I had same problem with my C50 it turned out to be the plug tracking down the outside instead of across the plug points. Changed it and no further problems.
If it you have a spark and you change the plug it could be loss of compression, fuel blockage jets or filters. Trial and error in these sudden faults.
sorry can't be more helpful. You could hope a m/c mechanic replies soon.
Nobby
August 2008
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