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Our 912UL (with 1000+ hours and perfect compression) starts reliably when cool or even cold (as long as we religiously follow our startup routine.

And if it's just warm  or barely warm after being parked for, say, an hour or two after a flight. starts instantly with one touch of the starter button... no choke,  no aux fuel pump on.

But if, say, I land, stretch my legs and pee, and in less than 5 or 10 minutes I get back in the plane with the engine HOT it often is a bitch to start. (Like scary enough to wonder if I'm going to be stuck out on some rural airstrip with no FBO, no other human beings, no cell reception, and a dead starter battery.)

If I remember right, in the ELSA forum I used to be in... where there are many Rotax owners... others report a similar "doesn't like to start when hot" issue.   On owner in England said his best solution was to go have a cup of tea and wait for it to cool down a bit. smile

Getting into the hotter weather... seems like it would be nice to get a handle on this. (FWIW we operate our 912ul essentially 100% of the time on mid-octane rated MOGAS with ethanol. Except winter when we use 91 Octane non-ethanol MOGAS.)

So three questions:

Have any of you noticed this "trouble starting when hot" thing?

Any solutions?

Any explanations why this happens?

 

 

  • Re: Hard starting when engine is hot?

    by » 2 years ago


    Your plane probably does not have a fuel return line back to the fuel tank and or header tank along with a electric fuel pump to purge the fuel lines of vapor/air before starting. Most likely the gas is boiling in the fuel lines after shut down. Search this site and the Rotax 912 installation manual for fuel return line information/discussion. A friend of mine just went through the same thing, he put clear fuel line in the verify bubbles/air/vapor in the return line was causing problem. The return line mounted high and tee'd with a small restricter .030" (to maintain pressure) in the between and just before the carburetors above the engine with an electric pump eliminated the hot start problem.


    Thank you said by: RotaxOwner Admin, Al C

  • Re: Hard starting when engine is hot?

    by » 2 years ago


    Thanks Garrett....

    I'll look into the return line thing you mention.... although I'm 99% sure (not being at the hangar can't check right now) we do have a fuel return line.

    But one other thing:  Were you suggesting that if I turn ON the auxiliary fuel pump for a hot start that that could clear the vapor and make the reluctant start issue disappear?  Normally, I'd think the last thing I want to do when hot is force more fuel in with the aux pump... I'd be afraid that might flood the carb.

     

     


  • Re: Hard starting when engine is hot?

    by » 2 years ago


    Al Said:

    ... I'd think the last thing I want to do when hot, is force more fuel in with the aux pump... I'd be afraid that might flood the carb.

     

    The fuel pressure does not control the amount of fuel entering the Carb.

    Once the carb bowl is filled,  the Float Valve closes and no more fuel enters.

    You should be able to operate the AUX pump for hours and never flood the Carbs.

    If you can't, then there is a problem with the Float Valve.

    With that AUX pump running and a return line installed, the Hot fuel and any vapor in the fuel lines will be returned to the fuel tank to be replaced with a circulating supply of cool fuel.

     


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


    Thank you said by: RotaxOwner Admin

  • Re: Hard starting when engine is hot?

    by » 2 years ago


    If you do have a return line installed the restricter may be clogged with debris which would disable the return line system. Take the line off and visually verify flow. It is on my checklist for the annual condition inspection. 


  • Re: Hard starting when engine is hot?

    by » 2 years ago


    Al absolutely use the electric fuel pump with a fuel return line for every start and takeoff, Regardless of engine temperature.  Rotax made the change to their install manual to add an aux pump and return line mandatory for a good reason. If you want to see an example as to why, here’s Jonas crashing his Kitfox with his 912 before he installed a fuel return line, then later, an electric boost pump.  His scenario was just as you described, warm weather, a brief stop, a quick takeoff, then he was upside down on the gravel bar when the engine quit about 20 ft above the ground. 

    https://m.youtube.com/watch?v=zPvA3Ymmqig


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