912 ul Back firing

[skyfox1]

Hi all you rotax 912 users could some please put me on the right track, my 912 ul is always back firing when shuting down ,l have tuned carbys, new plugs but still every time l shut the engine off ban off she goes.

 

 

[facthunter]

If you cut the ignition any fuel mixture unburned goes into the muffler and may ignite giving the backfire you experience. If so it's not through the engine and will do little harm. Do you have the throttle fully closed? Nev

 

 

[skyfox1]

Hi Nev yes l have the throttle fully closed l have even tryed shuting one ignition off at a time still back fires.

 

cheers Geoff.

 

 

[Guest Maj Millard]

By 'tuned carbys ..what do you mean ?. Generally a 'backfire' is caused by excessive fuel or rich mixture accumulating in the exhaust system. Frst time I have heard of this problem with a 912. Do you have a non-standard exhaust set up ?. Are you using the correct spark plugs, or ,are your spark plugs really sooty black....which would lead back to a carb problem or mis-adjustment. Have you modified or extended the carb bowl vent tubes ?...........................................Maj...

 

 

[skyfox1]

Hi Maj when l say tuned l bought new carbs for it had them balanced it is in a skyfox with standard Exhaust

 

i am using DCPR7E plugs and plus are light grey, have left bowl vents the way it was when l got the aircraft.

 

Engine preforms excellent its just on shut down.

 

cheers Geoff

 

 

[icebob]

From memory there was somthing on the rotax owners web site about this but well my reason is old age, can not remember how far back it was.

 

Bob.

 

 

[Guest Maj Millard]

Well you seem to be doing all the right stuff there Geoff. The only other thing I can think of is maybe your slide is resting too high at idle, and still allowing a good passage of fuel, which then ends up in the exhaust and ignites on the way out. What is your idle speed ?, and does it come right down to idle speed quickly, when you close the throttle ?.

 

Another possibility is you have a knob of weld or whatever in the exhaust itself, which glows red hot to ignite the fuel on the way out ,after the ignition is shut down. This would not occur with the engine running as the exhaust then is already burnt, so wouldn't want to re-ignite.......................................................................................Maj...

 

 

[Guest aussie carl]

Are you using low octane or fuel that has been stored in drums for some time.

 

edit sorry I mis read your post, Thought it was running on.

 

Backfiring is unburnt fuel in the muffler.

 

 

[facthunter]

It's been pretty much covered. Another thing might be to idle the engine for a short while before cutting the ignition to allow the muffler internals to cool a bit. You can also run it out of fuel . As soon as the motor starts to falter, cut the mags. Nev

 

 

[skyfox1]

Hi yes l am using 95 octane unleaded fresh fuel ,l will try idling down for longer period see how that goes .

 

THANKS FOR EVERYONES INPUT.

 

 

[geoffreywh]

this usually caused by an air leak in the exhaust somewhere, only has to be small. If you can richen the idle mixture you can make the mixture that gets into the exhaust too rich to burn...