Haven't experienced it myself but I would expect a check of the powervalve operation/RC servo and adjustment would be in order as well?

After you fix/replace the broken bit.

When you do get to discussing the jetting, make sure to check needle height, slow jet, etc and list ALL the sizes/positions.

Midrange is not generally main jet.

Andy has a handy tuning guide buried on the site somewhere which will help no end in establishing if there is something way off in the carb set up.