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Fuel in a K21S???

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Regular unleaded gasoline can be used in a B&S but
Unleaded gasoline is easier on the valves as there are less deposits on the stem.
this statement is wrong...lead was put into the gasolines as a lubricant after time the valves would hit the seats and mushroom the seat seal witch would cause valve seat problems after seats and valves were changed, unleaded gas was used without problems. B&S or Kohler does not run at high RPM's or for long period of times to worry about such problems but on overhaul or rebuild it may be something to consider. depending on what year the motor is.

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Leaded gas was an anti knock addtive. Did a lot of wire brushing on one of the valves to get white (lead) deposits off. http://www.uwsp.edu/geo/courses/geog100/Lead-Science.htm

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that is only part of it the horror stories about engine damage due to unleaded gas isn't pure hype - trying to get you to pay money for uneeded gas additives (which don't help at all, or at least not that anyone has been able to measure carefully) or to rush down to your machinist get get your valve seats changed. Just drive your tractor, and be aware that when and if you ever need a valve job, consider whether or not to throw in an addition $50 to get the hardened seats installed at that time.

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