Motorcycle Repair: Honda C70 motorcycle timing, honda c70 motorcycle, air fuel mixture

Question
QUESTION: Dear Rich, I study but cannot understand how the firing system of the Honda Cub C70cc motorcycle goes about. You see the contact is fixed inside with the magneto unit covering over it and by rotating the magneto will push the contact open and close. When it reach the point 'F' the contact open and firing occur. At the second round when it reach 'F' again, I guess there shouldn't be any firing occur as my bike is a 4 stroke engine but the contact gap will open due to the magneto pushing it. How to justify this point. Thank You.

ANSWER: Rick, those are really simple ignition systems and it fires the plug on every rotation of the crank.  It fires once on the compression stroke and again on the exhaust stroke.

Regards
Rich

---------- FOLLOW-UP ----------

QUESTION: Dear Rich, Thanks for the explanation but I still cannot figure out why that is possible for the firing to occur again because at that stage of the second rotation at 'F', the inlet valve is just opening together with the outlet valve just closing and there shouldn't be any disturbance in order to complete a 4 stroke cycle. Furthermore I used to read from books that firing only occurs once in every 4 cycle movement. Please help.

Answer
Rick, you have almost answered your own question. Yes, the system  sends a voltage spike to the plug again at the start of the exhaust stoke but it doesn't have any effect.  Since there are no combustible gases in the cylinder at that time, it doesn't help or hurt the engine performance. It's just cheaper to manufacture.

In this engine the plug sparks two times per combustion cycle, once at the end of the compression cycle which ignites the air/fuel mixture and make the power and it sparks again at the start of the exhaust stroke but it doesn't do anything to make power.

That make sense?

Rich