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Backfire

 
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george.mueller(at)aurora.
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PostPosted: Tue Sep 04, 2007 5:42 am    Post subject: Backfire Reply with quote

I am at the point of doing full power run-ups with my Zenith 701 with a rotax 912 UL 80hp installed with a Skyshops firewall forward kit. When I do a full power run up, I get a backfire after I turn the ignition key to "off". When I taxi around keeping the engine to around 2,000 rpm, this does not happen after shutdown, only after I do a full power run-up. Everything else seems normal as far as I can tell, the engine starts really well, seems to run smoothly, idles well. But after doing three full power run ups, I have had three backfires, all after turning off the ignition. Does anyone on the list have an idea about what might be causing this?



George in Milwaukee
Zenith 701 912ul [quote][b]


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Roger Lee



Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

PostPosted: Tue Sep 04, 2007 6:02 am    Post subject: Re: Backfire Reply with quote

Hi George,

Are you turning it off while it is at full power or are you brining it back down and letting it idle for a moment or two? At full power that may be normal, but not under idle.
Let it come back to idle after your run ups for 30 seconds or so before turning of the ignition. If you turn it off at high rpms you have more fuel and a more advanced spark than at idle.


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Roger Lee
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rparigor(at)suffolk.lib.n
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PostPosted: Tue Sep 04, 2007 9:02 am    Post subject: Backfire Reply with quote

I had motorcycles do this when you would get them hot by asking them to
make some BTUs, then shut off. Especial in warmer temps. I even blew the
guts out of a compression gauge by taking a compression test on a very hot
motor. I believe it is glowing piece of carbon that ignites mixture.

Try letting cool a bit more before shut off. Could try idle and pull choke
for 15 seconds to richen and cool, then go to no choke to turn off.

Also I have read that ignition modules when hot, first thing that happens
is you can't turn off motor.

Ron P.


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