The color of the spark plugs tells you about combustion conditions,
but there is another fun thing to try if you have an old oscilloscope
lying around. Don Mathis reminded me of this when he mentioned
tying neon lamps to the spark plug leads. Using an oscilloscope
tells a lot about the condition of the distributor, the plugs
and the compression. Many years ago Tektronix even published a
pamphlet on tuning your car with an oscilloscope.
The signal to the oscilloscope is picked up by wrapping one turn of wire
around the HT lead from the coil to the center of the distributor.
This is a capacitive pick up because the voltages are obviously too high
to input directlly to your scope. The oscilloscope trigger input is
connected to another pick up wrapped around No.1 spark plug lead.
Then adjust the scope sweep speed until you see 4, 6 or 8 spikes on
the screen which correspond exactly to the firing order of the
cylinders.
The spikes should all be the same height and not moving around with
respect to their neighbours. If they jitter in time it is an indication
of a worn distributor shaft or cam.
The height of the pulse represents the voltage to each plug. The initial
very high, very short duration spike represents the plug voltage before
the spark propagates. Once the spark initiates the voltage drops down
to a much lower level whose height depends on the plug gap and the
compression pressure. The heights should be equal for all plugs.
Unequal heights means wrong plug gaps, or a cracked plug insulator
or a fouled plug.
The length of the pulse during the spark propagation phase depends
on how much energy your ignition system had available to give each
plug. On non-electronic ignitions this is determined by the dwell angle
(points gap) since the longer they are closed the more enrgy the coil
stores.
At the start and end of the spark propagation phase there are a bunch of
oscillations (ringing). If these are excessive it indicates poor
function of the points condensor and/or the the coil ballast resistor.
To the practised eye you can see at a glance a great deal about the
state of your engine, under all load conditions and rpms.
Maybe this will make you want to run out and buy an old surplus scope
if you don't already have one.
The ultimate one-up-man-ship would be to buy a high tech portable one
and mount it under the dash of your SLBC!
Patrick.
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