18.5 HP B&S riding mower starts fine, dies suddenly

[Pahl]

I have a 2004 Murray 42” riding mower with an 18.5 HP Briggs and Stratton (Model #31N707) that starts without a problem, runs with absolutely no sign of problems, then after about 8-10 minutes of mowing, it dies suddenly, and will not even turn over when I try to start it. If I let it cool for about 15 minutes and try to start it again, it will turn over, but won't start. If I try again the next day, it will start as if there never was a problem , then die suddenly after about 8-10 minutes of mowing, and the whole cycle starts again. It will die completely randomly, whether I am climbing a hill, descending a hill or on level ground. It also does not matter whether I am cutting through thick grass, or making turn on a part of the lawn that has already been mowed.

When I first checked the engine, I thought the problem was a the wire from the magneto to the spark plug had been chewed by a mouse that lived in the engine while the mower was in my shed over the winter. I replaced the magneto, replaced the spark plug, cleaned the air filter, and changed the oil.. It seemed to fix the problem, until it died suddenly and backfired a couple times after mowing about 8-10 minutes.

Because of the backfires, I thought the carburator must need cleaning. I took off the cup of the carburator and the pin assembly. The inside looked completely clean and pristine, so I did not go through the process of taking off and cleaning the entire carburator . Besides, I don't think that it is a dirty carburator because the engine runs completely fine until it instantly dies.

Besides the “dead man” switch under the seat, is there some other kind of kill switch that may be the source of the problem? Does anyone have any other ideas about what might be happening? Any help would be appreciated greatly.

Again in summary, this is what I have already done:
- Used fresh gasoline
- Replaced the spark plug and magneto/wire assembly
- cleaned the air filter
- replaced the fuel filter
- cleaned the cup of the carburator – (it was so clean that I do not think that the rest of the carburator could be dirty enough to be the source of the problem)
- inspected the “dead man” switch under the seat – no signs of problems

[Walt 2002]

Well it almost sounds like an ignition switch problem to me but I would do a bunch of checking before replacing it. There will either be a screw terminal on the throttle box with some wires junctioning on it or a single kill wire coming out of the cooling shroud on the opposite side. Either way, disconnect the kill wire and see what it does. I would also check the solenoid on the bottom of the carb. Is there current to it when the engine stops? That would not explain the "no crank" problem though while a bad switch might explain both IF there is no current to the carb solenoid when it stops.

Walt Conner

[38racing]

On the switch idea maybe pull the plug off the switch and wiggle it back and forth as you put it back on in case it's just bad connections.

[dewaynep]

You said you had a mouse chew though the coil wire? There is probably a mouse nest still under the flywheel causing the engine to overheat and locking it up. It frees itself up after it cools and the cycle starts again.

[Pahl]

Thanks a ton for all the replies. I will take it apart further this weekend and let you know what I find. I need to get a new torque wrench so that I can put the fly wheel on at the right torque setting.

I have a jungle growing in my backyard now, so I hope I can fix it soon. Maybe I can sell tickets to the jungle and pay for the mower to be fixed by someone who knows what they are doing ...

[Walt 2002]

A mouse nest is very rarely under the flywheel and if it were, it would not cause overheating seizing and releasing repeated. This engine will seize the connecting rod to the crankshaft, almost never the cylinder before that happens and rod to crankshaft is fatal.
IF there was a mouse nest in the cooling fins/cooling shroud, surely he would have seen and cleaned it out when replacing the CDI.

Try the things I suggested before jumping into something that isn't needed.

Walt Conner

[Harryc]

Remove the fuel cap and drive it around for 15 minutes. Does it still stall out?

[jimw1949]

Remove the fuel cap and drive it around for 15 minutes. Does it still stall out?

You have me a little confused here, so I hope you don't mind me asking you a question. I understand how a clogged vent on the fuel cap would make the engine stop, but why would it prevent the engine from turning over when he tries to start it?

[Harryc]

For the same reason why it stopped in the first place. Vacuum pressure in the fuel tank (caused by a clogged vent hole in the cap or a broken vent/cap) preventing the flow of gas to the fuel pump. After the machine sits awhile, the vacuum pressure dissipates and fuel flows again. This is a very common problem, and the symptoms are almost identical every time. Not saying that is the problem here, but it could be ...just something to try.

[David]

Possibly two separate issues? (A) something causing it to conk out-like fuel flow restricted or ignition device getting too hot after it has been running awhile
(B) hot cranking problem when engine is fully warm starter will not turn it over until engine cools down some-like some Oldsmobile 455s would park in the rest area along the Interstate and they couldn't crank well enough to start up again until the car had cooled off for about 20 minutes ----- There is a big grade going up (Sideling Hill?) in Maryland I think it is Rt. 68. At the top is a rest area with some good views where cars pull in for the scenery and some of them used to have restart problems until they cooled off a bit-they had just come up the grade before they parked-