I put a 140 on a stock rear wheel. I bought mine new in ´83 and raced it on the WERA circuit in the stock class. If yours still has the stock sprocket on the rear, change it and go up one tooth. Get bigger jets for the carbs. If you can find a Kerker K2 ´off road only´ pipe, put that on there. Those three things will make night and day difference.