orite m8 i changed my sprokets originally it had 13 front and i changed it to 14 but i have not changed the back sproket but look how much play there is and if there isnt much undo the bolts on the back wheel and knock the back wheel forward till theres a bit more pay then put your sproket on and then put the chain on and tapthe bolt out till the chain is right tightness but if back wheel is already right foward you might need to add a link or get a new chain. hope i have solved your problem :)