I understand and accept your point redranger. If it were a two stroke snowmobile engine lean is a death sentence. Or if you had modified compression and timing, I might worry more about pre-ignition and detonation. I see this in sleds a lot in the winter. I don't think I've ever seen it on a stock Honda four stroke tho. They are amazingly resistant to excessive heat. I have seen lots of four stroke pistons destroyed by lack of or dirty oil, foreign matter ingestion because somebodies air filter didn't get oiled or fell off, but I can't ever remember seeing any sign of abonormal combustion (like piston holing, etc) like I do on sleds, or a failed top end part because of excessive heat alone. I've noted this glowing headpipe behavior on fine running and totally stock machines, including 85 big red, trx300, trx350, trx450. This question comes up when people use their machines at night. I've even discussed this with two different Honda reps over the years. They can do this and it doesn't necessarily indicate a problem. I bet your two machines will do the same thing under the right conditions.