RE: K&B .65 Sportster (yup, more from me)
None of the K&B Sportster .65 engines that I have ran (belonged to other folks) had a problem with a funky transition from idle to wide open throttle, once broken-in. The latter can take far longer than a normal engine. Remember, the piston is chromed, NOT the cylinder. So, we can expect the engines to run a little differently from normal.
In fact, I was surprised that such funky looking carbs could work so well, but work well they did. See if someone else that owns one of those engines will let you swap carbs for a float or two.
Good luck in your exploits and hang in there.
Ed Cregger