Backfire

do you shut it off at full throttle or idle it down? some machines prefer WOT and some want to be idled down at least 50%. never had my X500 do it and i shut off in pretty much whatever position the throttle is in, though 90% of the time that is WOT.

I had same issue but different model. Reducing throttle to low idle before turning off stopped the backfiring.

I have tried it in the WOT and at reduced throttle. There seems to be no rhyme or reason to when it backfires. I have noticed that after I turn the key off it actually takes the engine a few seconds to "get the message" and shut down. Thanks for your replies.

the delay in shut off is the ignition module and it is as designed.

Thanks davidg for the info on the shut off.

it took me a while to get used to it too. on my L130 i can kill teh key and the machine stops almost instantly. on the X500 i can actually DRIVE forward 15-20 feet after i put the key in the OFF position! i don't normally do this, but sometimes have been known to have a brain fart as i am parking and turn the machine off before i get stopped.

I have those brain farts quite often except I call them crainial anal disease.

A build-up of raw fuel in a hot exhaust is what causes a backfire, and is many times a result of being too rich. A backfire that comes from the intake, is a lean pop. The longer you idle the motor before shutting it off, the less likely it will be that it will backfire. my 318 will backfire out of the exhaust If I don't idle it down first.

Thanks saleen49, If this is the problem should I be able to smell the build up of raw fuel?

Nope....Whats saleen49 is trying to say is "when you shut off the keyswitch,you kill the spark,but the motor is revolving at a high/low speed,still sucking in gas fumes and fumes from the crankcase....so depending how the momentum(sp) stops(which stroke the piston and valves are at ) the raw fuel that has built up in the engine cylinder/muffler is now ignited by the hot exhaust ....causing a back fire)

This is as clear as mud isn't it???? ...Jim ....

Thanks JimC Vt, That makes perfect sense as far as what happens but how do I make it stop and what is the the term "too rich" mean when talking about the fuel? Is it an adjustment that can be mad in the carb or something like that?

BAMA10 said:

I have those brain farts quite often except I call them crainial anal disease.

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I work for a guy who has 'CRANIAL-RECTUM-ITUS".. :ROF

saleen49 said:

my 318 will backfire out of the exhaust If I don't idle it down first.

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Same for my 345. When I'm done working it, I throttle back to 1/2 and head for the barn. About the time I get there, it's mostly cooled off, and then I bring it to idle for about 10 seconds once I'm inside. Only then do I switch it off. That's made it quit.

Hope this helps.

JimC_Vt said:

Nope....Whats saleen49 is trying to say is "when you shut off the keyswitch,you kill the spark,but the motor is revolving at a high/low speed,still sucking in gas fumes and fumes from the crankcase....so depending how the momentum(sp) stops(which stroke the piston and valves are at ) the raw fuel that has built up in the engine cylinder/muffler is now ignited by the hot exhaust ....causing a back fire)

This is as clear as mud isn't it???? ...Jim ....

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LOL! :thanku:

BAMA10 said:

Thanks JimC Vt, That makes perfect sense as far as what happens but how do I make it stop and what is the the term "too rich" mean when talking about the fuel? Is it an adjustment that can be mad in the carb or something like that?

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Its quit possibly just the nature of the beast. You have a simple carburated small engine with a hot muffler. You shut the motor down while everything is saturated with heat, and raw fuel makes its way to the cherry red exhaust and explodes. Like others have said, try taking a slowdown before you turn the motor off.

As many have already said, it's the drawing in of raw fuel that is being ignited by a hot spot in the combustion chamber or exhaust. My X300 doesn't do it but my old Agway tractor will if it is hot out. It's a heat issue with that tractor as it never does it unless it is very warm out. That is another plus for fuel injection. When the power is killed there is no fuel going into the cylinder to ignite. It is best to let a carburetor engine idle down before shutting it off because as long as the engine is spinning over, it is drawing fuel into it. By shutting it off at WOT you're also pulling excess unburned fuel into the cylinders washing them down and causing excessive wear. It could also be caused by carbon build up causing a hot spot. Or maybe the cooling fins or fan screen dirty. Just a couple ideas.