A plug will always work best when it's new. If it's gaped properly, the engine is healthy, the proper heat range, the mixture is right, and getting good spark, it will last a long time.
But even under optimal conditions exposure to combustion will leave an ever so slight coating of carbon on the insulator. The carbon is conductive, so some of the spark charge will bleed off through the carbon. A too cold plug will accumulate more carbon, eventually to the point it will not fire at all.
If the original plug was too cold, then it could have been having bleed off problems, or if it was worn to the point the gap was too wide for the spark to reliably jump, a new plug will fix it.
But if the problem returns after a few miles, there could be a problem with weak spark at low RPM, or a too lean mixture.
If that has a carburetor, it is probably a slide type carb with a tapered needle under the slide. There is a snap ring with adjustment groves to set the position of the needle. Try raising the needle one groove. It's easy, and returnable to the original if it doesn't work, but I think you will like the results.
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Today's Featured Article - Earthmaster Project Progress Just a little update on my Earthmaster......it's back from the dead! I pulled the head, and soaked the stuck valves with mystery oil overnight, re-installed the head, and bingo, the compression returned. But alas, my carb foiled me again, it would fire a second then flood out. After numerous dead ends for a replacement carb, I went to work fixing mine.I soldered new floats on the float arm, they came from an old motorcycle carb, replaced the packing on the throttle shaft with o-rings, cut new ga
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