Export thread

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.

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 .

Linkage ?

bertsmobile1 said:

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 .

Click to expand...

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.

peejo50 said:

Checking the ignition shows it has good spark.

Click to expand...

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

VegetiveSteam said:

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

Click to expand...

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.

peejo50 said:

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.

Click to expand...

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

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.

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.

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

peejo50 said:

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.

Click to expand...

Not trying to offend here so hope you don't mind me asking, how do you know it has good spark? What was your testing method?

peejo50 said:

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.

Click to expand...

How about the fuel itself? You've eliminated the carb and fuel delivery as being an issue by trying another carb and gravity feeding fuel (assuming to the carb?) but was this all done with the same fuel?

slomo said:

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

Click to expand...

Tried that with no results.

VegetiveSteam said:

How about the fuel itself? You've eliminated the carb and fuel delivery as being an issue by trying another carb and gravity feeding fuel (assuming to the carb?) but was this all done with the same fuel?

Click to expand...

No, I had fresh fuel.

VegetiveSteam said:

Not trying to offend here so hope you don't mind me asking, how do you know it has good spark? What was your testing method?

Click to expand...

I set the plug on top of the engine and could see a strong visible spark. I also have a passive tester that shows ignition for the short time it's running. for

peejo50 said:

I set the plug on top of the engine and could see a strong visible spark. I also have a passive tester that shows ignition for the short time it's running. for

Click to expand...

One of those inline Christmas lights does not load test the coil. Only indicates spark at that time. All electrical circuits require testing under load.

So you have air, fuel and spark. Only thing left is compression. What do you have for compression?

peejo50 said:

I set the plug on top of the engine and could see a strong visible spark. I also have a passive tester that shows ignition for the short time it's running. for

Click to expand...

Is your tester something like one of these?

VegetiveSteam said:

Is your tester something like one of these?View attachment 65647View attachment 65648

Click to expand...

The engine has a compression relief so it will only show a rate of about 35 lbs., and I don't have access to a pressure tester. The spark tester looks more like a short pen with a light in a window and a metal cap. Something I've been using for 50 years or so. Also, this has a magneto for ignition, and I will need to find the specs on it to check. I got a reply on another forum from a guy that raced sprint cars. He said he had some strange experiences dealing with the magnetos in them and this sounded like one of them. Since I've tried a different carburetor, a different fuel source and even starter fluid, plus the fact that it does start and run, I'm ruling out fuel, valves and timing and going with an ignition issue.

peejo50 said:

The engine has a compression relief so it will only show a rate of about 35 lbs., and I don't have access to a pressure tester. The spark tester looks more like a short pen with a light in a window and a metal cap. Something I've been using for 50 years or so. Also, this has a magneto for ignition, and I will need to find the specs on it to check. I got a reply on another forum from a guy that raced sprint cars. He said he had some strange experiences dealing with the magnetos in them and this sounded like one of them. Since I've tried a different carburetor, a different fuel source and even starter fluid, plus the fact that it does start and run, I'm ruling out fuel, valves and timing and going with an ignition issue.

Click to expand...

That's why I was asking about the ignition testing method to try and rule that out. Did you actually test compression and get 35 psi because even with a compression release 35 psi would be extremely low. 70 psi would be more reasonable but a leak down test is the best way to check and engine with a compression release. On an engine without a compression release you'd typically be looking at 150 psi or higher.

Good luck and if you need a manual, you can get it a Kohlerengines.com. Go to Service & Parts. When you get to the manuals page simply type in M12 in the model number field, scroll down and you will find a service manual.

This is what you need.

So reaching deeper into the problem I checked the flywheel key, removed and checked the muffler, pulled the cover and checked the gap on the tappets. All was as it should be. What I did next was to eliminate the ground wire to the mag. Low and behold it ran and even revved up. I mowed for about 15 minutes without any sputter or miss firing but it seemed to only be running at about 90 percent. I shut it off but had trouble starting it again. It would continue to run but it was a struggle to get it to rev without backfiring. At this point I can only figure that it must be the magneto. I can't understand how I could go from having the best mower ever to the worst nightmare with just a burp!

Solid state electrical devices have a finite life and it sounds like your magneto has come to the end of it's life

bertsmobile1 said:

Solid state electrical devices have a finite life and it sounds like your magneto has come to the end of it's life

Click to expand...

Nope. He had strong spark pack on page 1. He has solid fuel flow and valves are in spec.

I can only count on one hand, the number of valves, that were in spec, on a used new to me mower engine. Think I have a couple spare fingers too. Have to trust your valve checking procedure.

Again something got missed.

Post up a video of it starting and running.

bertsmobile1 said:

Solid state electrical devices have a finite life and it sounds like your magneto has come to the end of it's life

Click to expand...

Exactly it. Though I had what appeared to be a good spark when I checked the plug, it was inconsistent when under a load causing the missing and back firing. It was odd that it would only run at idle until I cut the ground wire for the kill switch. Doing that increased the rpm's though running like crap. Put a new magneto on and I'm back in business.
Thanks for all the input, folks. Hope this helps anyone in the future with the same issue.

Glad you got it figured out and thanks for coming back and letting us know.

That's exactly why we were asking asking about your testing method. I don't know if you were using a tester that is something like the pic below but if so you're far from the first person to get fooled by it. It doesn't take much energy to make it glow. They can prove no spark but they can't prove good spark. A spark plug held against the block is also not a good test and potentially dangerous. You may see some fire jump the spark plug gap but if it's weak fire it will get blown out like a candle in the wind once the plug is back in the engine and under compression.

Hopefully that Magnum runs many more years. Definitely one of the best engines ever made IMHO.

peejo50 said:

Though I had what appeared to be a good spark when I checked the plug, it was inconsistent when under a load causing the missing and back firing. It was odd that it would only run at idle until I cut the ground wire for the kill switch.

Click to expand...

Main reason why, you need to test ALL electrical circuits under load. As in a good ol' PET-4000.