What's the compression like? Take the recoil housing off, remove the spark plug. Turn the 'pawl' by hand, you'll feel if its smooth when the pistons in motion with no resistance. If there's any stiffness or grinding, then you'll be looking at possibly replacing the cylinder and piston/ and rings. It could also be a crank bearing or con-rod bearing so if you're not mechanically minded id suggest taking it to a repair center.