Originally Posted by FauxMako

Everyone recommends the 19t pullies because it's next to impossible to get the belts loose enough using 20t pullies. Too tight belts can cause all kinds of issues.

You *can* run with different sized pulleys but you'll end up with one set of wheels being driven faster than the other -- which end will be faster will depend on where you put the pulleys. Drifters make the back wheels spin faster to make it easier to get the rear loose. Crawlers make the front wheels spin faster to tighten the turning radius (they don't use belt drive, but same idea). I doubt you'd want to do it in a car you're racing. You really want all the wheels being driven at the same speed.

Make sure there isn't anything in there. Clean it out good.

You could have had a rock or something similar get stuck in there to wear the edge of the pulley off.

Your rear belt looks wider in the pics than the front belt. It's hard to tell from just pics but is it the correct belt?

Basically, get new 19T pulleys and replace both sides.