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Strange Miss


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A couple weeks ago, my 1912 Abbott Detroit (Continental engine, L-head, 350cid) developed a slight miss while running on the mag. Switching to the battery eliminated it completely. It uses a low-tension Splitdorf system, in which the "battery" uses the same points, same coil, same distributor, etc. as the "mag", and neither side of the battery is grounded. When it happened, the miss seemed consistently to be on cylinder #4, and it gradually got worse. I checked many things to no avail. Last night, the miss occured on the battery side, and stayed. I ultimately found that the problem was the spark plug (new). Moving the plug to any other cylinder moved the "miss". I got good spark at the top of the plug, but grounding it made no difference in how the engine ran (so that cylinder wasn't firing). Installing a backup plug (80-year-old Splitdorf) fixed all problems, now runs on mag and/or battery the same. So, I set the "bad" plug on my lawn mower, hooked up the ignition wire, pulled the cord, and the plug sparked as it should! <BR> Can anyone explain that type of plug failure? It's not fouled or dirty, I wire-brushed it, re-installed it in the car, and it's still "bad"!

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