Weird plug failure.

This was in a lawnmower, but since there's no rec.mowers.tech group,
so I'll discuss it here.

Engine occasonally starts and runs for a few seconds, then cuts off
suddenly. If I let it sit for 20 minutes or so, it catches again.
It kept lookign like fuel starvation, but the float bowl contained
gas. I even squirted a bit into the plug hole, not a pop..

Checked spark with the plug removed and connected and grounded. I
could see a small spark,

Cleaned the plug, it started for a few seconds again. In fact, it
seemed that every time I messed with the plug, it started and ran a
few seconds.

So, with only $2.00 to lose, I bought a new plug and installed it, not
really believing it would work. First pull, fired right up.

My guess is that as soon as it heated up a bit, it opend up (or
shorted) when I cleaned it, I used solvent, which colled it down
again.

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The multi-fire high energy ignition system would fire a 35KV spark even
when the distributor rotor wasn't turning. It would arc inside the cap.
I knew that if my Dodge van's engine didn't start the first try on a
cold damp morning, it wouldn't start at all. The cap had carbon tracks
inside it that couldn't be cleaned out, so I replaced it.

 

Small high-rpm engines don't need much voltage to light the fire inside
if the fuel/air mixture is rich enough.

The cheap low voltage ignition system is trying to raise the voltage
high enough to overcome the air pressure inside the combustion chamber
and make a spark jump the gap and voltage may be leaking away across
carbon faster than the coil can make the voltage rise.

(I have also suspected that moisture may condense on the spark plug
electrodes inside the engine in a damp environment, shorting the plug
that way. I used to heat my Honda's two spark plug with a torch when
the engine wouldn't start.)

The ignition spark will jump from the sharpest point on the center
electrode to ground. It takes a few atoms of metal with it every time
it jumps the gap. The electrodes erode and become rounded. That's why
we file the center electrode flat.

Small high RPM engines with low voltage coils need a rather
uneconomically rich mixture to increase the possibility of burnable
mixture passing through the narrow spark plug gap. High voltage systems
can increase the gap and find burnable fuel in a leaner mixture.

 

Was it a Champion???