Change the plug first!
Then change it again, they are cheap!
If you have fuel, compression and ignition.......at the right time......it has to run, it has no option!
As Fish said check the flywheel key and the air gap between the coil and magnet if the above doesn't solve the problem.
I have seen both of those produce a good spark when the plug wasn't installed and NOTHING when you put the plug back in.
Mike
I haven't checked the ignition timing, and if its off that could explain what I'm seeing. I'm totally new to chainsaw repair, so I need a little coaching. Is the flywheel key a fixed woodruff key type of arrangement, where it would have had to physically break or fall out to change the timing, or is there something else that's adjustable that I need to check for proper alignment?
I'm pretty sure the air gap is correct, but I need to get a non-magnetic 0.3mm gauge to make sure. I used a piece of cardboard and micrometer, and it seemed to be just about right. Or, can I just put a regular metal feeler gauge in there?
Regarding some of the other suggestions:
Fuel tank pressurization:
I'm not sure exactly how to check this, but have done this so far: Removed gas cap, put it back in. Removed fuel supply line from carb, no gas shot out. Blew into supply line, waited a second, fuel then flowed out of line. I think this showed two things, one that the fuel filter was not plugged, two that the tank held pressure, but I don't know if this tells me anything or not. Any other tests I should do?
Fuel delivery:
I don't know if the impulse line is working properly or not, but I'm not sure exactly how to check that. I think that works off of crankcase vacuum to suck fuel into the carb diaphragm, is that correct? If I'm seeing fuel on the plug can that problem be ruled out?
Manually priming fuel:
I would like to do this, but I'm not sure how much fuel to squirt into the cylinder. Too much would probably flood the plug, too little may not ignite. How do you know if you've put the right amount in?
Pulling muffler:
I know this is easy, but I have one question. What's the chance that 8-year old bolts will just snap off in the cylinder when I try to remove them? I really don't suspect compression since the saw ran fine last time, and I have no reason to believe the piston/cylinder was damaged during the last usage. I don't want to create a new problem for no reason, so I've avoided this step for the time being. If this is zero-risk, and I can reuse the old gasket, then I guess I have nothing to lose. If I need a new gasket then I have to go pick one up. I also have to go buy a long allen wrench socket since the one I have isn't long enough to reach the bolt heads.
New plug:
I'm going to do this because its so simple, but in 35 years of experience with cars and motorcycles I've never seen a bad plug yet, but I guess there's always a first time.