Why do my Kohler engines backfire when shut off?

bczoom · Apr 20, 2005

I have 2 machines with Kohler engines.
Garden tractor with a 18hp 2 cyl Kohler and a ZTR with a 23hp 2 cyl.

If I run them at higher RPM's (mowing) but don't let it idle for a few minutes before shutting them off, they let off one heck of a backfire.

Why is that? I have no other engines (Honda, Subaru Robin, B&S...) that do the same thing.

Bob_Skurka · Apr 20, 2005

Thinking about the 4 cycle engines I have now, here is a rundown of what I can recall:

2 cylinder (opposed twin) 18hp Kohler that never backfires (on a Cub GT).

5hp Briggs that backfires every time it is turned off (on a Yardshark chipper) but runs fine otherwise.

2 cylinder (V twin) 18hp Briggs Vanguard that never backfires (natural gas generator).

Last spring I got rid of a MTD rear tine tiller with a Briggs 5hp (?) engine that backfired constantly when shutting it down but ran great otherwise.

I have a 6hp (?) Kawasaki on a 12+ year old JD lawn mower that runs perfectly and probably backfires less than once a year. I also have a Kawasaki (21hp) gas engine in the Ventrac GT and that has never backfired that I can recall.

briggs_man · Apr 20, 2005

( but don't let it idle for a few minutes before shutting them off, they let off one heck of a backfire.
)

Well you just answerd your own question,,,, Plain as day states in my Kohler shop manual and operators manual . Engines WITHOUT the fuel shut off soloenoid in the carbarator to Let Place throttle at low idle and let engine run 30-60 seconds then shut off engine ,,,,,,, My 25 hp twinn if not idled down will not only afterfire but also diesel up a storm so every operator now sees a tag taped to the dash about shutdown .

wushaw · Apr 20, 2005

I have a 15hp Kohler on a ZTR and I let it idel for about a minute before shutdown and it has never backfired yet. It's all in the shutdown procedure.

pajoube · Apr 20, 2005

I have a 25 hp Kohler and if I do not reduce the throttle and let it run for a few seconds at low RPM it will continue to chug, chug after key turned off. By doing the above it will shut down almost immediately.
PJ

B2400 · Apr 20, 2005

When you shut the engine off( especially from a high rpm) you load the exhaust with raw gas fumes. The exhaust is hot and ignites these fumes, thus you get a loud "pop" thru the exhaust.

KENSFM · Apr 20, 2005

A bad exhaust valve or too lean a fuel mixture will also cause an engine to backfire. For some reason Kohlers do seem to backfire more than other makes.
Ken /forums/images/graemlins/smile.gif

zapper · Apr 20, 2005

I also have a backfiring kohler on my ZTR Grasshooper (which only happens on shutdown). This has been going on since the unit was new so I contacted Kohler. The represensitive said when turning off the engine turn the rpms up (vs idle) and this seem to solve my problem.

GerardC · Apr 20, 2005

The represensitive said when turning off the engine turn the rpms up (vs idle) and this seem to solve my problem. 

On occasion my Briggs would backfire. I tried turning the idle up (all the way) and then turn the key off (right away). We never get a backfire doing it like this. Sounds like you got good advice.

Bird · Apr 20, 2005

I can't claim to know much about these new engines. All my life I've let engines idle briefly before turning them off. However . . . , when I tried out the Toro ZTR with a 14hp Briggs engine, I pulled the throttle back to idle, turned off the key, and was startled by a terrific backfire. /forums/images/graemlins/shocked.gif The dealer was standing there and said, "Yeah, they all do that; you're supposed to shut it off at full throttle." /forums/images/graemlins/shocked.gif Now I'd never heard of such a thing, but to quote my manual:

" Stopping the Engine

1. Move the throttle lever to Fast

2. Move the PTO to Off

3. Turn the ignition key to Off"

Now I've tried it a couple of times since I bought it. If you turn it off at idle, you wonder if it's going to blow the muffler off, but if you turn it off at full throttle, it dies quietly and smoothly, as it should. /forums/images/graemlins/confused.gif

JINMA224XR · Apr 21, 2005

If I run them at higher RPM's (mowing) but don't let it idle for a few minutes before shutting them off, they let off one heck of a backfire. 

Hey bczoom,

Explosion in the exhaust is a result of unspent fuel, hitting the hot exhaust.

It may be a result of the carburetor being set too rich. Or a dirty air filter. I would pull the plugs on a cold engine and inspect the Sparkplug insulator's color. Perfect color is slightly tan. Too rich and the ceramic insulator will be dark. If so, make sure that your air filter is clean and/or adjust your Fuel/Air Mixture.

On the bigger engines, it may also have something to do with the Exhaust Emission System as well. On some Motorcycle Engines, there is a fresh air inlet valve that leads into the exhaust chamber to fool the EPA EXPERTS, into thinking that the engine is burning cleaner, but in actualality, it is just diluting the unspent gas with more air. /forums/images/graemlins/smile.gif When that air inlet valve leaks a little bit, upon shutdown, it will provide fresh oxygen to a little bit of unspent exhaust gase and cause an explosion as well. On my Motorcycle, I stuffed a bearing up into the hose just prior to the air inlet valve going into the exhaust side of the head. It took me a long time to figure that one out. /forums/images/graemlins/smile.gif

At any rate, an explosion in the exhaust system is not helpful to any engine and a remedy should be found, before noticeable engine damage results.

Have a nice day,
Joe/forums/images/graemlins/cool.gif

ByronBob · Apr 21, 2005

What does the manual say about this? I remember in my Craftsman tractor with a Kohler it says you need to idle for 30 seconds before shutdown. I'd re-read the manual first and go from there.

bczoom · Apr 21, 2005

( What does the manual say about this? )

HUH? Read a manual? /forums/images/graemlins/shocked.gif /forums/images/graemlins/grin.gif

Other engines I have that are the same size (18-25 hp B&S or other) in things like generators do the shut down from full throttle and don't have the problem. I'll try it with the Kohler engines but I don't think it works on the ZTR as one of the main reasons I shut down from full speed is if one of my kids is approaching or calling me.

I'll read the manual but the fuel mix in muffler sounds right and will let idle for awhile.

Brian

frank_f15 · Apr 21, 2005

hey Brian: with my 18 hp Briggs, if i don't throttle down for a second or 2 , it will do the same thing.

valko · Apr 19, 2012

zapper said:
I also have a backfiring kohler on my ZTR Grasshooper (which only happens on shutdown). This has been going on since the unit was new so I contacted Kohler. The represensitive said when turning off the engine turn the rpms up (vs idle) and this seem to solve my problem.

I have a 25HP Kohler in a Scotts by John Deere. I purchased new in 2000. The serv, manual tells you to idle down before shutdown. It always backfired and several times wiped out the head gaskets. After checking several web sites, suggesting to shutdown at full throtle, which I did , it hasn't backfired yet. I was told that the electric fuel pump will continue to pump fuel into the system at full capacity when shut down quickly at low RPM's. I'm on my third set of head gaskets and when they blow you know it by the smoke filling up the neighborhood. I'll keep my fingers crossed.

grsthegreat · Apr 19, 2012

valko said:
I have a 25HP Kohler in a Scotts by John Deere. I purchased new in 2000. The serv, manual tells you to idle down before shutdown. It always backfired and several times wiped out the head gaskets. After checking several web sites, suggesting to shutdown at full throtle, which I did , it hasn't backfired yet. I was told that the electric fuel pump will continue to pump fuel into the system at full capacity when shut down quickly at low RPM's. I'm on my third set of head gaskets and when they blow you know it by the smoke filling up the neighborhood. I'll keep my fingers crossed.

i agree. my Huskvarna (sp?) riding mower will backfire if i idle it down prior to shut off. but wont do it if shut down at full throttle

tcartwri · Apr 19, 2012

Use high octane fuel and it will not backfire. The raw fuel is getting lit off by the hot muffler, and this is why you idle it down after use to give the muffler time to cool off. I can also see that setting the throttle to full will lower the vacuum signal to the carb and prevent it from drawing in more raw fuel after the ignition has been cut. I have never tried that though but it makes sense.

I will get the occasional backfire in my briggs twin if I don't idle it down for 30 seconds or so.

RandyT · Apr 19, 2012

How the Kohler carb works at shutdown. the fuel solenoid on the Kohler shuts off the main jet of the carb but not the low speed idle circuit. when at idle the engine can pull fuel from the low speed jet when shut off causing the afterfire. At the high speed position the governor opens the throttle when shut off because the engine is slowing down and tries to get fuel through the high speed jet which is blocked by the solenoid therefore no fuel can get through to the muffler to cause the afterfire.

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