Hard starting when engine is hot?

Hi Jim... Thanks for your useful reply.

You wrote ...

"...absolutely use the electric fuel pump with a fuel return line for every start and takeoff, Regardless of engine temperature."

I always do use the electric fuel pump on for every takeoff and landing. My reason was/is it's "insurance" that if, say, the mechanical pump decided to pick that particular time to fail or lose pressure...

I will add: Turn OFF the electric fuel pump during run-up. If the mechanical fuel pump that day is not feeding adequate fuel and electric pump is ON you would not detect that during run-nup... and you could take-off for a flight (or for many flights) not knowing you had only one properly functioning fuel pump. Of course after run-up and before take-off turn the aux pump back on.

Then once fully clear of pattern at  decent altitude turn electric pump OFF until setting up landing . Again, my reason is that if you routinely start, run-up, taxi and fly with auxiliary electrib pump on, if your mechanical pump fails or goes substandard you may never discover that.... until maybe one day when the aux pump fails or your battery/alternator decides not to feed enough power to IT.

Hi

Yes this is the safe procedure. You don't need the aux. pump running all the time.

Al Said:

I will add: Turn OFF the electric fuel pump during run-up. If the mechanical fuel pump that day is not feeding adequate fuel and electric pump is ON you would not detect that during run-up... and you could take-off for a flight (or for many flights) not knowing you had only one properly functioning fuel pump. Of course, after run-up and before take-off turn the aux pump back on.

Good point Al.  The other side of the question would be how do you know the AUX pump is operating.

If you have a fuel pressure gauge, you could verify fuel pressure from the AUX pump BEFORE Engine start and then confirm the Manual pump is functional AFTER Engine start by de-energizing the AUX pump.  Then, of course, operate both pumps until above the pattern altitude.

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JIM:

If you know Jonas, you might like to inform him that he mis-installed his fuel return line.

What he did was tie the pump output back to the pump input.  This will NOT clear a vapor lock, just pump the vapor and hot fuel around in a short circuit allowing it to get even hotter.

Ideally, it needs to be returned back to a fuel tank or at least to a header tank that is vented.

A return back to a non-vented Gascolator is just making a longer version of the short circuit.

The vapors need to be vented, not recycled.

His saving grace will be that the AUX pump will be able to keep the pressure up in the main pump's suction line, delaying vapor formation and also permit the vapor bubble to be pushed through the open float valve of the stalled engine and refill the carbs to make a restart possible.

But this is still not an ideal situation, because, in his incident, there was no time available for the vapor to clear or perform a restart.

Jonas has set himself up for a repeat, by creating a false sense of security.

Wow good catch Bill. No I do not know him personally, I remembered watching this video and my intent was to use it as an example as to what can happen if the updated Rotax engine fuel system installation instructions are not implemented. I did not notice that in the video his return line did not route to a vented fuel tank, which is absolutely how it should be. Actually after watching this video the first time (it was during my 701 build and 912UL installation) I also bought and installed a fuel pressure gauge. That is a very valuable instrument when running a 912. And, after a look at a few more of his videos, it would appear one may be prudent not to emulate his behavior but rather use him as an example of what not to do, as his nickname is Sketch, short for the moniker bestowed upon him by his flying friends, “the sketchiest pilot on YouTube” But, to bring us back on topic, Al please get ‘er hot, shut ‘er down, and try a restart after just a few minutes break using the electric boost pump and let us know how it goes. 

"....If you have a fuel pressure gauge, you could verify fuel pressure from the AUX pump BEFORE Engine start and then confirm the Manual pump is functional AFTER Engine start by de-energizing the AUX pump.  ..."

Definitely,  if you have a fuel pressure gauge, that's the way to go.

I don't have one. Had a Technam once. It had that gauge, and the POH specified exactly as you describe as part of startup. 

The only evidence I have that my aux electric pump is functioning properly is that is sounds right and that if I turn it on before hitting the starter button the engine starts more easily.