1987 with 36k with sidecar was averaging 38-40 all of a sudden dropped to 21, 26.2, 31.7, 28.6 , 29 no long idling no excessive speed rig coasts fine no brake drag i thought if there is a return line do to fuel pump maybe it broke or was leaking but there is no gas smell or residue. is there a return line line and any suggestions to look for for milage drop could it be clutch?doesnt seem to be slipping.