Agree with others in that it sounds like its running very rich in the pilot circuit.
Have you tried airscrew adjustments to see if you can dial out even a little of the hesitation?
Barring other problems like your float condition, and the cleanliness or otherwise of your carb, the condition of your reeds, engine compression, condition of your silencer packing etc sounds like going down a size on your pilot should make things better, maybe even more than a few sizes...
Make sure the other things I mentioned are nominal though before working on your jetting... otherwise you'll simply be bandaiding a problem that needs to be fixed first...
While your moving slowly in first or 2nd gear, do you hear a distinct "crackling" noise along with the hesitation when you roll on the throttle and accelerate? That's a definite sign of running rich...
Also, when you changed plugs and said it ran worse with the new one... This might be a stupid question, but did you use the same heat range plug?
I'm probably over analyzing the whole thing and you're just simply running "rich" :) But never hurts to be thorough :)
Good luck to you!
Feanor