Your first sentence says it all- - - - -"headers". Look at the gasket seal (or lack of it) around the exhaust ports. You've got a pretty major air leak there, which is doing the same basic thing as an extremely lean mixture- - - -overheating the valves. I'm a little bit crippled from a bike wreck a few years ago, but I think I could still outrun somebody who wanted to give me a set of headers!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!