Well, I changed the front sprocket today, thinking that it may have been the problem. Nope. With the new sprocket on, the problem is just as bad. Also I think I need to tune the carb some more, it runs kinda rough sounding below around 1/2 throttle.
When the front sprocket was off, I turned the splined shaft coming out of the transmission with my hand. It was very stiff, much moreso than it should be. I don't hear any grinding or scraping, just the sound of gears turning together. The new sprocket is a 15t, so my acceleration is better, but this bike should be a LOT faster than it is.
So far I have ruled out:
rear tire size/type
Which leaves the transmission itself or possible the clutch. I have absolutely no clue how to mess with the tranny/clutch of this thing, so I'm at wit's end now. I would like to take it to a mechanic, but I have already spent too much money on this thing. I'm considering just selling it and being done, I am really really sick of this.