I am trying to fix an ignition problem with my 1977 Yamaha XS 650 motorcycle. There is no spark at the plugs.

Try new wires then check the coils under the tank.
Also try http://www.650motorcycles.com/ for all sorts of info on your bike

Change your sys. To 1980 elec. Ign.
Its apain, but it ends there [if you do it right the first time.]

You may also want to check out www.xs650chopper.com (http://www.xs650chopper.com/2009/01/1970-yamaha-xs650-bhopper/) Use a voltmeter and check each component. Work your way through the systems one component at a time.