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More 914 UL fuel system fun. This time on a different aircraft.  To quote the pilot, "I had to adjust something and shutdown to leave the cockpit.  However, I neglected to turn off the fuel pump.  When I restarted the engine, I got a loud backfire.  Just after takeoff, the engine rolled back from full power at 5600 rpm to 4500 rpm.  I pulled back on the throttle to 4200 rpm and it ran smoothly.  Above that, the engine sounded like it was flooding and started to stumble and run rough.  I made it home, thinking fouled plugs, but that was not it." 

And now my comments.  This is a SeaRey and he was in the water so I don't know if he did an engine runup after this occurred. (I think I would have, but that's just me.)   

Anyone out there in Rotax land left their fuel pump/pumps on with the engine not running and experienced a backfire?  And if so how much damage was done to the carbs and/or airbox?  

This sounds like to me excessive fuel pressure overcame the needle valves in the float bowls and dumped raw fuel into the carbs causing the engine to backfire. A contributing factor might have been a plugged float bowl vent line or sinking floats.  A carb rebuild with special attention to the floats, carb vent lines, and fuel pressure regulator per SB-914-040-UL might be in order here.  

Any suggestions for where else I might find damage?  I'm thinking of the air filter...

Thanks in advance for any suggestions.  

Jeffrey Fritts


Jeffrey Fritts, USAF (ret.)

www.flywwlsa.com

"In aircraft maintenance, good enough is not good enough."

  • Re: 914 Backfire Areas of Possible Damage

    by » 6 months ago


    Jeffrey 

    When i bought my plane I had problems with carb flooding when the pump was on but the engine was not running. Fuel was coming out from the airbox drain lines after a minute or two of pump operation. It turned out that the return line from the pressure control valve was very restricted and the regulator was sending much higher pressure at the carbs. Now, the carbs were not able to handle the pressure and the float needles would leave the excess fuel to flood the carb bowls. When the engine was running this flooding was not happening due to fuel being consumed by the engine. Of course I changed the return line and also the float needles and since then you can have both pumps running for an hour with the engine off with no flooding whatsoever. I never experienced a backfire when I had the flooding , even when the fuel was going down at the vent lines and to the floor. But you’re right -flooding can cause backfires. 
     Air filter is something you wanna check for sure, but another point is the turbo itself. Specially the cold section. At least check visually in order to find any anomalies at the compressor. Most probably, you won’t find anything , but better be safe than sorry.


    Thank you said by: Jeffrey Fritts

  • Re: 914 Backfire Areas of Possible Damage

    by » 6 months ago


    Thank you Spiros,

    I'll be sure and check the fuel return line from the pressure regulator.  That makes perfect sense.  In their material Rotax spends a lot of time talking about fuel return lines and I'm not sure we give them enough credit for the important roll they play in the operation of the engines.  

    Jeffrey


    Jeffrey Fritts, USAF (ret.)

    www.flywwlsa.com

    "In aircraft maintenance, good enough is not good enough."


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