Your car has stopped running and all of your troubleshooting has
failed to find anything wrong with the ignition or the fuel system.
Your allotted repair time has run out and you're being towed.
Resist the temptation to beat your car to death with a worn out crank. Maybe it's the coil. Yes, it did make an observed spark when you tested it, but there are no good, quick tests for a coil except substitution of a known good one.
Coils seldom fail suddenly, except for the broken internal
connection which happens to the wood-cased Pontiac coil. They
usually deteriorate slowly, giving erratic and weak ignition.
It is an old axiom that coil problems always seem to be carburetor
problems. Hard starting, lots of "plug fouling," and
excessive sensitivity to mixture are the most noticeable symptoms.
Testing the coil by watching it fire a plug outside the engine
is not a conclusive test. The reason is that the voltage needed
for ionizing a 0.030 inch air gap in free air is much less than
that needed when the pressure is five or six atmospheres, and
a bunch of fuel and oil droplets - all insulators - are present
in the air gap.
A good single-cylinder coil should produce a spark across a 1/4-
to 3/8 - inch gap in free air. This spark length is largely independent
of voltage applied. However, the elusive "Fat Blue Spark"
containing lots of energy requires at least five volts at the
coil. For convenience, the vibrator
point adjustment procedure is repeated on the next
page.
Coils go bad due to broken internal connections or insulation
failure. Do not operate a coil without a spark plug in the circuit
to absorb the energy. If the points are vibrating, a spark is
being generated and will jump somewhere. If it occurs inside the
coil, the insulation will be damaged and will eventually leak
off energy or arc over, bypassing the plug.
Some coils used to have an intentional air gap inside as a safety
measure, but the Pontiac coils used on single-cylinder cars do
not. They do contain a condenser which helps increase the spark
intensity, just as in automotive ignition systems.
This coil should measure 3,000-4,000 ohms from the PLUG terminal
to the COM terminal. If the circuit appears to be open, a spark
can still jump across the internal defect, but the wire is being
slowly burned up. The coil should be replaced even if it seems
to work okay today.
One subtle symptom of weak ignition, possibly due to a bad coil,
is that the engine will misfire when the timing is retarded but
runs better when the timing is advanced. The reason for this is
that the pressure in which the plug must fire is greatest at top
dead center. When the timing is advanced, the pressure may be
only half as great, allowing the weak coil to fire. Reducing the
plug gap to half of the normal setting may help get you home so
that you can fix it properly.
The metal-cased, dual-lead coils used on twin cylinder cars are
not made by Pontiac. They also do not have a safety air gap inside.
These coils should make a 1/2- to 3/4 - inch spark from one plug
terminal to the other, and the resistance should be near 10,000
ohms from one plug terminal to the other. These coils don't fire
to ground except from one plug to the other through the engine.
Arcing to ground inside the case will bypass the plug and cannot
be determined except by bench testing with two 1/4- to 3/8 - inch
air gaps, each to the coil case. A weak spark on one side can
cause one cylinder of the engine to stop firing or to be erratic.
When it is firing sporadically, that plug tends to get all sooty.
This in turn causes the owner to install a hotter plug in that
cylinder, and to start researching ways to clean the old plugs.
By now, you have realized that the coil is at fault, not the spark
plugs. An RK engine with a good coil and clean fuel can run all
year on one set of D21 or C88 plugs.
Therefore, efforts to clean spark plugs do not solve the fundamental
problem. Some of the cleaning procedures can damage the porcelain
and cause the plug to fail again. This leads to a new round of
plug changing and cleaning. It's lots more fun to fix it properly
and then enjoy the scenery.
Don't forget that a good solid-wire plug lead is part of the coil
circuit. The high energy in the vibrator spark can quickly open
up the resistive wire used on automobiles, and this wire can also
be tricky to adapt to our coil connections.
Also, do not use an automotive spark coil. The Fairmont coil fires when the timer points close, while the auto coil fires when they open. This retards the spark by 36 degrees. If you can get it to run okay forward, it probably won't run at all backward.