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10/20/2017 1:01:18 AM
9/22/2017 12:11:25 AM
Posted: 9/10/2005 5:00:14 AM EDT
Just looking for the basic reasons/fixes.

Is it a fuel issue?
timing issue? Is that adjustable?
Air issue?
thanks.
Link Posted: 9/10/2005 5:13:46 AM EDT
How do you shut it off?
Foot off the gas pedel, run it for a few seconds before you shut it off?

Backfires are caused by unburnt fuel being combusted in either the intake or the exhaust system.

Before the days of electronic controlled ignitions I could push the gas pedel to the floor of my car at speed, shut off the ignition and then turn it back on.
It would make a hell of a bang!
Link Posted: 9/10/2005 5:33:56 AM EDT
Link Posted: 9/10/2005 5:35:21 AM EDT
It's because you touch yourself at night!
Link Posted: 9/10/2005 5:38:32 AM EDT
[Last Edit: 9/10/2005 5:40:14 AM EDT by jmarkma]
It’s most likely the centrifugal motion of the motor turning over a few times after you turn off the switch, witch only kills the ignition. With carbureted, non-fuel injected motors, the turning of the engine will drawn a vacuum to pull in the new fuel and without any spark for the last few turns the un-burnt gas is forced into the hot exhaust system. Then BANG!

BTW: it shouldn't be a problem.
Link Posted: 9/10/2005 5:43:26 AM EDT
I refer to this phenomenom as "FM"...


fuckin' magic



Seriously, I believe it is because of unburned fuel being ignited by the cherry red hot
exhaust system.


LB
Link Posted: 9/10/2005 5:45:16 AM EDT
Don't know but I'm sure it's Bush's fault.
Link Posted: 9/10/2005 5:49:10 AM EDT

Originally Posted By PanzerOfDoom:
Don't know but I'm sure it's Bush's fault.



+1
Link Posted: 9/10/2005 5:49:19 AM EDT
Mine does the same thing but only when the blade is engaged, not if there is no load on it
Link Posted: 9/10/2005 5:52:10 AM EDT
[Last Edit: 9/10/2005 5:53:46 AM EDT by YardDogOne]
.
Link Posted: 9/10/2005 5:53:09 AM EDT
Solution:

When you're done mowing and ready to put it away, run the throttle down to low idle for a bit over 1 minute. Then shut it off. Running at idle allows the engine to sync cool & burns off excess fuel.
Link Posted: 9/10/2005 5:58:29 AM EDT
25HP Craftsman does the samething since new, but does not do it as often now that it is 3 years old. I also run it real slow before I shut it down.
Link Posted: 9/10/2005 6:20:16 AM EDT
Here's the deal. Inside the muffler is glowing hot after running at high RPM. When you shut down the engine, it is spinning down with no ignition and it still continues to draw fuel from the carburator and exhausts the unburnt fuel vapor into the muffler which to an extent cools things a bit. When the engine is still, the heat in muffler heats & ignites the residual fuel vapor and causes the backfire.

Now, there are two issues at hand for preventing the back fire.

If you have an older engine (Kohler Magnum for example) without a high speed circuit/fuel shutoff solinoid screwed into the bottom of the carburator float bowl (has single wire) then the best bet is to allow the engine to idle for two minutes or longer to cool the exhaust and then shut it off.

If you have a newer engine (Kohler Command for example) with a fuel solinoid in the float bowl, then you are supposed to shut these off at Full Throttle. The Solinoid closes off the high speed circuit fuel flow when the key is turned off stopping the flow of fuel. As the engine spins down, the exhaust is purged of fuel vapor by a very lean fuel/air mix.

If you shut down at idle speeds, the idle circuit in the carb is active and dumps fuel into the exhaust even thought the high speed circuit is inactive.

All that being said, there are some dumb ass manufacturers that couldn't wire a flashlight and subsiquently there are a number of engines wired such that when they are shut off, the DC voltage from the charging system backfeeds into the wiring keeping the fuel solinoid open until the engine spins down (remember, the alternator is under the flywheel) thus causing a backfire no matter what. The wiring can be modified or a different ignition switch can be used to eliminate the problem.
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