bike has zero pulling power ?

hi my piaggio zip 50 4 stroke has lost all power it has new belt rollers spark plugss carb is clean but it will rev on stand but as soon as you sit on it it will barely set off then reaches on 10mph .its is has if under load it doesn't want to rev up as if fuelling is wrong and whilst running it surges like it is trying to get going then stops revving like an on off switch ! any help much appreciated