That one spark when you let go of the key is a good clue. Could be a bad ignitor but also very likely is low voltage to the ignitor not giving it the energy it needs to do it's job. The cure for that is electrical trouble shooting. A wire straight from a good fully charged battery to the ignitor should isolate the problem to poor connections somewhere or problems with the ignitor, coil, plug wires, or plugs. Always nice to do a two stroke tune up (buy new plugs). Since I am cheap and very anxious to hear the vroom vroom noise I clamp the spark plug in a vice and heat it with a torch, not mamby pamby warm it up, but HEAT it the ground strap should be glowing red, get the flame down around the insulator then vigorously wire brush it. Set the gap to what Yamaha recommends. While I have the plugs out I check compression.
PS do not crank the engine with a plug wire disconnected. Go grab a chunk of house wire get out about a 1.5' of bare ground wrap each end around a plug base and make sure the wire is in good contact with ground. Possible sources of low voltage are all the involved connectors wires and switches. keep us informed on how it goes. and yes needed parts are available, post in the classys here to find out who has them and how much they want. Unlike most ignitors the XS650 ignitor has some accessible replaceable parts, the how to is documented on this board.
