Two ways to approach this. Change the accel pump cam to strech out the pump shot to cover the flat spot during transition off the idle circuit or install a larger squirter which will dump more fuel into the transition mixture. This may result in a pump cam change as well. Make sure the accl pump operates and squirts just as the throttle is moved first.
What happens is when you step on the gas large amounts of air get into the carb and intake faster than the gas gets there. You get a temp lean condition and the accel pump squirts extra gas to over come the lean condition.
I should have asked if this was a vac secondary carb as the secondaries opening too soon can cause the same symptoms. A heavier sec diaphram spring would be in order if this were the case.