How do the spark plugs compare, soot-wise? The flat spot and pilot screw symptoms say rich on that side, but it could also be that the idle is set higher (out of sync), and the needle jet is contributing more fuel, so the pilot setting is set to less.
The flat spot may be to a too-full bowl, or another path making the mixture a bit rich, like a loose jet. Basically, when you open the throttle, a CV carb is temporarily richer, as the air velocity over the needle/main jet is higher until after the piston lifts, causing more fuel to pull through the jets until the velocity drops to 'normal'. If the carbs are out of sync, then the left one will pass through this rich zone more than the right, and make for an imbalance. So, verify your idle and throttle sync are very close, before chasing mixture issues.
You may also have a weak spark plug or spark on that side, and the inrush of air is blowing the spark out. Once the sync and mixture is checked, this would be next on my list.