Going with Champion Spark Plug numbers, the original standard plug was a D16. That was then changed to D15Y. The plug has about the same effective heat range because it is one heat range colder but with an extended tip plug. The plug for light duty was D18Y. After leaded gasoline was phased out Champion changed the recommendation to D15Y for heavy use, D18Y for standard duty, and D21 for light duty.
Because the D21 is a pretty hot plug it can causes problems if used for heavy duty use. It can cause preignition if it runs too hot.
I use D21s in my I2444 with good results but that engine (C-153) is rarely used at more that about 2/3 throttle and light load. On my Super C with a 6 foot Wood mower I run D18Ys because that engine runs under much more load.
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Today's Featured Article - Restoration Story: 1929 McCormick-Deering 10-20 - by Randal Simmon. On July 4th 1998 my dad and I was sitting on the front porch when he said that he was going to sell his two 10-20's to a junk man. He said that the junk man was going to give him $500 for both of them but he hadn't received the money for them yet. So I went into the house and wrote him a check for $500 and bought them myself and said now they are mine. He had bought them about five years before thinking that he was going to restore them himself. After five years sitting in the barn, he changed h
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