This could be many different things man. Its not your clutch slipping, or you would hearand feel it rev. It simple to take the jets out of your carby and check them. Something i used to do when my jet got blocked, under the bonnet, rev it up a bit, then put your hand (causing a vacuum) over the intake of the carby, this will suck out any shit in your carb, but i stress, its only a temporary fix. Grr.......