250-500cc Starts but Immediately Dies

My son's 1996 250 WXC starts but dies instantly; more kicking brings little to no run time until it absolutely won't even fire. Ran fine up to now, on our last ride, when after a rest it did this: would start but then died, popped once or twice thereafter, then never again. We use the lean-it-over-until-it-leaks and just-past-TDC-method which worked so well before.

• Compression is as strong as ever; low hours on top end. Check
• Plug looks fine (not fouled, carbon or oily); bright spark seen in strong sunlight. Check
• Fuel flows out of overflow as usual; carb internals (float height and movement) good. Check
• No jetting changes from before. Check
• Reeds look new and seal well. Check
• Everything else tight and snug. Check

What to check next?

Is the plug getting wet with fuel?

Check and make sure your fly-wheel is not loose or sheared the key.

Thanks for the suggestions.

Yes, the plug shows signs of fuel being present.
I can see through the pilot jet and side-spray nozzles, and nothing came with it when it was removed. Air pressure can be blown though the circuit. Same with the main.
Fly wheel looks tight on from never having been removed or loosened. The key pin cannot be seen; if it in the correct position on the crank - I need a flywheel puller to get it off. There is corrosion on the surfaces from not seeing air/being washed though since it has adequate spark I might discount the condition but the position - exaggerated advance/retard - would produce spark though at the wrong point; enough for it to initially fire up?

Pulled the fly wheel and cleaned up the surfaces. Reassembled. No start.

Next step was disassemble to replace the CDI. In the process I saw in the harness where one of the female leads had backed out of the connector. Light bulb!

Pushed it back in and it fired/ran great on the second kick.

Great,good fix