could be a number of things. need alittle more info. How many miles on the bike? If it has high miles could be time for a top end rebuild, they don't last forever. Have you done a compression test on it.
has the bike ever been swamped? if so this is not good, to be done right the hole motor will need to be torn down and cleaned out.
has the bike ever been swamped? if so this is not good, to be done right the hole motor will need to be torn down and cleaned out.