What causes a 'Backfire' ?
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it is not harmful ...before shuting of the motor let it idle for 20 or 30 seconds and it will not backfire ..
Or try 89 octane gas.
Try leaving the thottle up when you shut down. 1/2 throttle or more. It seems wrong but it works on many engines.
It's not the Octane level. Take it to lowest idle it has before shutting down. And, YES, it can be harmful. It is unburned fuel (due to your ignition being shut down and the spark plug therefore not "sparking" to burn the fuel) being dumped into your exhaust port/header and/or muffler, and encountering an environment sufficiently hot to suddenly ignite. It can blow your muffler up...I think that's probably the most-likely damage that will occur, so, it's not a disaster, but, it's not a good thing to let happen.
JTK
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Annoying isn't it? My ~2003 model year Briggs 17.5hp did it, as does the 20hp Kohler Command twin on my Cub Cadet 2544. Both will REALLY bang if s/d at idle once they are hot.
It's right in my Kohler manual that the engine needs to be s/d while throttled up to prevent the 'afterfire'. All the idling in the world wouldn't do a thing for mine.
Joel
Joel
mine did that untill i richen up the idle jet by 1/4 turn.
i think these newer carb certified engines are set to run slightly lean from the factory.
i think these newer carb certified engines are set to run slightly lean from the factory.
Like Master ACiD said, if it backfires only when shut down from an idle, I'd guess it was running lean (and therefore hot) at idle. In THAT case, you could avoid the backfire by shutting it down from it's normal "cruise" speed.
I have a Kohler 20 HP single (or, is it 21 HP? I forgot already), and I avoid the backfire on mine by idling down before shutting down.
I have a Kohler 20 HP single (or, is it 21 HP? I forgot already), and I avoid the backfire on mine by idling down before shutting down.
What you are describing is after fire, not back fire.
IF your engine is equipped with the fuel shut off solenoid on the carb, then you are supposed to shut it off at higher speed to purge gasoline fumes out of the exhaust system.
IF your engine is equipped with the fuel shut off solenoid on the carb, then you are supposed to shut it off at higher speed to purge gasoline fumes out of the exhaust system.
Dang, Bill, that makes sense. When did small engines get sophisitcated/complicated enough to have a fuel shut-off solenoid? I'm obviously way behind times on these things.
JTK
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Fuel cutoff solenoids on the carb bowls have been around since the early 1990's maybe late 80's. The carb bowl stays full of fuel, the solenoid just pushes the main metering needle shut. I know our JD GT262 w/ a 17hp Kawi single had one. That was ~1992. Funny thing is, that engine never afterfired. I do think today's 'leaner' OPE carburetors are a major contributer to the idled s/d *BANG*.
Well, that 'spains it, Lucy! I haven't dug around small engines since the 70's. I'm WAY behind...
JTK
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Yup. It takes a bit to get used to, but it's the only way to consistently prevent the kaPOW! on my 2005 model year kohler command 20.
Joel
Our JD 22hp twin never afterfires. I never knew this happened to anyone else.
I think the JD's have a delay circuit in the ignition system to prevent this problem. A good idea but sometimes they fail leaving you with an engine that won't run...
JTK
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I thought about that too JS, but the ignition is out of the picture anyway on the Kohlers that pop on idled s/d. It seems like a case by case thing. Some do it, some don't. IMO, it's dependent on the carb and exhaust system design.
Not sure how a delay circuit would have any effect on after fire since the ignition is OFF.
Some Kohlers have a Spark Advance Module (SAM)
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