picture host ru

There is some residue on the side of the overflow tank. And the MAP and calculated boost fluctuate a little on a wot pull, while my MAF steady climbs. Just looking for confirmation before I buy a belt from someone who's experienced this before. I'm running a 3 inch pulley and current belt is
653 - GTP/GS/SS supercharged with 3.1 - 3.5" pulley (65 7/8")

Should I drop to a 65 1/2"? That's the next size down but it's only 1/4" difference.