914 Low Power on Takeoff Attempt

I’m no expert but seems fuel pressure is dropping off 

do you cycle the main and aux pump during runup to see if one drops ?

Yes, I test both pumps individually during the runup.

I notice that both fuel pressure and MAP (Airbox really) drop unexpectedly in the middle of the runup and when pushing the throttle forward for the takeoff.

I think the airbox pressure drives the fuel pressure, so I think that's the root cause (but, I'm not certain).

Craig

Hi Craig,

Short of hearing from a real expert, I offer this insight.  

Your fuel pressure sender looks to be connected after the regulator, which is correct. However, it appears to be referencing ambient pressure rather than airbox pressure. This makes the information less useful, or at least more complex to analyze. What is important is that the fuel pressure to the carburetors is maintained at approximately 3.6 PSI (7.3 hG) above airbox pressure. If your fuel pressure sender was a differential type and referencing airbox pressure (and installed after the regulator) than you would expect to see fuel pressure remaining steady at about 3.6 PSI. You might consider making this change.  

That said, your fuel pressure seems to be tracking manifold pressure correctly. There are a couple points where the fuel pressure seems to be the same as manifold pressure, but that is just an effect of the fuel pressure units being PSI and and the manifold pressure being hG, and how that scales on the graph.  When you do the math at that point on the graph you do have sufficient fuel pressure to overcome airbox pressure. 

You have not said the engine was running rough, so I assume that what you are describing is the engine running smoothly (normal) but not creating full power.  If this is the case I would guess fuel flow is OK, and you should look at the turbo wastegate and related controls.  You can see that at several points in the graph you accelerate to 5000 RPM and the manifold pressure falls off like it would on a non-turbo engine.  Maybe the wastegate is getting stuck open, or maybe a problem with the turbo servo.  

The engine was definitely running smoothly, just sapped of power.

I do need to fix the fuel pressure measurement. I have no idea why the installer did not follow the Rotax guidance on it.

I'll look into the turbo path. Thanks.

There is an enrichment solenoid on the airbox. Might want to check that.