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 - Identifying Tractor Noises - by Curtis Von Fange. Listening To Your Tractor : Part 3 - In this series we are continuing to learn the fine art of listening to our tractor in hopes of keeping it running longer. One particularly important facet is to hear and identify the particular noises that our
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