It should take about 1 second to smoothly transition from idle to full throttle. Given that the high speed needle is correctly set then Mike's recommendation is right on. To do that, have a long screw driver with a blade that fits the low speed needle adjustment screw on the inside of the throttle arm. To lean it out, turn the low speed needle in (clockwise) 1/8 turn (no more). Let the engine idle for 5 or so seconds. Advance the throttle off idle to full. If it was rich on the low end the throttle should respond more quickly. Repeat the sequence of 1/8 turn, rest at idle, advance throttle test until the throttle response doesn't improve. Back it off the 1/8 adjustment and retest. At the end, your engine should smoothly accelerate from idle to full throttle in about 1 second.

Hth,

Btw, after tuning, my OS 61 FX performs this way.