Of course - I thought of that as I posted.
So a four cycle engine has:
inlet valve open on a down stroke
Inlet valve closes compression on up stroke
Spark fires BANG (timing point for TDC at number 1)
Both valves closed
Down stroke because of BANG
Exhaust valve opens as stroke come up
So while number 4 is TDC and firing when the dots line up, number 1 is just starting to open the inlet valve?
Yes, more or less. But during the overlap period @ TDC the exhaust valve is just closing too, that's why you get a BANG! if the timing is 180° out at the dizzy- you are firing the unburned charge which has just been pumped down the exhaust port.
