Check that the piston and values are lined up at TDC first. Whilst the ignition timing is fired by the flywheel as the previous reply said, the piston and valves must be lined up correctly so that the spark plug fires at TDC. To do this remove the upper engine cylinder panel on the side to show the cam chain. There will be a mark on the wheel which when at 12 o'clock should see the piston at the highest postion. You will know this for sure if you also remove the circular panel below on the same side of the lower engine. It too will have a mark which should also be at 12 oclock so that everything lines up. If those are fine then look for restricted fuel flow next...so clean carb, air filter, fuel lines etc. Failing that then look for sticking valves and poor compression. Poor compression can be a bit more complicated to solve. Split gaskets or failing piston rings are usually a good place to start. In rare cases the piston rings can line up so that the gaps are in line causing blowback.
Angela, February 2008
