Let me apologize right off for the way this post is going to sound. It's not meant to be harse, I think more diagnosis is really needed here.

Remember he mentioned that this is only on takeoff. He did not mention that it was at any other time.

Normally a tank problem where the tank is too low would create a lean condition so in the air it would only be worse all the way through the operatin range. but since it appears that the problem goes away in the air 2 things could exist here

1. Low speed needle is slightly rich, giving the hesitation
2. Tank position.

I would like to hear the results of this test. Fly it, get it up good and high to prepare for a possible dead stick. Cut throttle to idle, wait a few seconds then quickly go to full throttle. If it hesistates then I agree it could be the tank or even still be richon the low end. If it goes right to full throttle then I stand by the low speed needle being to slightly rich by an 1/8th or even a 1/16th turn.

I have had 2 of these planes, one with a 2 stroke and the current one with a 4 stroke and have had no problems at all. I did look at mine last night before I made the post and mine is also low in relation to the carb on my OS91 FS and it just purrs right along without a care in the world.