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1983 Honda vt750 Shadow
20,541 Posts
Discussion Starter · #1 · (Edited)
Insanity of spark operation and the importance of a good strong battery.

I worked on foreign cars for years and learned ignition in the old points and condenser days. Then came along "transistor ignitions" and Capacitor Discharge systems. These were a great improvement over points. Then in the 1980's many cars manufacturers put on Waste Spark systems where 1 coil fired 2 plugs but on a car it is on 2 different cylinders and helps clean exhaust emissions. Our twin plug motorcycle engines also use CDI and 1 coil to fire 2 plugs but it is all in one cylinder. And so it fires each revolution on the models with a crank sensor but no cam sensor. Firing on the power stroke and then the exhaust stroke and back and forth. That adds to the complication of it all.
I keep hearing many guys with weak or no spark, and starting problems,-so I thought this info. from GM explains it well---just remember our engines fire 2 plugs alternating on one cylinder. (The newer bikes with computer control is a step up also.)

Spark Polarities
One spark plug in the pair of DIS running mates always fires from the center electrode to the side electrode, while the other fires from the side electrode to the center electrode. One cylinder’s firing voltage rises in a negative direction, relative to engine ground, on the way to its final spark plug gap breakdown voltage. It then breaks quickly in a positive direction, back toward ground until the spark line is established. The running mate cylinder’s firing voltage rises in a positive direction, relative to engine ground. It then quickly breaks over in a negative direction, back toward engine ground until the spark line is established. The polarity characteristics of the spark break over events are one part of the information reflected in the CSI signal.

As a DIS ignition coil releases its stored energy, an electric field is created in both secondary circuits of the paired
cylinders. This growing electric field simultaneously creates a voltage potential across both spark plug electrode sets.
Over a period of 5 to 10 microseconds, this voltage level becomes high enough to reach the spark plug gap breakdown voltage level. Each spark plug gap breakdown level is determined by the
impedance value of the individual spark plug gaps. At the breakdown point, the air gap between the spark plugs’ electrodes will ionize and quickly conduct current, break over and establish an arc across the electrodes.
One of the important characteristics of this event is the voltage level at which any spark plug gap impedance
will reach its breakdown voltage level. The greater the spark plug gap impedance, the greater the voltage
level that must be reached for the plug gap to break down. The greater the voltage level required in order to
reach breakdown, the greater the required time to reach this voltage level. The spark plug gap impedance
is affected by several factors. The greatest variable is the pressure in the cylinder when the spark is generated
by the coil. A cylinder on its exhaust stroke has less incylinder pressure than a cylinder on its compression stroke. Remember, a DIS coil releases its energy to two cylinders simultaneously—to one cylinder on its exhaust
stroke, to the other on its compression stroke. However, due to the pressure-related uneven plug gap impedances, both spark plug gaps will not break down at the same instant. Under most engine operating conditions, the spark plug gap in the cylinder on the exhaust stroke will break down before the spark plug gap in the cylinder on the compression stroke. The order of the spark plug gap breakover events is reflected in the CSI signal.

Sorry this is so long winded but it has a lot of explaining to do, and we know that "knowledge is power" and what do we want for our bikes?=

· Registered
1983 Honda vt750 Shadow
20,541 Posts
Discussion Starter · #6 ·
Ignition operation

Thanks for more info. on the subject. I am studying my old 1983 750 shadow to understand the twin plug CDI ignition. It only has one trigger per cylinder on the crank so it must fire each revolution. But more sophisticated computer systems can change this a lot. Especially with better triggering.
I am just so intrigued by the dual spark system with one plug powered forward and the other is backward, to complete the circuit.
I actually proved this with a Fluke 88 test meter and a clamp-on pickup for RPM readings. I had to switch the direction of the clamp-on pickup to read RPM from one side of the cylinder to the other. If I had on old ignition scope maybe I could see the pattern differences.

· Registered
1983 Honda vt750 Shadow
20,541 Posts
Discussion Starter · #7 · (Edited)
Spark operation

Sorry but I must be flogging a dead horse here, but I am studying this dual spark system because I see so many guys have spark and starting troubles that I want to know how to diagnose this if I have a failure.
I looked over the Honda Common Service Manual ( published in 1988 ) which is a bit generic on Honda bikes but it explains some of the workings of various systems. I found a puzzling test section on ignition spark tests.=
Page 23-10 says to check spark with a plug against he head to see if it sparks, OK normal stuff, BUT then it has a paragraph that says this=

For multi-cylinder engines, remove spark plug from each
For Some models with the CDI system, there is a circuit within
the CDI unit designed to turn off the spark at low cranking
speeds below 200-500 rpm. In this Case, leave the spark
plug in the cylinder head and try the spark lest with known
good spark plug.

Not only is a bit confusing but May mean with a poor battery or starter drag or very cold weather the spark may not be sent there.You could test and find no spark-- Anyone heard of this before about RPM ???
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