The large amounts of unburned fuel coming from the exhaust would seem to indicate an over-rich condition.

A couple weeks ago I finally got my Saito .91 running well in the inverted position. The main problem was the low-end setting; it was too rich. It took some experimentation, but now it transitions well and doesn't die when the throttle is advanced after it's idled for more than 20 seconds. BTW, the low-end on mine is a bit tricky to adjust. You have to be careful not to push with the screwdriver while turning the screw.

Cheers,
Richard