First off, thanks for looking at the FAQ, at least you looked before you asked
You need to be a little more descriptive about what you dont understand.
Without knowing specifics, I'll take a stab at helping you.
An engine is like a air pump. the pump (engine) works based off of three basic ingredients- air, fuel, and spark. For basic purposes, spark usually doesn't need to be adjusted regardless of mods (it could benefit a litttle bit but thats getting pretty technical). that said, that leaves us with two ingredients, air and fuel.
For the engine to run properly, a certain amount of air needs to mix with the given amount of fuel flowing into the motor. this mixture is called your Air to Fuel ratio- commonly abbreviated as A/F or AFR. for best gas mileage, the ideal AFR is 14.7:1 , but if you're trying to make maximum power, you want roughly 12:1.
So let's say you're running lean (not enough fuel to mix with the air) , that would mean you have a high (example: 17:1) AFR. not good for the motor, you'll experiencing overheating, backfiring, poor performance.
A rich mixture will lead to bogging, poor performance, etc. This will lead to a low AFR (e.g 10:1 or so). It means too much fuel being mixed with the air.
So the whole point of rejetting, is to provide enough fuel to appropriately mix with the air coming in. stuff like airbox mods, exhausts, aftermarket air filters, all that stuff will increase the airflow coming into the engine. so you need to add more fuel. Rejetting does this.
edit: It is totally a DIY job, have no fear!
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