I had a similar problem on my twatsoo 70.
It turned out to be the plastic conections on the throttle linkages that had partially jumped off.
the symptoms were that it would rev to aprox 4K rpm normally and then the motor felt like it was "searching" for fuel when trying to go faster. In this case I recon that at full throttle the timing advancer arm would have been at its max position but the carbs would have been only 2/3rds open because the plastic links were not in the correct position. (does that make sense?)
It was an easy fix though, I just cliped em pack on and then I wrapped some fuse wire around em so it wouldn't happen again.
Im not sure if this is of any use to you but it is worth a quick check.