I had a similar issue on my GS500. The screws holding my timing plate in place came loose, letting the timing advance in odd ways.
Didnt figure it out unitl they both came loose and the timing jumped 90 degrees
. Bike didnt like that, at 40mph on a back road. luckily, i happened to stop in the driveway of a real nice fellow with a well stocked garage
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Current bikes; '99 R1100GS ABS, '80 CM400A Hondamatic