backfire after shutoff. rotax 912f3 80hp engine

[katanajer]

Hi guys,

Recently after flying my katana...when shut down, after the prop stops, there's a second then there's some backfire noise. If I fly for 10 min a quick flight in the pattern, then there's nothing, but if I fly for 40 min or more. I hear a bang after the engine shuts.

I use chevron mid grade gas and never had a problem. However, the plane did sit for a month without flying.

Any ideas what it could be and how to fix?

[krellis]

Hi guys,

Recently after flying my katana...when shut down, after the prop stops, there's a second then there's some backfire noise. If I fly for 10 min a quick flight in the pattern, then there's nothing, but if I fly for 40 min or more. I hear a bang after the engine shuts.

I use chevron mid grade gas and never had a problem. However, the plane did sit for a month without flying.

Any ideas what it could be and how to fix?

This is not uncommon, especially if the idle speed is a little too high. With the dual carb setup on the A1 model and the bowden cables between the throttle and the carbs, you might find it necessary to hold the throttle (using some pressure) all the way back against the stop before you turn off the ignition. The Rotax carbs are spring loaded to go fully open if the bowden cable breaks/comes loose, so there is spring tension towards the throttle open position.

It has happened several times to us, more commonly in the summer when it's warm here in Georgia. Hope this helps.

Krea Ellis
DA20-A1 N853DF Princess Amelia

[katanajer]

hi krea,

thanks for the reply, is there a solution or should i just hold the throttle back on shut down?

also is this going to harm my engine?

[krellis]

hi krea,

thanks for the reply, is there a solution or should i just hold the throttle back on shut down?

also is this going to harm my engine?

On your next annual, check the idle speed, the throttle stops on the carbs and the bowden cables for slack and wear. Also, double check that the throttle friction is not set too high preventing you from retarding the throttle to idle.

There is some natural hysteresis in a bowden cable setup - so even if everything is set perfectly, the throttle can creep up a bit. I just made a habit of pulling the throttle back and holding it at the stop as I shut down the engine.

I don't think there is any harm to the engine, but it probably is not good for the muffler. The baffles inside the muffler are a weak area and are difficult/impossible to repair. Unlike a Lycoming or Continental engine with a mixture control, we have no way of starving the engine of fuel - other than closing the fuel shutoff valve and waiting for the engine to die - which I do not recommend.

Krea Ellis

[katanajer]

Thanks for the help and advice.

Last night I losened the tension and held the throttle in idle position, and no more backfire after an hour flight. I flew for another hour and did the same thing.

Wahooo. no more backfire!!!