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The mower ran great a few days ago. Cranked it up this last time and it idled rough and stumbling. The throttle did nothing, and it wouldn't idle up, but it will run indefinitely like that at a rough idle until the ignition is cut. But when I cut the ignition, for a moment before the engine dies, it revs up and sounds great. You can repeat it reliably - running rough, cut ignition, revs up and sound good, turn key back on, it runs rough again. I checked the spark plugs first. They were a little fouled, but nothing too bad, so I brushed them off and replaced them. Made sure all the wiring I could find was intact and the connectors were tight. No damage I can see anywhere.

My amateur guess is that something is going wrong with the ignition system or timing, and only when it's turned off does the motor level off and run well those few seconds until it dies. It showed no warning signs before though, so I'm wondering if it's an electrical component that's just failed. I don't know what to begin troubleshooting though, since it's a bit out of my depth as a mechanic, beyond what I've already looked at.

There is an old addage that goes like this
90% of ignition problems carburettor and 90% of carburettor problems are in the ignition.

When you turn off the ignition, the fuel solenoid shoves a plug into the main jet pick up tube.
So for an instant the engine is running lean.

This is the instant of good running you hear just before it stops.

It also tells me that the magneto is not shutting off and the engine is starving off.

So you have the jackpot , both a carb & an ignition problem.

Carb is flooding so it needs a clean and perhaps a new float needle & seat
Photo instructions for that are here http://outdoorpowerinfo.com/

Also your kill wire is not grounding the magneto so pull the blower housing off and check the thin coil wire.
It should be open circuit to ground with the ignition on and closed with the ignition off.
Being this is the beginning of the season chances are some thing has had a munch on it during winter.
Shame on you for not feeding your local critters over winter :laughing:

Thanks for the help! I'll look at those the next chance I get.

It had crossed my mind that it may be a carb problem, but I was unaware of it having a solenoid. The only real experience I have with carbs is rebuilding the one from my yfz 450 quad, which has no electronics to speak of. But all of that on the mower was due a check and clean at any rate, so it's par for the course. And it absolutely would make sense about something chewing the wires, since I've already had a rat or a squirrel build a nice, big nest under the hood of this thing a couple of years ago. And I recently took down a lot of trees on my property, so there are a lot of homeless, vagabond rodents around here now, I'm sure.

No , no they found very comfortable lodgins in your mower but would appreciate a little more tucker next year thank you .:laughing:

Well, turns out my choke was sticking! The spring that's supposed to return the little plastic lever was sticking. Unfortunately, I didn't discover it until I'd taken the carb off and cleaned the inch of dust away. I put it all back together, and seem to have created a new issue somewhere by messing with it.

Now, the throttle is surging up and down regularly. The little lever on the back of the carb, going to the throttle plate, keeps going up and down. I'm fairly sure I put everything back together properly, as far as the springs and such, and my carburetor was in good condition, so I didn't replace or change any jets or disassemble anything except for the bowl and float to check the float valve. Could the float cause the throttle to open and close like that? Or some kind of vacuum issue maybe?

First thing I'd check is to make sure the choke is opening completely. A rich mixture will cause the surging, might smooth out when the motor has a load on it?

The carb is not letting enough fuel through to sustain high speed running, so the engine slows down.
This causes the governor to relax and the throttle to open WFO and that puts a stronger than steady speed running suction ( we call it signal to sound sofisticated ) so fuel gets sucked up better.
The engine over speeds then runs out of fuel again and the cycle continues.

Two reasons for this
1) air leak after the main jet
2) not enough fuel available to the main jet

So easy bit first.
Saturate the manifold region with WD 40 r similar from a trigger pack, not a spray can.
Engine faulters = air leak
the check the float bowl for debris and in particular little bits of rubber from the fuel line.
Also with the float bowl off work the float with your hand to check the fuel does not shut off till the float is horizontal.
Some carbs have a rubber seat in the float valve that can come loose during cleaning.