I have a 25HP Kohler Command on my Grasshopper 725K. After its gotten hot, it will run-on (diesel) or backfire upon shutdown. Is this timing? And if so, how does one set it?

Check the cooling, is every thing clean and getting good air flow... If so then,
Try a higher octane rated fuel...
Ethanol fuels will loose some of the octane rating as the alcohol evaporates, so your fuel could be lower than you thought... KennyV

All the cooling fins are clean...it feels like it has good airflow. The engine does feel hot to me (compared to other engines, but this is the first bigger Kohler I have ever owned).

My gas is fresh, as in less than a week old with stabilizer...the gas should be OK?

I did find out that these Commands are supposed to be shut down under throttle, not at an idle...so that might be my whole problem right there.

Yep... too lean and you could have this, a ny thing that would contribute to lean mix, like a slight air leak between the intake and carb... OR closing the throttle.

I have never cared for shutting down an engine with the throttle still open .. but if it works... It will save you having to replace the muffler from the cumulative backfire damage... KennyV

I did some thinking, and remembered the PO told me he replaced the plugs recently. Someone mentioned that one of the only ways timing changes on these engines is by spark gaps...turns out the plugs were gapped at 0.040" to 0.045" as close as I could tell, they are supposed to be 0.030". I regapped, but haven't had a chance to run it...

I also added some Seafoam to the gas, and sprayed the carb out with carb cleaner. Probably mow the grass tonight or tomorrow, pending rain. Since I sprayed the carb out and the additive in the gas, I'll probably mow one time then change the oil again.

Originally Posted by steved

Someone mentioned that one of the only ways timing changes on these engines is by spark gaps...

Plug gap will not have an effect on timing.. It will have an effect on the arc quality... KennyV

Wider gap requires more spark energy to jump that gap, which requires a longer duration of the magnet passing the coil, therefore...it affects timing. The larger gap retards timing...the plug is firing after the point it should fire.

We (the Wife, I watched) used it last night and it is running cooler, which indicates to me that the spark gap was causing an issue. I could actually touch parts of the shroud, and the oil cooler and filter were blazing hot either.

Originally Posted by steved

Wider gap requires more spark energy to jump that gap, which requires a longer duration of the magnet passing the coil, therefore...it affects timing.

Theoretically correct But since the velocity of the magnet is not going to be changed... the saturation of the coil will remain the same... Put a strobe on it and you will find the timing remains constant ...
you will however effect the spark quality ... and if stretched too long you do take a chance of a misfire because it is too wide ...
In any case if you are satisfied that it had a positive effect that is all that really matters ... KennyV