Kohler M12 won't go over an idle

[peejo50]

I have a 1990 Wheel Horse with a Kohler 12HP Magnum engine. I started it up, let it idle and warm up while I hooked up my small trailer to go out back and do some work. I started out about half throttle and within 20 feet it sputtered, backfired and quit. All it will do now is start, idle for 5 seconds and quit. I cannot get it to rev at all. I checked and I have great fuel pressure and volume, I pulled and cleaned the carb, no help. I replaced the carb for the one on my other running mower, and even used a remote, gravity fed fuel tank just to be sure. Checking the ignition shows it has good spark. It would be easier to diagnose if it just wouldn't start at all.
To sum up, good spark and fuel, nothing above an idle and only for seconds at that.
I welcome any suggestions that would aid me in the solution to this problem.

 

[bertsmobile1]

When was the last time you checked the valve lash ?
SV engines go tight over time so the exhaust valve ends up being open all the time
Eventually it get to the point that the engine will start then stop or won't start at all .
If you are realy unlucky the head of the exhaust valve will be burned beyond use .

 

[Bertrrr]

Linkage ?

 

[peejo50]

When was the last time you checked the valve lash ?
SV engines go tight over time so the exhaust valve ends up being open all the time
Eventually it get to the point that the engine will start then stop or won't start at all .
If you are realy unlucky the head of the exhaust valve will be burned beyond use .

I will check the valves out but I would find that odd since this has been a good running tractor that all of a sudden became a bad running tractor. I wanted to investigate all external solutions before getting into any internal ones.
Thanks for the suggestion.

 

[VegetiveSteam]

Checking the ignition shows it has good spark.

How did you check ignition? And you didn't mention anything about trying a new spark plug.

 

[peejo50]

How did you check ignition? And you didn't mention anything about trying a new spark plug.

I do have a good spark but replaced the plug just to be sure. So, it starts, will only run at an idle for a few seconds and dies. Start, run, repeat.

 

[bertsmobile1]

I will check the valves out but I would find that odd since this has been a good running tractor that all of a sudden became a bad running tractor. I wanted to investigate all external solutions before getting into any internal ones.
Thanks for the suggestion.

Well basics
Fuel + compression + spark = bang
You have eliminated fuel as the problem
So that leaves compression & spark.
It would be odd for an engine to suddenly go out of time without making a sudden forced stop to shear the timing key
However as it is an old engine , the magneto could have given up the ghost
If it came into my shop I would pull the blower housing off , remove the kill wire from the coil to eliminate the mower's electrics
If no joy there then it would be a leak down test or perhaps even shove the bore-o-scope down the plug hole while shinning a strong torch in the inlet then the exhaust

 

[slomo]

You blew right past Bert's suggestion. Keep looking for the easy stuff to buy and change out.

You claim to have everything you need to make it run. Something got missed.

Load test the ignition coil. Talking about taking your old plug and gapping it way out to 1/4". See if the coil can jump that gap. Or get a PET-4000 even better.

 

[peejo50]

Time to go out and start delving deeper into this problem. I find it hard to believe anything major could be wrong since it still continues to start, even if for only a short time.

 

[slomo]

Take some starting spray. Shoot some into the carb when it dies. See if it will run on carb like spray.