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Carburetor jet sizing and how a carb works. 

The fuel system is designed to provide the optimum mixture of air and fuel to the engine under varying conditions. This optimum air/fuel ratio is called the stoichiometric ratio and in theory is approximately 14.7 parts of air to 1 part gasoline (14.7:1) based on weight, not volume. However, this air/fuel ratio must be modified under different atmospheric and running conditions. A ratio numerically higher than 14.7 would be considered a lean air/fuel mixture, numerically less than 14.7 would be considered a rich mixture.

The jets on a carburetor meter the amount of fuel that enters the throttle bores of the carburetor where it mixes with incoming air. If the engine shows performance problems such as stalling or sluggish acceleration, you'll need to change the jet sizing. Jets are threaded and have a small orifice through their center that varies based on the size of the jet. For example, a jet may be stamped with the number 115. Installing a smaller number size jet will reduce fuel flow (like a 110), is ideal for high-altitude conditions where air is limited, where a larger jet size (120) increases fuel flow for oxygen-rich, low altitudes.