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 - An AC Model M Crawler - by Anthony West. Neil Atkins is a man in his late thirties, a mild and patient character who talks fondly of his farming heritage. He farms around a hundred and fifty acres of arable land, in a village called Southam, located just outside Leamington Spa in Warwickshire. The soil is a rich dark brown and is well looked after. unlike some areas in the midlands it is also fairly flat, broken only by hedgerows and the occasional valley and brook. A copse of wildbreaking silver birch and oak trees surround the top si
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