaminants in the fuel lines restricting flow.

Hi Dan,

"@Sean Griffin: That indead sounds very similar to my problem. However, when I do nothing, the pressure stays low and after around 10 seconds, the engine starts sputtering.
I have very large cooling openings in the cowling, I don't think that it gets to hot in there...however, I have not meassured it."

Just remember, exit air will control , to a large extent, cowling through flow. Enlarging entry air may not improve ventilation.

I am thinking about reducin the return orifice to #30...

Reducing the  restrictor jet size will, improve fuel line pressure but will also impact on the return line efficiency for removing fuel vapour/keeping your fuel cool. 

Check out the RV 12 fuel problem/fix using a higher capacity Facet pump. This will not only improve pressure/volume but will preserve the return line scavenging performance. (Rotax have accepted this fix.😈