back-fires all the time

[marcuswayne]

I'm new to the list and have been reading some of the post, interesting.
Maybe you can help.
I have a 27 hp Kohler Command CH18-740S that I use on a small homemade wood haul truck that I built. The motor runs good, always starts up good, has great power but when ever I turn the motor off ( throttle at idle ) it back-fires out the exhaust. Sometimes when I turn the key off the motor will start dieseling for 10 seconds. I removed the carb. and cleaned replaced gaskets, all looked good. Put new plugs in, old plugs looked good. The inside of the exhaust looks black and sooty. I would think that means the motor is running rich.
marcus

 

[Fish]

Does the carb have a solenoid, and is it hooked up?
Post all of the numbers off of the tag on the engine.

 

[CR888]

Gas igniting in the muffler? I'd slowly reduce throttle before shut down as to not promote it. But I have no direct experience with these Kohler units, just a general guess.

 

[Al Smith]

In some cases"run on" is caused by carbon build up that kind of acts like a glow plug .I've never seen it on a small engine though .

 

[Al Smith]

Another cause could be a carbon build up that is causing the exhaust valve to not fully seat once it gets heated up .

 

[Stump Shot]

I'm new to the list and have been reading some of the post, interesting.
Maybe you can help.
I have a 27 hp Kohler Command CH18-740S that I use on a small homemade wood haul truck that I built. The motor runs good, always starts up good, has great power but when ever I turn the motor off ( throttle at idle ) it back-fires out the exhaust. Sometimes when I turn the key off the motor will start dieseling for 10 seconds. I removed the carb. and cleaned replaced gaskets, all looked good. Put new plugs in, old plugs looked good. The inside of the exhaust looks black and sooty. I would think that means the motor is running rich.
marcus

Sounds like the backfire preventer on the carburetor is not functioning properly. Check the wires for breaks and if good may need replacement.

 

[Fish]

The newer, quieter mufflers have more baffles that get hot and ignite the unburnt fuel sucked in after the key is shut off, is why the newer engines have a "afterfire" solenoid on the carb, which shut off the fuel when the key is turned off. When an engine is put onto another piece of equipment, the solenoid is often disabled for the engine to work without a lot of re-engineering. So I am guessing this is what happened.

 

[marcuswayne]

The carb. dose have a solenoid and is working. This is the solenoid that shuts the gas off. I unhooked the wire to the solenoid and it will not run.
#'s on motor Model CH754S Spec. # CH740----0007 Serial # 3227007441 Displ. # 725
I always let the motor idle before shutting down.
I think that I will remove the muffler to see if the motor still backfires.
Maybe I should check the valve clearance when the motor is hot, anyone know what the clearance should be when hot?
thanks for your help

 

[Fish]

Should be hydraulic lifters.

 

[Fish]

Can you snap a pic of the model number tag?

 

[Larry B]

If the engine diesels after the key is shut off then either a bit of carbon or the ground electrode of a spark plug is is so hot it is acting like a glow plug. Are you running it hard? Do you throttle it back with no load for a few minutes before you turn it off? If the engine is dieseling for 10 seconds then it is getting fuel somehow. The rubber tip on kohler anti backfire solenoids go bad a lot. Thank you ethanol
What color were the insulators of the plugs you took out? With the engine idiling unplug the solenoid and see if it dies immediately.

 

[blades]

Note : JD mower engine twin cylinder don't remember if briggs or koler- in manual states not to shut unit down in the idle mode will lead to backfiring- counter intuitive eh . so I played around a bit and yep at idle backfire every time when shut down - 1/4 throttle almost never. Go figure.

 

[JB-PlantHeirloom]

I had a Kohler 8 HP magnum that would back fire when shutting off, so, I would move the idle towards off, while choking it slightly, then when almost ready to die, choke it, then shut off.

 

[Jetwrench]

Kohlers do that. Shut it off about 1/3-1/2 throttle. The solenoid shuts off the fuel when power is removed. The coils are also grounded which kills spark. Whatever is still in the carb and the intake will cycle thru the engine. I had a 25hp on a ZTR mower and it’d scare the heck out of you in an enclosed trailer.

 

[AVB]

Why don't you just use the anti after fire solenoid as the shutdown device instead instead of killing the magnetos? I have seen several mowers recently where the OEMs are now using this system as the shutdown method. The engine runs a bit longer than with magneto shutdown but just a little bit longer.

 

[r7000]

the solenoid fuel shut off valve on the bowl of the carb... an anti-runon valve if u want to call it that, is exactly that. They are used primarily for engines that have no throttle and would often experience shut off at throttle or running at ~3600 rpm. When ignition key off happens, spark is stopped but engine is still running at 3600 rpm and WILL draw air/fuel in from the carb and belch it out the exhaust. Actually it's probably more of an epa thing, but whether it ignites or not is another matter but if you know that solenoid is functioning then... I would lean out the idle mixture screw on the carb, if that's overly rich then that would be next likely source of air/fuel to come from- through the idle passage and pilot jet and being rich it's enough to keep a hot engine running on for a bit, it's also probably why it starts really good. And if the fuel level in the carb bowl is higher than normal, because of the float setting, that would also cause richness and increase likely hood of run on.

And if the air cleaner is overly restrictive then I suppose it's possible it can act like a choke and increase the odds of run on pulling just enough through the idle circuit on carb, you might try removing the air cleaner prior to a shutdown as a test and see if the run on does not happen. The run on can only happen with an air/fuel mixture, and it has to be coming from somewhere somehow. I doubt it's engine oil you would know because of blue smoke as well as non clean spark plug.

and easy way to test run on carb solenoid, idle your engine down to reasonable speed then disconnect the solenoid, it should shut down the fuel enough to kill the engine, if engine keeps on running then that's a good indication what is happening.