I have a craftsman riding mower, model 917.273634 with a 3
1P777 18.5 Briggs motor. It starts and runs fine when its cold. I can cut the lawn for 1 hour or sometimes for just 10 minutes and it will stop as if i turned the key off. It seems the hotter the motor gets the more often it happens. I have gas and spark. When i try to restart it, it fires like it wants to start. After a few minutes or more i can get it running and black smoke will come out of the exhaust at first.
Where do i start?
I've blown out the gas line, checked the gas cap, i have power to the fuel shut-off sol., and spark
Someone mentioned it might have a bad thermostat that automatically shuts down the engine when it gets too hot. Never heard of anything like that.
I need help, the grass is growing as I speak

 

There is no "thermostat". I would suspect a bad coil, since this is what commonly happens when a coil starts failing. The black smoke is fuel, so it could be just the fuel that has accumulated in the cylinder while trying to crank it that is burning off.

 

So when the coil gets hot it will fail until it cools down? By coil do you also mean Magneto??

 

Might be muffler bearing,,,

 

That's what often happens when a coil goes bad, yes. I don't mean the magneto. The magneto is the entire system, including the magnet on the flywheel. The coil is what fails. It may also be called an armature, but the armature is not what fails, it is just integrated with the coil.

 

Thanks for the info, I'll try it.

 

Ok cheese.
I tried a new coil. Engine started, ran good. I shut it off, put the top cover back on, went to start it again, wouldn't start, no spark. I took the cover back off, checked everything, tried to start it again, still won't start, no spark.

 

Remove Kill wire that goes to the bottom of coil and see if it runs,,,you may have a wire that has had some of it insulation rubbed off or chewed off by our little furey friends. Good luck,,,also make sure air gap is about .010 to .012.

 

Yep, try unplugging it as mentioned. An intermittent problem can be hard to diagnose, so it's probably good that it failed hard now so that you can find the problem.

 

Ok guys, this is what happened. I bought a new coil from the local NAPA store, put it on, mower started, turned it off, would not restart, no spark. I returned it to NAPA as defective, they gave me a different one. Put that one on, same thing, started but would not restart, no spark. Took the kill wire off as you suggested, still no spark. I took that one back as defective and started getting questions from the NAPA people that it couldn't be their coil causing my problem. They gave me another one, this time i put it in and didn't connect the kill wire. It started and restarted all afternoon. The next day i cut my lawn with no problems. I still have the kill wire disconnected. So far no problems (Knock on wood). I think i'll leave that wire off if it won't cause anymore problems in the future.
THANKS guys for all your help!!!!!

 

That kill wire must have a bare spot on it and is grounding out some where. How do you shut it off??

 

I'd say you are sending voltage to the coil through the kill wire. If voltage goes to the coil, it will fry it. Use a voltmeter to check for voltage at the kill wire with the keyswitch in different positions and while turning the key to start. A bad keyswitch will sometimes do this.