I was having the shudder and growl problem when accelerating between 0-7 MPH. Just picked up the scooter yesterday morning. The dealer replaced the gearing weights (is that the right name for it?) and said a few of them weren't perfectly symmetrical (or round). That could have been from wear and tear, or the way they were made.
Anyway, 95 percent of the problem has been eliminated. I experienced some shuddering, but no where near what it was before, and it was only a few times, and not every time I accelerated from stop.. Just rode in from South New Jersey to University City in Philadelphia. No problems. It's great to have my scooter back, though the engine feels different -- like the timing I've grown accustomed to is off, but I'm willing to become reacquainted again with my bike.
-Wolf