Belt and lifter getting hot right away on start up.

I replaced the impeller on my Kirby G 3. Now when I install the carpet head on, the belt and belt lifter start to get very hot right away. The screw that holds the hook to the plastic body of belt lifter, got so hot that it melted the surrounding plastic. I shortened the screw, because I thought it was rubbing on the new shaft that came with impeller, but that did not solve the problem. It appears to me that the belt is rubbing on the belt lifter because it wants to ride on the edge of the shaft. If I turn the brush in opposite direction by hand, then the belt seems to be moving away from the belt lifter hook.For the brush to turn backwards, I have to reverse the rotation on main motor. I am confused because there is good suction when I use the hose, meaning the rotation has to be correct. If I am looking at the shaft from front end, it is turning anti-clockwise. I would really appreciate if someone could guide me to the cause of this problem and how to fix it. Thanks.