gravely 1238H (old) drive belt listed as 73" by 1/2 put on 74 and it comes off after 5 or 6 hours. Put on 73" by 1/2 and it starts to smoke after 6 hrs when pto (mechanical) is engaged. Inside edge of belt is frayed. On the 73" belt, idler by engine pulley is so tight that it is hard to pull spring to set it. Would a 74" by 5/8" belt work better. (have one). Could I replace the spring on that idler arm to create less tension. Might that work. Don''t see any obstructions on belt route. Changing this drive belt is becoming a chore. Any help would be appreciated.