400EX Cam/Ignition Timing

[kennymac131]

Those of use who work on our own bike and have taken our cylinder heads off probably know that on four strokes the crank turns 2x for every 1 turn of the cam.

What we may not know (and here comes my question) is if the ignition sparks on every crank revolution.

Obviously it sparks on the beginning of the power stroke (a down stroke) but what about the beginning of the intake stroke (the other down stroke)? I'm wondering if you can postiion your cam 180 out of phase with the crank?

I'm thinking the 400EX does spark on every stroke. I'm certainly no expert but I don't know of anything on the 400EX that would tell the ignition to spark on only every other crank revolution.

I haven't experimented and I can't because my bikes are in pieces right now having the heads ported.

So can anyone tell me if the 400EX sparks on every crank revolution or on every other?

Thanks
Ken

[zrpilot]

It sparks every revolution.

[GPracer2500]

^ Yep. It's called a "lost spark" or "wasted spark". The pulse generator (the thing that triggers the CDI to sent spark energy to the ignition coil and then on to the plug) is triggered off the flywheel. It triggers on every revolution of the crank regardless of which up-stroke the piston is on. The only thing that distinguishes which TDC is which is the camshaft being oriented 180 degrees one way or the other. So, it's impossible for a 400EX to be exactly 180 degrees out of time. 90 degrees is really the maximum amount it could be out of time. Once you go past 90 degrees you're actually moving closer to getting back in time. 91* out really = 89* out; 92* out really = 88* out and so on until 179* out really = 1* out. Go one more degree and you're back in time. All you've done is flip-flopped the two TDCs and since there's spark at both the engine never knows the difference.

Some of the old XR's (like the XR185) located the pulse generator on the camshaft. No lost spark there since the cam spins at 1/2 the speed of the crank.

The only downside to lost spark systems that I'm aware of is increased spark plug wear. That has little impact though since spark plugs last a very long time.

[wilkin250r]

Originally posted by GPracer2500
^ Yep. It's called a "lost spark" or "wasted spark".

The term I've heard most often is "Phantom spark".

[GPracer2500]

Ahhhh...."Phantom spark", got it! :cool:

[JOEX]

But does a phantom spark really spark?:blah: :devil:

[kennymac131]

Thanks guys... pretty much what I thought just couldn't prove it to myself.