Timing, that is, the movement of the piston is well coordinated with the opening and closing of the valve. Each model has a timing mark or a specific timing tool. As for how to judge whether the timing is normal, it is easy to have a timing gun. If not, try the engine. The engine works normally, with rapid acceleration, rapid deceleration, idle speed, medium-high and low speed, no backfire, no ignition, strong power and rapid throttle response, which can be judged as normal.