Why the backfire

[BairleaFarm 340]

Ive got a K?, well a 16hp kohler. The guy who had it before me has it running great. I can use any position on the throttle. I always throttle down for a few minutes before i turn it off and it usually back fires. Why? i wouldn't mind i I do all my in town errands in this tractor and its loud when you part next to a building. People jump high B)

[nylyon-(Admin) 7,263]

Are you talking about BSS the backfire several seconds after turning off the key? My K-Series C-160 does not do that, but the Magnum on my 414-8 does. I have found 2 ways to eliminate that.

1. Allow the engine to idle for 2 minutes prior to shutting off the key

2. Now this DOES work, at about half throttle, turn off the key. You can have it idle then go to half throttle and turn it off.

Either of those have worked for me, I do not know the cause, but this is common for the Kohler's

[BairleaFarm 340]

I wouldnt say several seconds but yess there is a delay. It usually happens at the time im completely off the tractor.

[T-Mo-(Moderator) 4,496]

Usually the Kohler's backfire after shut down due to unburnt fuel in the carb. Idling down usually helps this as the fuel then has time to be used off before the engine dies. Also, if you don't idle down before shutting the engine off, the next time you start it, there probably will be a puff of black smoke for a moment when it starts. Again, unburnt fuel in the carb is the culprit.

[TT-(Moderator) 1,131]

Here's my theory:

Because it takes a long time for the engine to actually stop (heavy flywheel / low compression) it is still drawing the air/fuel mixture in through the intake after the ignition is turned off. The large canister mufflers seem to store up the unburnt vapors until the heat from the baffles ignite the contents of the muffler. (a carbon flake may cause the ignition)

I have rarely seen this with the old "pepper pot" mufflers, or other types of free-flowing exhaust.

Think back to the days before catalytic convertors when intentional exhaust system "explosions" could be acheived by turning the ignition off for a brief time while driving down a hill. Turning the ignition back on again would produce a rather large "bang", sometimes powerful enough to blow the seams on the muffler wide open. B)

[kpinnc 11,877]

My 310-8's Magnum backfires if I switch it off for a few minutes and then crank it later before it's completely cooled down.

In other words, it backfires when it STARTS....

WTF?

Kevin

[T-Mo-(Moderator) 4,496]

Kevin,

Probably unburnt fuel. Now don't quote me on this, but I heard Kohlers are notorious for this occurrence, especially when you don't idle down. TT also might be on the right path with the exhaust theory - but I'm leaning towards the unburnt fuel theory. Maybe there was a second gunmen - oh, wait a minute, wrong conspiracy - but maybe it's a combination of both theories. B)

[kpinnc 11,877]

Thanks Tmo, I'll give it a try this weekend. I do have a bad habit of shutting down wide open....

It's rained for the last 7 days. Got a LOT of grass to mow!

Kevin

[Curmudgeon 28]

due to unburnt fuel in the carb.

Uh, I hope all fuel in the carb is unburnt, otherwise you have a serious problem my friend!

[kpinnc 11,877]

Uh, I hope all fuel in the carb is unburnt, otherwise you have a serious problem my friend!

How right you are Dale. Never thought of it that way....

Kevin

[T-Mo-(Moderator) 4,496]

Let me clarify or possibly save face - unburnt fuel/vapors in the CYLINDER CHAMBER. When you shut down at full throttle, all you're doing is dumping/leaving unburnt fuel in the chamber causing the backfiring.

[Curmudgeon 28]

I know I've had several tractors with this backfiring tendency, and it's the ones with the larger mufflers. I've always attributed it to fuel mixture being moved through the engine to a hot muffler with enough capacity to accumulate sufficient said mixture to ignite in a rather large way.

By letting it idle some, the muffler may cool slightly, plus from a dead idle less mixture will be drawn through the engine and expelled into the muffler thereby resulting in a reduced chance of said backfire.

It worked for me at least...... Let it idle, no backfire, kill it just as it's barely getting idled down, count on some fire coming out the exhaust!!

[Curmudgeon 28]

Let me clarify or possibly save face - unburnt fuel/vapors in the CYLINDER CHAMBER. When you shut down at full throttle, all you're doing is dumping/leaving unburnt fuel in the chamber causing the backfiring.

Face saved Terry!!!!!!!! :banana:

[T-Mo-(Moderator) 4,496]

I know I've had several tractors with this backfiring tendency, and it's the ones with the larger mufflers. I've always attributed it to fuel mixture being moved through the engine to a hot muffler with enough capacity to accumulate sufficient said mixture to ignite in a rather large way.

By letting it idle some, the muffler may cool slightly, plus from a dead idle less mixture will be drawn through the engine and expelled into the muffler thereby resulting in a reduced chance of said backfire.

It worked for me at least...... Let it idle, no backfire, kill it just as it's barely getting idled down, count on some fire coming out the exhaust!!

Dale,

You just supported TT's theory. And I think it might be a valid theory with a lot credibility. Now, about the second gunmen on the grassy knoll theory...... B)

[combatmp29r 27]

Terry you don't have to kiss up Dale already said you saved face :horseplay:

[T-Mo-(Moderator) 4,496]

Kissing up to Dale wouldn't bring me much now would it. Besides he was just :whistle:

[Curmudgeon 28]

No matter how I I'm always found out!! B)

[linen beige 14]

Let me clarify or possibly save face - unburnt fuel/vapors in the CYLINDER CHAMBER. When you shut down at full throttle, all you're doing is dumping/leaving unburnt fuel in the chamber causing the backfiring.

The question begs, what ignites it? If it is igniting in the cylinder, then it would be muffled, even if the exhaust valve is open or partially open. There should not be enough temperature in the cylinder to ignite unburned fuel several seconds after shutoff. The only ignition source would be glowing coke deposits, therefore decoking the head and cylinder ridge should eliminate this IF it is what is happening. TT's theory about fuel igniting in the muffler is more than likely right on.

Liquid, unburned gasoline WILL NOT BURN but gasoline vapors are explosive. That means that if fuel is finding it's way into the muffler, and that big flywheel turning several rotations after the switch is off will draw it in, it lays there for a moment until the heat causes vapors to rise. These vapors are then ignited by either the heat itself or, more likely a glowing ember of coke like TT said.

One answer may be for you to remove the muffler and let it run for a while. If it still backfires then it is time to pull the head and get to cleaning. If it turns out to be the muffler, time for a new one.

While we're talking backfires, anyone remember the old Mazda RX 7 rotary engines? They were notorious for backfiring and starting back up a few moments after being shut off. Owners quickly learned not to park one with it in gear or it would go for a ride on it's own. Seems the overhead carbs would leak fuel into the combustion chambers and be ignited by coke deposits. I saw this happen once at a service station. The guard rail next to the pumps stopped the car long enough for the owner to get it out of gear.

[nylyon-(Admin) 7,263]

In my opinion, it is obviously unburnd fuel in the mufler, after all what else could it be? Question is, why shutting down at half throttle does it not happen? Try it you'll see. Must be something with the idle circuit being too rich? Letting it idle is probably letting the ignition source cool enough to not ignite the vapor.

[Curmudgeon 28]

I'd make one slight adjustment to your statement.

The gas is already in a vapor state, already mixed with air, when it gets to the muffler. It would have to be. It's vaporized by the time it gets to the cylinder, otherwise, it wouldn't run. Even at slow cranking speeds, it's ready to fire.

More likely, it just has to accumulate enough mixture in the muffler, possibly finding that piece of coke, before it gets set off.

[T-Mo-(Moderator) 4,496]

There is validity for this when shutting down, but not for backfiring on start up. My LT133 sometimes would back fire on start up when I didn't idle down properly when shutting down the time before. Again, I think the points are well made on the exhaust being the culprit on shut down, but start up.....Let's hear some theories on this. B)

[linen beige 14]

You are right to a certain extent Dale, it is vaporized as it enters the cylinder. But some condensation occurs before it gets to the muffler and is probably enough to cause the delay in ignition.

Karl, running at half throttle is likely allowing the piston to move the exhaust out at enough velocity to force the fuel/vapor mix out of the system before it can ignite.

[T-Mo-(Moderator) 4,496]

Not meaning to , but I found this - it's from a Toro rep on one of the lawn care sites - I'm not vouching for it, one way or the other.

http://www.lawncafe.com/t21767-engine-backfiring-advice.html

[linen beige 14]

Not meaning to , but I found this - it's from a Toro rep on one of the lawn care sites - I'm not vouching for it, one way or the other.

http://www.lawncafe.com/t21767-engine-backfiring-advice.html

I would ask the Toro guy one question. Why, then, does Kohler instruct you to let the engine run AT IDLE for 30 seconds before turning off the switch?

[TT-(Moderator) 1,131]

One word explains the no backfire at part or wide open throttle....

VACUUM

If the throttle plate is closed, more vacuum is created in the cylinder, which could potentially draw oxygen into the muffler through the outlet. It might be amplified because of the ACR on the Kohlers, but it still exists in all engines. (especially on multiple cylinder engines)

Turn an engine over by hand and observe.... you will sometimes be able to hear air being pushed out of the carburetor, and there is also suction (negative pressure) in the exhaust. (at very low rpm)