My bet would be that something has been done to upset the run or seating of the throttle cable while the bike was being worked on. Check the cable is not snagged anywhere along its length and that it's seated properly at the twistgrip and carburettor. Also check the top of the carburettor has been fitted correctly (can't remember if the Beta uses a threaded, screw-on top or the type attached with two screws. If the latter the top should be attached with the cable slightly forward of centre - it's possible to fit it back to front in error) and that the twistgrip is not binding on the end of the handlebar. As mentioned above, when you let the throttle snap shut it should do so instantly, not gradually, and you should hear the "clack" of the slide bottoming in the carburettor. Check these last two points with the handlebars on both left and right lock.