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 - Harvestin Hay: The Early Years (Part 2) - by Pat Browning. The summer of 1950 was the start of a new era in farming for our family. I was thirteen, and Kathy (my oldest sister) was seven. At this age, I believed tractor farming was the only way, hot stuff -- and given a chance I probably would have used the tractor, Dad's first, a 1936 Model "A" John Deere, to go bring in the cows! And I think Dad was ready for some automation too. And so it was that we acquired a good, used J. I. Case, wire tie hay baler. In addition to a person to drive th
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