Spare spark?
#1
I think i am correct in saying that a lot of Jap bikes - possibly/mainly inline fours which have only two HT coils - operate on a `spare spark` ignition system wherby the plugs fire every time the piston is at TDC because the pickup/`pulser` coil from which the CDI gets it`s signal to fire is operating off the flywheel rather than the camshaft at half engine speed (such as on a car with a distributor for example)?
This will provide a pulse to the CDI to fire a spark plug once per revolution giving a spark at TDC on both the exhaust stroke and compression stroke, hence `spare spark`.
I fitted an LCD tacho to my Big today (just for the hell of it, it was a spare one i had lying around, and i have a predeliction for useless gadgets) which is made for two-strokes - one revolution of the crankshaft = one spark = correct rpm for this particular tacho.

Now, if the Bigs pickup/pulser coil is also driven off the flywheel (which i`m sure it is) then it is also firing the plugs every turn of the crank so i thought that my two-stroke tacho would work OK and display the correct rpm, but it doesn`t, it shows 1/2 the engine speed.
This leads me to the assumption that the Big only does fire once per two revs of the crankshaft - which is what you would normally expect on a four stroke engine, but how does the Bigs` CDI do that if it is getting it`s signal to fire from the crankshaft on every rotation?

It`s not a problem, i`m just curious and want to understand exactly what`s going on....

I`ve just had a thought - because the sensor wire for the tacho is wrapped around only one of the HT leads, is the 2 spark plug configuration of the Bigs` cylinder head anything to do with this?
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#2
I'm pretty sure it's called a "waste spark" system - but why your tacho isn't doing the expected is a complete mystery to me, what you say seems obviously right

I'll be interested to hear the answer when it comes.
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#3
Today i hooked up the tacho to my CBR1000 which definitely does operate on a wasted spark and the tacho read the correct speed (inline 4, 2 coils, triggered off the crank, one spark event per revolution per cylinder) even though it is a two-stroke tacho....
So i don`t really know why it isn`t working correctly on the Big, but i can`t be bothered to spend any more time on it so it will now forever remain an enigma.
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