Has the motor been ported. If so, some port jobs can have a "hesitation" on the low end caused from improper port timing. Some carbs make the hesitation worse than others. For example a Mikuni has better low end adjustments than a Kehin so you can generally tune through the bog better. Not saying this is the problem, just something to check. You may need to go to a different needle to help the transision between the pilot and the needle. Good luck!