Is it timing belt? ?

I have an old Singer 247 sewing machine that serves my sewing needs. After changing the belt located at the bottom of the machine, I noticed the needle hitting metal part. I believe that timing belt needs to be adjusted. Could anyone please help me? It is driving me crazy trying to find out how to make my good old machine sew again!
Two belts were included in a package: regular size, and a small size belt. Since there are no instructions given, I have no idea what the small size belt is for. It perhaps may not be for Singer machines.
Thank you.