| This is no informed engineering opinion BUT: it happened to me once. The bike (900SSie) had been standing for a while. After starting-up I left it running on quite a fast tickover to get it really warm. I panicked a bit when I saw the rear pipe a dull red, but rode off anyway. With the bike warmed-up and on the move, the next time I looked at the pipe - after about a 20 minute ride - it was grey again. Never happened again.
I've used the bike far more regularly since, all year round and no problems at all. Made me think: people say Dukes are temperamental but I reckon they're as good as any other make as long as they're used. Servicing's a bit pricey but the only thing that's ever gone wrong in 13k miles is the Datatool! | 
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