I just put a new belt on mine, an original style black v-belt, and the new one is a bit tighter than the old one was. Per the manuals, I just let the loosened motor weight the belt and tightened it there. Which was fine for a few minutes of use until it stretched out and lost grip and I had to do it again. And will probably have to do again when the belt wears in a bit more. I basically made it just a bit tighter than "just tight enough to work" but it still has play (dropped the motor a little bit more).
Otherwise, make sure you've put the hand wheel and stop motion assembly back on correctly, stop motion washer tabs facing out. Mine was on backwards when I got it.