Backfiring after engine shutdown?

[PawNature]

I have an 18hp onan. Runs perfect except when I shut it down. It will wait about 10 seconds then backfires.
Any Ideas what may be causing this?
Thanks

[scsmith42]

When you shut the engine off the spark is killed, but not the mixture.  As the engine spools down to a halt, fuel-air mixture is sucked through the intake and into the cylinders, and then pumped out unburned into the muffler, where the hot metal will ignite it.

Not to worry about.  You can minimize it by allowing the engine to coast down to idle and cool off for a minute before shutting it off.  You may need to adjust your idle down somewhat if it still does it.

I once blew the muffler off of my old 1960 Ford pickup when I shut the ignition off at 80 mph, and then turned the key back on 10 seconds later.  The sound of that muffler blowing up scared the bejeebers out of this then-16 year old kid!

[easymoney]

 i have a nissan pickup that does the same thing. it idles a little too fast that might contribut to it. it is annoying to pull up to someone and shut the motor off and about a second later it sounds like a gun going off. that could get you hurt with someone thinking you was shooting at them.

[PawNature]

Yea that's my main problem, when I shut it off my mind or what left of it immediately goes to something else. Then KaBoom. I feel like that scardie cat on the cartoons that the dog sneaks up behind and starts barking.

[jrokusek]

I've also heard that if the engine is running too rich it does this.  Excess gas pools just like scsmith42 says and BANG!  

I had a Harley that did this.  I fixed the exhaust leak, swapped to a smaller intermediate jet in the carb, lowered the idle and changed plugs and wires.  I'm horrible at tuning carb's so I had a buddy of mine adjust the fuel/air mixture.  Can't tell you what fixed it, but it seemed to fix my shotgun sound.

[jkj]

The engine on my Woodmizer did this so I went back and paid more attention to the instructions.  Seems like I'm not supposed to turn off the engine while it is idling. 

Now I run the engine up to about 1/2 throttle, hold it there, switch off the engine, wait until the engiine stops, then release the throttle.  This stopped the backfire.  If I forget to do this, it backfires every time.

JKJ

[sawguy21]

I once blew the muffler off of my old 1960 Ford pickup when I shut the ignition off at 80 mph, and then turned the key back on 10 seconds later.  The sound of that muffler blowing up scared the bejeebers out of this then-16 year old kid!

:D :D :D :D I pulled the same stunt with an old Jeep Wagoneer at night. The guy tailgating me must have panicked as his headlights suddenly shone across the road.
The backfiring from your Onan can be minimized by shutting it down at 1/3 to 1/2 throttle.

[IL Bull]

My 318 John Deere lawn tractor does this.  It has a 18 HP Onan also.   Drives me nuts(short drive). ;D :D

[jimbo]

   A well tuned engine will do that evey time on a kohler  engine  on sawmills and lownmowers    just live with  it                             

                                                                                        jimbo                                   

[bitternut]

I have a Kohler engine on my lawn mower and it will backfire after mowing if you shut it off right away. If you let it run for a minute or so at idle it will never backfire. My neighbor has a Kohler engine on his also and his backfires every time he shuts it off, but of course he does not let it cool down from high load.

[jkj]

A well tuned engine will do that evey time...just live with it

I don't think I'd just live with it.  A backfire can cause damage.  Concerning backfire, Kohler's web site states:
Your owner's manual has proper shutdown procedures to avoid this occurrence. If your engine is equipped with a fuel shut-off solenoid, be certain to set the throttle between half and full throttle before turning the key switch "off." 

I have a Kohler engine on my lawn mower and it will backfire after mowing if you shut it off right away. If you let it run for a minute or so at idle it will never backfire.

The manual for one of my zero-turn mowers says to run the engine (Kawasaki twin cylinder) for three minutes at idle before shutting it off.  Idling for any length of time, however, will not prevent backfire on the Kohler on my sawmill.  The difference between the two procedures may depend on whether the engine has a shtudown solenoid, according to a Kohler manual:

2a. For engines without a shutdown solenoid:
Move the throttle to the ''slow'' or ''low'' idle
position. Allow the engine to run at idle for 30-60
seconds; then stop the engine.

b. For engines equipped with a shutdown
solenoid: Position the throttle control somewhere
between half and full throttle; then stop the engine.

BTW, the Kohler web site has PDF files of the operating manuals and service manuals.

JKJ

[ARKANSAWYER]

  Wanda started doing that when she was about 6 months old.   I asked the Kohler guys about it at the WM 20th party and they told me there was a valve that needed cleaning  in the carb.   So for about 4 more years every time I killed the engine there was a "POOF".   This spring sometime I notice that it did not do that any more.  ???   Not sure why but none the less it does not do it.   Maybe it is because gas is so high or she has a kazillion hours on her now and is to tired to "POOF" any more.

[jackpine]

Arky, my Kohler did the same thing for about the first 1000 hours and then it became more infrequent and lately haven't noticed it at all. It has gone away so gradually that it's hard to remember just when it stopped.

[Bill]

FWIW  ( sometimes measured by what you pay for something )

There's a school of thought that says that its not good to leave unburned gas on the walls of the cylinders - like killing the spark before the fuel stops - as it will "wash" the oil off the cylinder walls before you start it the next time . The metal to metal contact ( between rings, piston and cylinder w/o any oil ) is supposed to wear the engine - when you next start it - more then actually running it. If there's a solenoid ( or electric fuel pump ) to shut the fuel off then there should be no problem - otherwise people ( auto mechaninc types ) have been telling me to let the older mechanical fuel pump type motors to come down to idle before killing the ignition/spark. I've also heard that a slow ( as possible ) idle speed and a correct fuel mixture help.

Hope some of this rambling helps  . . .