Thursday night I had it running until I cracked open the throttle. The motor bogged (lean?), burbled (rich?), backfired (WTF?) then quit running. Tried restarting it, but nothing aside from a few pops in exhaust. Okay, quick check of the basics. Air? Yep, still breathing. Fuel? Check! Spark? Hmmm, kind of weak. Pulled the flywheel cover and noticed something strange...
According to the book I have there should be a nut holding the flywheel on, not a bolt. So I removed the bolt and noticed the woodruff key was missing. Also the threaded end of the crankshaft has been broken off. A hole was drilled and tapped for an 8mm bolt in the end of the crank.
I pulled the flywheel off by hand, which usually requires a flywheel-puller.
Found what was left of the woodruff key stuck to the magneto.
None of the local Honda dealers (tried three) had a 3mm x 12mm woodruff key in stock. Same key is used on current model XR's and CRF's. So I bought a standard 1/8" x 1/2" woodruff key and, using a bastard file, re-shaped it to fit the crank and flywheel.
Before CDI was all the rage, this is what Honda used to advance the timing on the XR75. Simple, but effective.
Once I got the woodruff key shaped to fit I used a series of bolts and washers, along with a soft-blow hammer, to press the flywheel onto the crankshaft. Once it was all the way on I used a new 8mm bolt with a hardened fender washer and medium strength loctite. Then re-checked the timing, which was dead-nuts on.
It's difficult to tell from the photos, but it appears a catastrophic failure occurred to the left side of the engine at some point. Most likely the chain broke and bound up around the countershaft sprocket. The ignition is no longer water-proof, so I'm going to have to get crafty with some aluminum-stock, devcon and a file. If I ever have to split the cases, I'll have a piece welded in then mill it flat on a Bridgeport.
Now it's time to fix the leaking petcock!