Kohler Magnum M18QS runs but dies, need help

I have a Kohler Magnum M18QS engine, probably from the early 90's which is installed in a Bobcat 440B loader. The engine will start (sometimes) and run, but will eventually die. Sometimes it'll run for 30 minutes or more and dies, other times it'll run for 5 minutes then die. Once it dies, it's near impossible to restart. Give it a day or so and it'll start up only to repeat all this crud again.

We checked for spark early on when it wouldn't start, and it didn't have spark. It didn't appear to be the grounding/kill wire, so we bought a new coil. Today I installed the new coil, but nothing. It had spark, but the engine would not start. I installed the old coil and it fired right up. So I installed the new coil again, and it wouldn't start....so I installed the old coil again and it fired right up. I suppose the new coil isn't good?

Anyhow, I put it all together again and started it up. It ran for no more than 5 minutes and died. I haven't been able to start it since, but it has spark. I removed the carburetor to check the fuel bowl - it was full of fuel, so I assume the fuel pump is working. Since I had the carburetor removed, I went ahead and cleaned the carburetor. I reinstalled....and nothing happens.

I'm lost at this point. This should be an easy engine to diagnose, but I can't seem to pinpoint what's wrong.

Any ideas?

Battery or mag ignition system?? If battery try a hot wire from the battery to the coil and see if that works. If it does then you have a bad switch or wire

Does the carb have a solenoid valve on the bottom of the bowl?

If so, be sure it is getting power and the tip hasn't come loose and blocking the fuel.

Also check the fuel for contamination, water, trash. Disconnect the line from the carb, pump out a sample in a clean glass, see what you catch, see what kind of fuel delivery you get from the pump.

Is the gas tank vented?

Old may be onto something. I have seen terminals come loose on switches and cause the problem you are describing.

does it have a low oil pressure kill switch?
remove the wire and see if it starts.

If it is TRULY a Magnum 18 HP, it doesn't have a "coil", it has an "ignition module" that is basically an electronic version of a magneto in that it is self-contained and requires NO battery/12 Volt power for operation.

If it is REALLY losing spark, there's two things that will cause this... a bad module, or something that is loading or shorting the "kill" wire.

Have you tried disconnecting the wire from the "kill" terminal on the engine to see if spark returns?

(WHATEVER you do, DO NOT apply battery power to the "kill" terminal! You will let some very expensive smoke out!)

<img src = "http://oi68.tinypic.com/35d48iq.jpg">
Link to official factory manual

Still would not start this morning. Both plugs have spark. I cleaned the carburetor yesterday, but it wasn't too dirty to begin with. The fuel bowl was full of gas, so I assume the fuel pump is working. We even sprayed starter fluid into the carburetor this morning, but it would not start. This is frustrating me to no end!

Steve mentioned a fuel shutoff solenoid.

Does your carburetor have one?

(Covered on Page 53 of the manual I posted a link to.)

You say you have spark but is it a good blue/white and jump a 1/4 inch gap or more? Also did you by chance get the plug wires switched if you did it will not start

(quoted from post at 12:46:09 03/14/18) You say you have spark but is it a good blue/white and jump a 1/4 inch gap or more? Also did you by chance get the plug wires switched if you did it will not start

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old, that's a "waste spark" ignition system. BOTH plugs fire once per revolution, swapping the wires has no effect on timing or firing order.

Speaking of timing... you don't suppose the flywheel key has sheared and the flywheel and timing are slipping around, do you?

I had a lawn tractor that would make two rounds and quit,let it sit and it would do the same thin the next day. It had a steel pick up line in the tank that had a 90deg bend in it. There was a very small cocoon in the tank. It would slip in the pipe and stop the gas at the 90, when it sat a while the cocoon would wash out. I was ready to check my fire insurance before I found it. (25 hp Kohler)

The M18's have electronic ignition, and not a traditional coil. There is a fair amount of confusion because the nearly identical K17 engines used breaker points, condenser, and an externally powered coil. The M18's have a few other differences such as spin on oil filters and a different air filter arrangement. I'm not sure what internal changes were made to squeeze out an extra HP.

In any case, I had a similar experience with a Kohler engine with electronic ignition. The ignition module had developed an electrical intermittency. Started fine, ran fine ...for a little while. Then it would stop. After it cooled down, it would re-start so I could put it away.

I went through a fair amount of frustration, and a couple of small engine shops before I found a mechanic who, based on my description immediately identified it and fixed it in an hour or so. That was seven or eight years ago. Still running fine.

Had a similar problem with an '87 Cub Cadet M18. Replaced the ignition module.

(quoted from post at 18:22:26 03/16/18) Had a similar problem with an '87 Cub Cadet M18. Replaced the ignition module.

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Allis, in the first post of this thread the OP stated he replaced the ignition "coil".

If he truly has an M18, the "coil" is the electronic ignition module.

Wonder what the REAL story is?

Did you ever figure out the problem? I have a Bobcat 450c with a m18 engine in it. Same problem. Have replaced the ignition coil, problem remains. Am wondering about the fuel pump, seems to pump but it might just be siphoning from the tank as it sits higher than the engine.

(quoted from post at 18:36:01 04/26/21) Did you ever figure out the problem? I have a Bobcat 450c with a m18 engine in it. Same problem. Have replaced the ignition coil, problem remains. Am wondering about the fuel pump, seems to pump but it might just be siphoning from the tank as it sits higher than the engine.

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Ian, you need to start a new thread. YT has two separate forums running side by side, one is modern, the other archaic, with half of the members on each one. The archaic version buries old threads where no one sees them again, so only the "modern" viewers will see your post.