First, what size are your pullies? Then, if your setting on the seat looking down, which side of the chassis is your clutch on? Those are going to be my two guesses. Obviously pullies are a simpler place to start, but you say that you have the belt 'really tight', it leads me to thinkin that your clutch is on the 'draw' side of the belt. Settin on the seat looking down that would be the left side. It needs to be on the 'slack' side which would be the right side looking down from the seat. Let us know what you find.