I done some reseach on this, the consensus seems to be too retarded timing.
Other factors of course were spoken but I'd say 90% of the stuff I read was that putting more timing in and having it come in sooner was the cure, that way the combustion event was good and over before it entered the exhaust.
If you got a locked out dizzy ignore the above, if not I'd recurve it so that it's all in by at least 2200 rpm.
Another strong possiblity is a way lean carb and or big vacuum leak.
How does it idle?
Goodluck>Cp