I am prone to vibration in the same 3400-3500 range. When I balance my carbs (mercury balancer), I check them at idle, but also at a steady 3400-3500 range where the vibration feels the greatest. If it's off, I compromise between that and idle. That did help smooth mine out a little.
Also, I check the balance after any pilot screw adjustment. I've heard you don't have to, but I've noticed that even the smallest screw change does change the balancing. Be sure to re-adjust the idle after any screw adjustments before checking balance.
