It warmed up enough today that we removed the hood. The rear spark plug hole is a problem. Can't start a new plug by hand. It's too snug. For the most part, the thread doesn't look bad, so I'm sure it only needs tuned up except for the bottom. The bottom two threads are caked full of gunk, consistent with how we found the gasket rusted to the plug two threads from the shoulder. Amazing that the tractor ran that way. The next plug forward is just fine. We didn't look at the front two plugs, it was getting too cold and dark. I've used that emergency tap from a bolt a few times. There's no way that it will clean out the gunk from the bottom of the rear plug hole. I'll have to find a tap. I can't believe that they are metric in a 50 year old tractor, and bastard metric at that. The normal thread pitches for 14mm threads are 1.5 or 2 mm. A plug is 1.25. Grainger does not carry them. Where else should I should look? Maybe an autoparts place, but I'd expect to find some Chinese junk there. I'd rather buy a decent one and have it work more than once.
Once the hood was off and we could see all the wires, we found that bottom terminal of the resistor is broken off of the rest of the unit and not really holding the wires together. I say resistor, but it seems to be labled as resistor, resistor block, or terminal block depending on which diagram I look at. It is mounted directly below the ammeter on the engine side of the dash. Going by what I can make out from the wiring diagram that Tom linked me to, this bottom terminal is just a binding post, and not connected to anything. Can anyone confirm this? A loose connection like that could explain why the spark was hot and blue sometimes, and lame at other times.
By the way, going back through the posts, often when someone writes the word 'cut', it gets underlined. How come?
