check that the slides are both at rest at the bottom of the carb. and then open the throtle and check the slides lift at the same time. and rise together. if not adjust the screw that is in the centre of the linkage between the two carbs.
also have you checked the compressions. they should be close to the same on both pots. if the cylinder that is slow to fire up is a lot lower, i would pull the top off and have a look.