In other news, I rebuilt the engine a while back and replaced the crankcase because I couldn't get the bearings out of the old crankcase, and ever since then there's been rather a lot of oil on the side of the engine after each run. For whatever reason, I only just now examined the engine carefully enough to notice there's a casting defect on the underside of the exhaust mount. So I rebuilt the engine *again*, using another crankcase that I know has no casting defects. Maybe that will fix the idle-fluctuation problem; if nothing else it will at least keep the inside of the vehicle cleaner.
While I was in there, I modified the clutch shoes again; I shaved the middle of the clutch shoes flat, to reduce contact area, so they engage more like a 3-shoe clutch. Here's what it looks like now:
