It was "popping back through the carb/intake". I mentioned that I tuned up unit last month. I decided to replace wires/cap/rotor to see if it makes any difference. Surprisingly the problem went away... It has to been a bad wire or something.

If you don't mind... What is the purpose of the "valve, vacuum transmitting"? It looks like a filter that goes from carb to governor and has a red end and a black end. Why the color difference? Both directions appear to flow air the same way. How can you tell if its no good?
Thanks again.