I am completley rebuilding the z 400 i just got and I put the Wiesco high compression piston in. It is 13.5:1....... The little bit I got to ride it (now putting a better carb in it) after it warmed up sometimes when I turned the key off it kept running for a couple seconds.... I put 92 grade gas in it... I am assuming its detonating... I am going to put the carb in and re torque the bolts on the head and base. Then I will see if it keeps doing it. What do you think it will do ?