Back to basics,pull the muffler off and check the condition of the piston through the exhaust port, if ok, drain out fuel tank and clean re fill with fresh fuel, clean/replace the filter in the fuel tank, check fuel hose for splits or cracks, strip carb, single screw cover houses the pump diaphram and fuel screen, clean/replace, check condition of the spark plug, gap 0.5mm. More indepth, pressure test the crankcase looking for air leaks in gaskets or crank seals.
Husqvarna man, February 2010