When the exhaust goes down, then come up, as soon as it up all the way the inlet immediately starts to open. I've always been taught `the valves are on the rock`. So if you turn an engine over slowly, you will see number one valves go on the `rock`. Number one is the front of the engine, opposite end to the clutch end. Now with number one cylinder on the rock, slowly turn the engine and watch which is the next valve group to go on the 'rock', keep going until all the valves until all have 'rocked' and number one comes up again after two full revolutions, that's your firing order.