Modern Vespa

I'm dialing in my p210. I've got malossi cases matched to the malossi cylinder, and a 24/24 premix carb with the polini vortex attachment where the standard filter goes and an external pod filter.

The bike runs well at medium to full throttle, but lugs when going from total stop, i.e. opening up the throttle from fully closed leads to a near-stall situation unless it is delicately feathered. It also slightly backfires through the SIP big-box exhaust at closed or closing throttle, not all the time but some of the time.

(Timing is set correctly I am 99% sure).

Wondering if this is a mixture-screw issue or a jetting issue. I am using one of SIP's lemarxon slides that is supposed to deal with the transition from closed to open better.

My Stella with the PHBH is so perfect, I am very tempted to go that route with the p200, but there is something that makes me want to make it work with the stock airbox, carb, and oil injection.

Thanks for any input!

It does.
Are you using a Lemarxon atomizer?
The slide and atomizer work together.

The mixture screw must be adjusted on any modified bike.
Try turning it out a half turn. That slide gives you more control over low throttle, as it delays the opening of the progression hole at barely open throttle.
The channel under the slide also opens a bit earlier.

Brilliant. That worked. Thanks! Lag on opening throttle is much reduced. (I do have the lemarxon mixer tube installed along with the "L" slide).

Had to turn the mix screw out 1.5 more turns, I'll start playing around with a richer idle jet to see if that lets me screw it back in a bit.