I bought this leaf blower which has a 31cc, 2-cycle engine. It has only about 20 hours of useage and stopped working. I have been very diligent about using only fresh gas and the proper 3.2 oz. oil to 1 gal. gas mixture. The repair shop said I must have used straight gas without oil because the top of the piston is burned. As this was not the case, does anyone have any idea how the piston would have burned?