I have a 1999 yz400f that I took on a series of desert rides over the last week. All week the bike ran perfect, started in 1-3 kicks, and was a genuinely joyous bike to ride. Coolant always topped off, ran on non-ethanol 92 gasoline. Oil changed every 12 hrs. On the last day of riding, I started the bike first kick when I unloaded it. It ran for around 10-20 seconds before I hit the kill switch. around 5 minutes later, I started it first kick again to warm it up. It ran for 5 seconds then died. It has not started since. Fuel was on, air filter clean, and I have tried kicking and bump-starting repeatedly. I have tried choke and hot start on, off and every combination of the two. Nothing. It will not start. Why is this? I suspect a fowled plug, although I have never had a 4-stroke do this before. Bike makes good compression. Any thoughts? Thanks in advance.