[forchetto]

I would first try unscrewing the idle mixture screw about 1/4 turn to richen the bottom end a bit. This adjustment only affects carburation up to 1/4 throttle, so if it doesn't make any difference try raising the slide needle one groove by removing and replacing the circlip one groove further down.

For easier manipulation of this sort of screw I used an old electricians screwdriver and cut most of the handle and blade off. Then I re-ground a new tip on it. The total length depends on how much (or how little) space there is between the adjuster screw and any other engine bits.