managed to fix mine - found the woodruff key had sheared knocking the timing way off. A bit disconcerning to begin but turned out to be reasonably easy to fix.
Strip down so that the gearbox assembly (where the blade is connected to) is away from the main motor assembly - you dont have to disconnect any pipes or cables as you can leave the handle lose.
(covers, front shield and handle, screws holding springs, gearbox panel, gearbox)
Undo sparkplug and make a pencil note on the rotor where the rotor is now compared to TDC.
Remove the 'nut' - rectangle piece of metal which holds the clutch and rotor onto the driveshaft.
Remove the rotor and look inside the mounting hole - I could see where the key had sheared so matched that to the same on the shaft. Mine was about 40 degrees out.
Reassembled, 3 tugs on the cord and it bursts into life - well chuffed.
Hope this helps someone.
Norma Snockers
May 2011