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ChickenLips

Backfiring after shutdown

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ChickenLips
Almost every time I turn the key off on my AC720 (Onan 20hp), the engine backfires. I have owned the tractor for about 8 years and it has alway done it. I have tried slowing the throttle to idle before I turn it off but that didn't help. Anybody else have this happen?

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hufhouse
Within the last couple of months, there was a pretty long thread on this board about backfiring. That was probably late Spring. I learned a lot from that, because I was "trying out" the 7117 that I eventually bought, and it backfired. I was concerned, but got over it after I read all the posts on that thread. I guess it's perfectly normal, and I remember someone even had a name for it...the brown seat syndrome. I've used that one with my neighbors a couple of times! I guess it has to do with some gasoline getting into the muffler while it is still hot at shutdown. I have found that I can almost always eliminate the backfire by cutting the engine back to idle for about one minute before I turn it off. That allows the muffler to cool down enough that the gas doesn't ignite. At least that's what someone said on the other thread. Try searching for that discussion. I think you would find it interesting.

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cranemech
I've noticed on the ac 620 that I just bought does the same thing. I've had success with letting it idle prior to shutting it down, but I'm going to throw a set of splitfires in it to see if that will eliminate it. I just got started working on leveling my driveway, I'm very impressed with the way this thing works! The driveway is hardpacked dirt and gravel and it cuts down 1 1/2 -2 inches. I replaced the drive belts when I went in to free up the clutch pedal and I dont think they're tight enough, I could here them squeal when I went in to deep. Is there a procedure for tightening them or is it just by feel? Later, Chad

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gretsch
If it only backfires when you kill the ignition, it might not be due to this. However, one thing that can cause it is a lean carb mixture. It sounds backwards, but a lean mixture doesn't ignite all the fuel which allows some of the vapor to exhaust into the muffler......New plugs can cause it to temporarily subside because they fire stronger when fresh.

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HubbardRA
I have a 14 Hp Vanguard engine that does this, primarily because the anti-diesel valve is unhooked. This lets the air and fuel mixture that isn't burned after shutting off the ignition hit the hot exhaust and ignite. Not a problem, just a nuisance. None of my other engines do this. Rod H.

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Dutch
Can also be caused by worn throttle shaft in carb, or leaking carb or manifold gasket. I had the same problem with my CH18. I would idle it down (same as a K series or Briggs) and it would back fire or afterfire every time. Jeff Nemes gave me the perfect solution........ READ the manual. The CH18 should be shut off at HIGH idle.

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