Hi
On Sun, 23 Mar 1997 Trmgafun@aol.com wrote:
> I've got another question. For the past year, my MGA has been running
Time means very little; how many miles on those plugs??
> be ok. I finally decided that maybe I had a fouled plug, so I started
> pulling plug wires one by one. I found that when I pulled the number one
> plug wire nothing changed, but when I pulled the others, it made a big
Classic symptom of a fouled/worn out plug.
> difference. When placing the end of the plug wire near the end of the number
> one plug, it would arc (of course) and the engine would run smoother. But
By pulling the plug wire off, and holding it away from the end of the
plug, you are forcing the coil secondary voltage to increase until it
can jump the gap. When it does so, the voltage is so high that it jumps
the spark plug gap, rather than bleed off through the material fouling
the plug. 99% of the time, the fouling is composed of a high resistance
carbon mixture that is allowing the spark to short to ground.
> when I reconnected the plug wire the engine would run rough again. Could
> someone shed some light on this? Why would the engine run smoother when
> drawing an arc?
If you have the original ignition system in the car, you have a spark
that is 1. weak (low voltage) by modern standards, and 2. has a SLOW
rise time compared to today's electronic ignitions. This allows the
voltage to bleed off through the high resistance fouling as the voltage
rises from the coil. Put in a capacitive discharge conversion, and the
rise time is so fast that the voltage peaks before it has a chance to
dissapate, plus the voltage is MUCH higher. It has to jump the plug
gap, and fire the mixture.
The reason that a NEW plug will do better is that the electrode metal
gradually changes (due to the heat, and chemicals) and becomes more
resistant to the electricity. When you file the plug, it removes the
surface layer of contaminated metal, as well as restoring the sharp
edges (which electricity loves to jump from) of the electrodes.
I've been running the Bosch Platinum plugs quite successfully in our '73
"B"; but following the advice of "Mopar Madness" magazine in setting the
plug gap. They dyno'd the plugs, and found NO ADVANTAGE at the standard
gap, when compared to a NEW plug. They started to open up the gap, on
the dyno, until performance fell off. They found that the gap could be
opened up to almost twice the reccommended gap, which gave increased
dyno results.
Not having a dyno, I did the next best thing. I started opening up the
plug gap, and went for a boot up a hill. I kept on opening up the gap
in 0.010" increments until I had miss at 6,000 rpm at wide open
throttle, then went back down 0.010". Would you believe 0.055" works?
With a stock Lucas ignition system!!
Scott, if you want a quick and dirty way to check your state of tune,
try hand starting those beasts you drive. Once a week to stay honest...
My '60 mga mkII had a hand crank, and I didn't have the money to replace
the two 6volt batteries, so I used to hand crank it to start it. I made
sure I kept it tuned up...
TTUL8r, Kirk Cowen
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