I got a pretty basic engine question, what triggers the spark plugs to fire?

I understand the magneto generates the power. On the flywheel there is a contact patch that trips the trigger coil. The flywheel turns equally with the crank, the cams only turn 1/2 the speed of the crank. For every 2 rotations of the crank there is only 1 combustion stroke. So that means for every 2 crank rotations the spark plug only fires once, but that would also mean that flywheel would trip the trigger coil twice to generate 1 spark. Is the CDI responsible for when the spark is delivered. If the CDI is responsible, then how does it know if the piston is in the compression stroke or the exhaust stroke, technically they are the identical position just 360* off.

Please point out any errors I made to help me better understand our motors.

At least on a distributor you can physically see how it works! And its running off the cam gear so there is no 2:1 stuff.

 

Have you been reading my mind?

I've been wondering this for the last 6-mos. Have asked a few people and none have the answer.

I've put a few cams in and never have I had the issue of it not firing because the piston was on the exhaust stroke and I had the cams set with th lobes inward.

I gotta think it is firing every TDC but, I'm know expert.

 

It fires every other TDC.

 

That pretty much sums up my question, actually I think it fires a few degree before TDC. But what device makes it fires every other revolution?

 

The CDI?

How does it know which revolution to fire on?

I've never heard of anyone having to rotate the cams because they put the engine together on the exhaust stroke?

 

It seems it would be so easy to do, although I don't think there would be any damage to the valves because the piston to valve clearances would stay the same. The engine should not run because it would spark while the cams are both open in their overlap phase

 

I'm definitely still missing something here [img]i/expressions/face-icon-small-confused.gif[/img]

 

Obviously I am still missing something too!

 

this is my gess as well
sam

 

it's a wasted spark system - fires every TDC.