I agree with Brian’s comment about your ignition timing, have you tried a strobe lamp on a timing mark on the flywheel?
I would be interested to know a little more about your Boyer ignition set-up, which amplifier box are you using, 6 or 12 volts and where is the trigger plate mounted?
My interest is due my own bike fitted with a BSA/Triumph twin four stroke kit, which I am informed by boyer that in this application will produce half the advance at half the revs. This is evident when using a strobe lamp. Their solution is to use an amplifier box from a model where the trigger plate is activated by the crankshaft rather than the camshaft system usually employed. Food for thought. Hope this is useful.