Hi Ed,

"if you are just
tapping one spark plug you would get 1 spark per 360 deg."

I'm not quite convinced of that.  Every piston fires once every 720
deg.  Compression/(fire plug)/power stroke is 360, then
exhaust/intake is a second 360.   If I sample one plug I will see one
spark for 720.  From Jeff's explanation, the tach mentioned samples
two spark plug wires for a 4 cylinder 4 stroke (although I think this
is 1 pulse per 360 rather than 2).  For a two stroke, only one plug is
sampled, so still 1 pulse per 360.  At least I think that's what I
understood Jeff to be saying.   On a 4/4 engine, the tach's that work
with a coil and distributor will see two pulses per 360 as two
cylinders will fire in one 360 and the other two in the next 360.

If I still have this wrong, then I need more help! :)

Bob W.