Keep in mind that although all of the above is true...it could also be that you are to rich (this is rare with a stock setup as the are ALWAYS lean from the factory) or that your letting the bike sit idleing to long...this will also build up heat in the pipes and give them no "cooling" air movement....but its most likely to be the AI and to lean...
as Erwin pointed out idleing to long (no cooling) with choke (misnomer it actually richens the fule circuit) to rich
