Super Hot Expansion Chamber
 

So hot the rubber mounts baked off last year, dry leaves that get stuck on it start to smolder and last weekend the rubber coupling started on fire! If I hadn't been in a sand pit I would have lost the whole bike!
It's been hot since I got it (used, barely), but not so hot that the coolant boils over. I've retarded the ignition, jetted it richer and it still runs hot, much hotter than other bikes after the same ride. Please someone help me or I'll have to go back to riding my Alpina.

I think you're going in the wrong direction.
A rich mixture will make the pipe hot, retarding the timing makes it worse.
What is your jetting specs?

Lean fuel mixture is the reason so many motorcycle engines seize when ice raced: they run too hot. When the temp gets colder and the air gets denser, richen the mixture accordingly. That was my reasoning anyway. The excess fuel absorbs heat. As for the timing a retarded ignition produces less power, hence less heat. If you can quote a text book to the contrary I'd be grateful.
Jetting as received (I bought it used and the bike was dealer prepped) was a 122 main (I think 118 is stock) and I went to a 126. The plug color with the 118 was brownish black so I figured it could go richer and might run cooler. Coolant level after the fire was up there; it didn't lose a drop. Pump impeller is OK and I checked the thermo-switch in hot water, it's OK too but I don't know at what temp it opened. Radiator was clean and unobstructed.
I've since installed a manual switch for the fan instead of the thermo-switch. After I start it and it warms up a little I'll turn it on. See if that helps the pipe run cooler.
Thanks for your interest.

Actually gasgasman is correct. A rich mixture and/or retarded timing can increase exhaust temps due to the mixture still burning when the exhaust port opens. Not saying that this is definately your problem, but the man is correct in his statement.

Your not really doing yourself any favours in the please help dept by posting such a limited amount of information. But hear goes. if your coolent temp is ok, plug reading looks good, you A might have a exhaust leak causing a lean condition post exhaust port or B as suggested allready, ign timing is to retarded ,C You have some strange in pipe ceramic cat or emisions air injection thing going on.

OK, I see your point. I'm going back to a stock main jet and timing.