Yamaha XS650 Static Ignition timing

The spark advance unit can be accessed under the cover on the opposite side of the head. Blocking the advance weights open with a bent piece of welding rod enables you set the timing in the full-advanced position.

Before you spring for a Boyer setup, give this a read: $19 Junkyard electronic ignition | Welcome to the XS-650 Garage USA . I put one of these setups together on My XS650, started & idled great, pulled like train. The Boyer needs to have a full 12 volts to behave properly and full advance doesn't occur until about 5000 rpm. The GM HEI ignition module makes reliable sparks down around 9 volts and the stock advance curve gives the bike a little more mid-range grunt .
You can also run the GM module with points, using the circuit described here: http://www.geocities.ws/loudgpz/GPZheiModForPoints.html