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4xbill

What causes after burn? Onan 20hp

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4xbill
My AC 720 continues to fire after I turn it off and usually ends with a backfire. It only happens when it's hot. If I just start it up for a few minutes to move it, it doesn't do it. Is this a timing issue? Picture added because... well, we like pictures here:


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ambler
My understandingis that when you turn key off and the engine is turning, there is no spark but some gas is still being sucked into engine. This is expelled through exhaust valve and afterfire is ignition from the hot muffler. Drop the revs to idle for a couple seconds before shutting off.

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andrewk
Could be a couple different things. Ambler's explanation is a good one, although it can also happen because the carb circuits are too rich. Timing can do it, although that is not as likely on a small engine as it is in a car. Carbon buildup is a big cause too. Basically what is happening is that something is staying hot enough to cause the combustion reaction without the plug firing. It may also be detonating when hot. hope this helps- Andrew

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HubbardRA
This is one of the primary reasons that many of the newer engines have the electronic fuel shutoff in the carb. That unit shuts off all fuel flow thru the carb when the key is turned off to prevent run-on, and backfiring. It is a common problem, otherwise they wouldn't have fixed it on the newer engines.

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Simplicity314
A too lean of a mixture and an incorrect plug and/or gap could be the problem too. Make sure you have the right plug in there and it's gapped correctly. You should pull it to inspect it anyway. Let it get hot and readjust the mixture also. Edit: plugs...not plug.

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2burning
I usually idle down my air cooled motors to let them cool a minute before turning off the switch. Wasn't another technique to turn off the switch and push the throttle all the way open to keep it from running on?

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DMedal
Another word for what you describe is dieseling. Assuming your switch kills the spark, something else is igniting the gas in the cylinder. If it was only in the muffler the engine wouldn't keep trying to run. Dieseling is really rough on the engine so you don't want to just let this go on and on.
quote:
Originally posted by ambler
I think some people pull choke out to kill engine to stop this.
I sometimes do that too, and then give myself a good bop on the forehead. killing with choke leaves the cylinder walls bathed in gas, not oil. not good. Culprit is combination of overheat (let's not forget the ever suspect rodent nest) and carbon build up from a too rich mixture. Timing can do it but it's unlikely your timing changed. Quick half-fixes: gas shut off valve where you can reach it from the seat, kill the engine that way, leave key on til it dies. or adjust idle low enough it dies, and use your throttle cracked slightly open when you want to idle. If it is a standard trans, pop the clutch just as it is dying. real fix: pull heads, clean carbon (read up before you attempt this) from cylinder heads, then get carb adjustment right on, not too lean nor rich. Good luck. If this tractor gets to be too much trouble for ya I'm sure there are club members close by who could give it a good home.

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UCD
Spraying water into the carb with a windex sprayer at half throttle will clean out the carbon with out pulling the heads. Will also clean fouled plugs.(old drag racers tick) Spray just enough to make engine run rough not stall it.

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4xbill
Thanks all! I don't think it's heat, I had the engine out last summer and totally cleaned it, shields off, slight wash, compressed air. I did the Idle down trick, worked ok, engine didn't run on, but got a small back fire. Tried the choke and I think it just flooded it and made smoke come out of the muffler. When I restarted it, later on, got a huge flaming back fire. Scared the kids and sent the animals running:O. I will try cleaning the heads etc. next. Will keep you posted.

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