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I can get a carb from a good running B50 and try it.  grabbing at straws...is it possible that in some weird way the compression release is causing the problem?  For starting I can stand on the kickstart lever and it doesn't turn over without bouncing on it.  Perhaps it comes on and off randomly while running?  I positioned it way off the ex rocker and the current adjustment gives a lot of clearance at the handlebar lever.