1st check the  oil  level  (little nut at back of engine )
check your brakes arent worn through to the metal plates ,no? then 
remove the rear brake  2 bolts , slide off ,and spin the wheel  still making the noise?
if yes , take the side plate off your engine 12ish bolts
check your belt is ok , remove the belt see if the wheel spins ok ?
no then its something to do with the rear drive, there is a gear in there may be shattered , clutch may be stuck on , 
spluttering at full throttle , sounds like wrong  mixture , either to much fuel or too much air , tighten the mixtuer scew all the way in , make note of how many turns 
it should be around 1 1/2 turns  anticlockwise from fully closed position  unless youve modified  it
                
 jASE
                
 April 2008