Take a guess (I know this is a car site)
Pulled out the XR400 to go riding in the woods after being stored for a long time w/ occasional starts... it is a 1997. It starts up, sounds ok @ idle, no head noise, good compression. Overall, this bike looks like a virgin prom date & has very low hours. Fresh 100 octane fuel, clean air-cleaner too.
In less than in a minute, one of the two header pipes is very hot... in two minutes, it is glowing orange (both are hot) A little popping on downside of rev. Never had a problem with her before... it is air cooled & a 4 stroke which is hard to kill. I have never pulled one apart... it is forien to me. It has 4 valves. Anyone ever heard of this??? Stuck valve???