A carbs jets are what allows the fuel into the carb itself. Changing the size of the individual jets changes the amount of fuel allowed in. There needs to be the correct amount of fuel to air ratio inorder for the engine to run at peak performance.
Say for instance you wanted to put a new pipe on your sled. Most after market pipes allow a consideral more amount of air to flow freely through the "system". Since more air is allowed through, the engine will burn hotter. If you didn't "re-jet", you'd end up melting a piston.
So you'd need to get a larger jet size to allow more fuel into the carb to compensate for the increased flow from the new pipe. You would of course have to rejet each of the carbs your sled has.
This is kind of a general description, But I hope it helps.
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