Air filter was my first guess. If you ever run into a mower that keeps speeding up and slowing down by itself, that is probably the air filter.

Spark plug is not my second guess, but it is relatively easy to check compared to other things, and if it turns out to be your culprit, you win. It has probably (statistical likelihood) not been inspected since it was new. Check not only the condition of the gap, but also that of the wire and boot. Unfortunately, if the wire or boot are cracked, that's the whole magneto, since they're all one part.

Since it's probably not the ignition system (that would tend to be more reactive to moisture than to operating temperature, but all bets tend to be off with these things), next guesses lead to the fuel system. You're looking for things like cracking or collapse of old fuel lines, primer bulb, etc. or perhaps a clogged fuel filter. Those things degrade (cracking, hardening, loosening) over time, especially if you don't use stabilizer in your gas, but even if you do.

That's the easy stuff. From there, you're into checking to make sure the shear key which keys the flywheel to the crankshaft hasn't sheared (from hitting a tree root or something) (thus throwing off the ignition timing), rebuilding the carb (similar effects to the fuel line discussion), etc. The aforementioned shear key inspection pretty much means taking the top of the engine apart.

It's hard enough to troubleshoot these things when they're right in front of one.

How old is the gas? It may have broken down. Also, it could be dirt in the line. If you don't want to mess with it, a local small engine repair may be your best bet. A Toro dealer will be able to fix it right up, and charge you for the name. Look for some old guy that's been doing it forever you'll get a better deal, IMO.

I had a friend borrow mine & kill it completely. ran it out of oil, and then filled it back up as far as I can tell.

Good Luck

we were just having the same problem with our mower.

on the bottom of the "bowl" (behind the air filter) there's a little bolt with an air hole in it. It took Tony a while, but he had to clean all the gunk out of it and then the mower ran fine. He says this piece isn't replaceable or he'd have just run to Ace for a new one.

I only sort of know what I'm talking about, but that's the best I can explain it.

See if there's any way to flush the fuel line(s) through, preferably back towards the tank. There may be some crud in the tank and/or line(s) which is initially settled so the engine runs fine but the flow of fuel drags the gunk up into the lines where it can restrict or block the flow and stall the engine.

I've worked on cars on and off at home for the last ten years or so and that's my guess. I had an old diesel Land Rover Discovery, which would start and run fine but whenever the fuel flow was increased by pulling away from standing or hard acceleration it would choke and stall, almost killing me at junctions several times. Some bacteria got into the sedimenter (a coke-can sized pot in the fuel line between the tank and engine/pump) and formed a really manky disc of crap at the bottom of the can like a solid pineapple ring. When the flow rate of fuel would increase the disc of crap would get sucked to the top of the sedimenter where it would block the fuel in and out pipes. It was some vomworthy crap to handle even with latex gloves on, I tell you.

Hey, unconnected to this: did yo ge my message that your package arrived? Dude, that was great, funny stuff. Much thanks!

No duder, I don't think I did! How did you send it?

I assume that your oojit got to you OK and that you'll put it to good use? :D

Replied to the original FB message. Anyway, yes, he's sitting here on my desk, staring back at me. I have a feeling he'll hold my dice and live inside the esoteric dicebox, waiting for the right moment to spread madness and random rolls.