Trx450ES runs fine when cold, starts misfiring quickly when warmed up. Hooked up timing light to watch for
Misfires and discovered that there is no light after initial start-up. Will, however, "light up" when I hit starter switch. I have a 1996 that lights up normally. Wondering where to start...

 

Sounds like the start switch would be the place to start. If you need a service manual here's a link that may help.http://redirect.viglink.com/?format=...air_manual.pdf If the free site doesn't work here's one for 10 bucks.
http://www.reliable-store.com/produc...-2004-download

 

May check the blade fuse in harness that goes to charging system,,May want to check charging system before any trouble shooting..also if not charging may want check voltage regulator which is very common issue on this atv.