914 Backfire Areas of Possible Damage

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 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