Thanks guys for the tips. I'll check the carbs but I had felt the mechanic would have checked/adjusted the sync when he rebuilt them. As to the oil inj, it had been disconnected before I bought the motor. I've changed several sets of plugs, and tried different brands (NGK, Autolite, Champion). I've also bought two more switchboxes and tried those.

My plan is to check the fire at the problem speed, under power with these "spark testers" I bouht at H Freight. These are visual indicators of the charge to the plug. I've already tried these at idle with the muffs on and all cylinders looked the same. I guess most of you know that under load things can look different. Hopefully I can narrow down the problem to one or two particular cylinders. I really think it's electrical. It idles OK (as smooth as a typical 2 stroke :-)) and it now seems to run fine at mid and top end but I'm thinking it could also be one carb or one pair of carbs or one of the reeds. Maybe?