Camshaft Install and Ignition Timing - Curios about how it all works !!

[paulbaker1954]

Just on my final stage of engine rebuild and about to put back Cam but I have some confusion/questions around how all this works. This is really about satisfying my curiosity more than anythinig else............

So you line up the 1.4 timing mark so pistons 1 and 4 are at TDC and then line up the marks on the cam - I get all that but this got me thinking -- cos as an engineer I like to understand how all this works so here's my confusion.

Lets say Pots 1 and 4 fire at TDC , crank then  rotates 180 on the bang stroke, then rotates another 180 on the exhaust stroke and 1 and 4 then get to TDC again at the end of the exhaust stroke. Now surely the 1 an4 points will open again and fire the plug again. I would have thought the crank needed to rotate 360 before firing 1 and 4 again.

Clearly I am missing something obvious here or dooes it fire again at the top of the exhaust stroke.

Looking forward to demystifying this !!!

I am a bit mystified 

[SteveD CB500K0]

You are absolutely correct. The system fires twice in every cycle - it's called a wasted spark.

It saves having four points/coils (or a distributor!)

Steve

[Bryanj]

As steve sez, cheaper to fit two double ended coils than four single ones and can you imagine the clutter with four sets of points in there!

[paulbaker1954]

Thanks guys all now makes sense, at least my logic was correct 

[Johnwebley]

and if you fit a Boyer ignition you get all 4 plugs sparking every 180 degrees of the crank


 but it works !!!