Hi, Ian, sorry to hear the troubles have not abated. I'm sure I read a thread in which Andy indicated that it is possible for "portions" of a coil (i.e., certain funtions like advance) to fail. On the other hand, thinking that your coil has been working fine until the moment you wrenched on another part of your 30 year-old saw...it is possibe but it sure wouldn't be the first possibily I would address. I am reminded of the time that everyone here told me that I was being a bit goofy to think that I got 2 bad coils in row on an 025. Once a few of these guys ran the odds for me, I jolted up in my chair, took a good hard look at the empty Dewar's bottle, and went and checked my spark plug which, by the way, was the culprit (along with the Dewar's). Oh, if you haven't done this, swap out the spark plug.
I guess I'm saying that although its possible that you have a timing issue, its more likely that you have a fuel issue. Try this....forget the carb for now.... disconnect the fuel line...put a tablespoon or so of raw mix down the carb throat, maybe two (hold the throttle open to make sure it goes into the engine). I find that if I overdo this a bit (intentionally), it will be a bit hard to start (fuel loaded), but once it does, I get 5 - 8 seconds of run time. This gives you a few golden moments to diagnose your coil. If you can accellerate the saw cleanly once or twice before it leans out, you know the coil is doing its job. You've pretty much narrowed things down to a fuel issue.
Lest I receive credit for this gonzo whopper of an idea, I must admit I just took this excerpt out of the "Bad Chainsaw Mechanics Guide," Chapter 6, entitled "Your Carburetor, Precision Fuel Metering Device or Your New Hacky Sack?"
I also feel a bit stupid for not suggesting this earlier, but I think its possible that your kill wire might be grounding out... maybe not enough to outright kill the saw every time, but it might be robbing enough juice to give you a crappy or inconsistent spark. I'd disconnect it from the switch and temporarily wrap the wire loosely in electrical tape to take it out of the equation.
