Hi All,

I have a friend who is having problems with his 914 ULS backfiring in his Kitfox 6 when shutting down. The engine is quite new, about 100 hours now, and a replacement of his old identical 914 ULS engine that was replaced at 1000 hours. It has been backfiring ever since the first change of the spark plugs at about 25 hours. The engine runs perfectly, starts easy and runs smooth.

The backfire occurs on shutdown. The engine is idled to about 1800 for a few minutes, then the mags are switched off one at a time. Once the prop stops turning there is a delay of about 6 seconds, then there is a loud backfire from the muffler every time. The fuel is always fresh and high octane - Shell V power 98 Ron unleaded. The plugs are genuine Rotax plugs and are put in with the recommended heat paste.

The plugs have been changed a few times, the carb needles have been adjusted from rich to lean, and the throttle has tried in a range from 2200 to 1400 during shutdown, but the backfire is consistently there. The idle screw is set to idle at 1400 when pulling the throttle fully out.

The only way to avoid the backfire, is to starve the engine of fuel by closing the valve and switching off the pumps, and letting it shut down on its own.

Could this backfiring be related to the mixture settings? Or an air leak in the exhaust? If it was still happening with needles set at full rich, surely it cant bee that the mixture is too lean? Any tips or guides on what to check would be appreciated.

P.s. I have the same engine and settings, and both my carb needles are leaned out same as his, and I get smooth lots of power and no backfire on shutting down.

Many thanks in advance

Amer