What does the choke on a grassland mower adjust?

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Answers:    The choke "chokes off" some of the air coming in the carburetter. The choke closes a flapper spigot over the intake. If you remove the filter you likely can see it.

This changes the fuel/air mixture.

When cold or running slower most engines involve a richer mixture for burning and when warmer, or more rpms, it wants the air/fuel mixture to be leaner, (more air).
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The choke is used to draw more gasoline into the cylinder by restricting the upper air flow. Before automatic or manual chokes came along, society used the palm of their hand to cover air intake or the first of the carburetor which was pretty risky if the motor backfired resulting contained by a nasty burn.