Carb balancing can be a very fickle issue! Start by being absolutely sure, both throttle plates are adjusted identical....A feeler gauge works well.
Be sure both idle mixture screws are both turned out the same amount also, say 1 1/2 turns. Try not to let idle speed get above 600 rpm, if it does, use idle speed adjustment too get it around 450. I have found using my old motorcyle carb balancing/ vacumn gauge works great, but a standard vacumn gauge and tachometer does the job also. Patience and many, many adjustments may be required......good luck !
