When I purchased the house I'm currently living it, it came with a bonus: a 1979 John Deere 111 (yeah, that's not L111) lawn tractor. Theoretically, it ran great. In practice, however, things haven't been so wonderful (yet).
It started fine and ran very well for some time, but as the engine warmed up, it would run more and more poorly, until it finally would barely run well enough to drag it's sorry carcass back to the shed. I could mow maybe 1/3 to 1/2 the lawn before I'd have to quit and let it cool for several hours. The prevailing theory (assuming the fuel tank cap vent wasn't clogged, which it wasn't) was that the armature for the ignition (the "coil") was bad. Luckily, Briggs and Stratton produced roughly one quintillion engines that use the same coil, and still produces a replacement coil that also replaces the points in the system and converts it all over to electronic ignition. I installed it myself, and afterwards it started up and ran beautifully. I then proceeded to mow the lawn.
The next week (this week), I mowed about half the lawn (which makes a total of 1.5 mowings since the last major repair, a new record!) before the deck quit on me. The PTO switch (the '79 111 used an electric PTO clutch, nifty when it works) didn't do anything at all. I figured the clutch had failed, and I was going to be out at least $150 for a new one, assuming one could even be found. I put the front wheels up on blocks (after blocking the rear wheels) and mentally prepared myself to dig around underneath the engine.
As I was looking, I happened across the electrical connector for the clutch. This was fortuitous, since I was going to need to disconnect it anyway. As I was unplugging it (which was extremely difficult, stupid little side-clip connectors) I noticed a whole lot of bare, frayed wire. I realized that I may only have an electrical problem, not a failing clutch.
Removing the clutch itself is quite a pain. It's bolted only to the engine shaft, and so turning the bolt merely turns the engine. A strategically-placed screwdriver between the flywheel and the engine block, an end-wrench, and a hammer, however, and off it falls. Sure enough, the outer insulation (quite heavy-duty), inner insulation, and the wire strands have all been destroyed by dragging on the belt that goes from the engine to the transmission for who-knows-how-long. I wiped the grease, oil, and lawn crud off, soldered some new wires, taped up everything with gratuitous amounts of electrical tape, and reinstalled it. For the first time in a long time, the PTO engaged immediately and decisively.
I'm now at .5 lawns mowed since the last major repair, and I'm hopeful this trend will continue for some time. Maybe I'll get a couple complete mows out of this.