Needle jets are controlled by the floats and only keep a constant level of fuel in the float bowl for the carb. The pilot jet controls the fuel flow at idle and low rpm's and the main jet takes over at mid range and top end. If you are looking for mileage, you will want to look at your mains and pilots, also a good way to get mileage is clutching. If you change your jets, its going to make it richer or leaner depending on which way you go. Be careful....rather be rich than lean.
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