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 - Tractor Hydraulics - The Basics - by Curtis Von Fange. Hydraulics was one of the greatest inventions for helping man compound the work he can do. It’s amazing how a little floor jack can lift tons and tons of weight with just the flick of a handle. What’s even more amazing is that all the principals of hydraulic theory can be wrapped up in such a small package. This same package applies to any hydraulic system from the largest bulldozer to the oldest and smallest tractor. This short series will take a look at the basic layout of a simple hydraul
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