Do you still have the stock airbox that pulls air from the rear? I've always liked those on garellis
i would tinker with that. add a layer of pantyhose, take out some shit. try another filter element idk. with those tiny carbs with tiny jets, small changes make a big difference. It takes a good bit of trial and error to dial it in but then you wont have to mess with it until you start changing other parts.
There's a very good chance the pipe is full of gunk and build up and is restricting the air flow through the already small holes. If you haven't done it yet, i would pull that apart and make sure it's not all gakked up