i have had this problem on a number of occasions and find that it is something to do with the automatic valve lifter that reduces compression so that the starter motor can spin the engine . if you sit on the bike and feel with your fingers under the right side of the dummy fuel tank at the front of the bike you will feel a ve rtical wire at the top of this wire there seems to be a small hexagonal metal slide if you put a little copper grease on to this slide it shoiuld eliminate your problem