I had a similar problem with my YS170 with a CDI conversation. It would miss fire on take off.
I traced the problem to a faulty CDI unit.
To find my fault, I disconnected the CDI unit spark plug cap and the three wires from the rear crankcase.
Replaced the spark plug with a new OS F glow plug. Set the needle rich and flew. For me the problem went away on glow.
Going back to spark again. I tryed another after market CDI unit borrowed from a friend, and the engine ran perfect
Bought another CDI box, and all is good.
Hope this might be of use.
Mike