The 257 was the semi pro version of the 254XP,it ran an open transfer port cylinder, and suffered quite badly with piston wear due to the size of the open ports. The air gap between the ignition unit and flywheel is 0.3mm, the timeing is done by the flywheel key, the key is an integral part of the flywheel, and if sheared it is a new flywheel. If the key is ok it is not a timeing issue. Does the plug get wet after cranking? I think i would lift the cylinder and check the piston condition before going too much further.
Husqvarna man
December 2009
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