was the belt you installed a jobber belt or one from Honda? if it was a
jobber belt you probably got the wrong size, if it was Honda then you may
have installed it wrong or they sold you the wrong one. (don't hurt to have
them double check if there is any doubt) some other things to look for are
any belt guides on this unit to keep the belt on are they adjusted properly
or missing? did you check the owners manual on belt replacement? Is any
other part damaged or missing (i.e. idler pulleys etc)