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The usual problem is poor crankcase compression. The smaller the piston , the more sensitive it is too ware. A thousandth or two is hardy noticed on a big piston, but is a large amount as a percentage of size on a small one. Singles and alternate firing twins load the forward part of the main bearings which causes them to became egg shaped. This coupled with a lack of seals also causes a loss of pressure. The 2-½ horse Johnson is also sensitive in that respect. You can compensate somewhat by using a heavier oil. ie. SAE 40 or 60 weight. This would probably require a hotter heat range spark plug and will carbon over the exhaust ports. Some have knearled or stretched the piston skirt to increase sealing, but you don’t have much material to work with.
Garry in Michigan
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