Could be a dodgy spark plug, a weak spark cant ignite the fuel, could be air restriction (blocked filter) fuel restriction (blocked fuel passage) or a blocked exhaust, thease are the first few areaa to look at. For air, try a quick run without the filter (test only) for fuel try laying the bike right over and see if fuel pours from the carb overflow pipe (it should in most cases) try raising the revs slowly and see if the exhaust is blowing reasonable pressure from it, or see if it sounds particularly quiet. its a start for you anyway
Rob, South Wales, October 2009
