Rotax 912ULS ?fuel pump failure?

A very similar incident happened to me a couple years or so ago. Turns out an o-ring was in the process of dissolving. The auxillary fuel pump didn't help but it cleared up when I reduced throttle. I had to disassemble the carb to find it. The only thing visible in the bowl was a tiny black speck.

What does your fuel pressure look like, before and after turning on the aux pump?

The 912ULS in our RV-12 acted the same way.  My wife noticed it first then had me fly it.  On the warm ,but not hot, day I climbed to 4500.  Leveling off, I turned off the electric fuel pump.  The engine sputtered and tried to quit.  Turning the electric pump back on cured the problem in a few seconds.  I thought  it was vapor lock.  After 390 hours, our plane had just came out of extensive shop service and annual inspection, that included a new mechanical fuel pump.  After talking with several Rotax service people, I'm convinced that the new mechanical fuel pump fails under elevated engine temps.  A mechanic friend has offered to swap out the new pump.  I'll take her up on that soon.  Van's recommends leaving the electric fuel pump on at all times.  But, how would you know if the mechanical pump was bad.  In that scenario, we'd be cruising along when the electric pump dies, only to find that the mechanical pump had already died some time ago. Not a good day.  By the way, before that recent shop service, I used to fly using the mechanical pump primarily, rarely adding the electric pump.  I know, not a good procedure.

Ron, do you have a fuel return line with a calibrated orfice routed to a fuel tank? What is your fuel pressure gauge indicating on mechanical pump alone when the engine stumbles? Is your mechanical pump vent line routed to avoid any slipstream or ram air entry?

I fly with my electric pump on at all times, but verify proper mechanical pump operation during my pre-takeoff check (I momentarily turn off the electric pump during the 4,000 rpm ignition circuit check), and again during engine shutdown checks (I turn off the electric pump as part of my shutdown check and verify fuel pressure and that engine still idles smoothly).  If you do these things and despite that the mechanical pump fails after takeoff and the electric pump does too, well I’ve still got gravity feed with my high wing aircraft but yes, your RV you will become a glider, but the probability of that dual failure occurrence is remote.

 RON DUNN wrote:   "Van's recommends leaving the electric fuel pump on at all times.  But, how would you know if the mechanical pump was bad.  In that scenario, we'd be cruising along when the electric pump dies, only to find that the mechanical pump had already died some time ago. Not a good day.  By the way, before that recent shop service, I used to fly using the mechanical pump primarily, rarely adding the electric pump.  I know, not a good procedure."

I turn on my aux electric before startup, and for takeoff, but other than that I leave it off.  As you say, if you run it all the time it may mask problems with the mechanical pump, not to mention extra wear and tear on the aux itself.

But regardless, what makes you say flying off the mechanical pump is bad procedure?  Because it might fail?