I looked at that bike about two months ago, at Chad's place. Are you in the twin cities area?
If you can get the machine up to 30mph, that would tend to indicate that the carb was not plugged up. It is still delivering enough fuel to produce useful power.
Sounds like the problem is too much air.. Do you have any vacuum leaks, at the joints on the intake maifold, or at the base gasket? Do you have a complete (and working) original air filter / filter housing?
If everything else is good, start thinking about the crank seals. They could be doing this.