No start: Have spark, have fuel, have air

[isaac]

Hey guys, new to here but have had my 1999 GZ250 for nearly 8 years now. It ran great that whole time aside from eating its starter clutch at 2,000 miles (never heard of this ever happening) and its most recent no start. It has 12,000 miles now.

It ran fine before it was parked a few months ago. It's been parked before, gummed up carb, cleaned it out and all was fine. This is not that problem.

It has fuel, air and spark. It has compression (it didn't just disappear while the bike was parked). The intake and exhaust are sealing as well as they ever did. It ran before just fine with the exact same configuration it has now.

Why I'm completely stumped: It has seemingly everything necessary to run, yet it doesn't even kick. Cranks over just fine, but no firing. It will on rare occasion backfire through the intake or exhaust. It seemingly refuses to fire inside the actual cylinder.

Fuel: I've tried spraying starting fluid, spraying gasoline, putting oil in the bore to rule out a gas washed cylinder. Of course I've already cleaned the carb. It seems to flow fuel just fine. I also had changed out all the fuel from the tank, and replaced the air and fuel filters. I

Spark: Changed the plug 4 times to rule out fuel or oil fouling. Changed the ignition coil (which seemingly helped nothing). Cleaned all grounds. Cleaned ignition switch contacts. It has throughout all this had a nice blue spark, regardless of gap, which I have also changed repeatedly. It seemingly fires in regular intervals while cranking.

Air: I've sealed it all off, replaced the intake manifold and seals. Tried different filters. Tried just having my hand over the intake and varying it. Varied throttle and idle stop all over the place. Varied the mixture screw all over the range. I've adjusted the valves and they're well within range. It compresses just fine.

So what else could it be? Does the CDI ever "half break"? Does the ignition pickup in the side case of the engine ever send a signal that's completely off timing? Those seem about all that could possibly be left assuming it's not just cursed.

Ideas? Thanks!