Replace a motor belt for sewing machine

Question

When replacing a motor belt on my Singer Futura 920, are there any markings that need to be matched? Any other adjustments that might need to be made after a belt breaks? Out of time? I have replaced the belt and a shaft washer, new needle, but cannot get a smooth stitch. changing the upper tension does nothing to change the stitch. Bobbin winds smoothly. needle to hook looks right as do the timing marks on the needle bar.
Thanks for any help.