Thanks for the reply. The plane was bought used so the engine is broken in (if that is what you mean by run in?)
Also this happened after the engine was already warmed up. Do you think it is a too rich mixture? Thanks again.

A little more background: Engine tuned after warm up (1/2 throttle for about 30 or 45 secs), high speed adjusted from 4 to 2 cycle, lean until smoke almost disappears and richen by 2 to 3 clicks. Fuel is new.