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.
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.