Ok, so I want to added on a GYTR air filter with a prefilter and removed the airbox lid, my friend says I should rejet since I took the air box lid off. Need to rejet?

 

Most likely.
Go by the drivability symptoms. Is it hard to start when cold? Is there a bog or backfire? Does the engine miss? Does the engine surge at steady throttle openings?
If you have any of those, or others, then it needs rejetting.

 

You'll have to rejet. The stock 350 airbox and snorkel is pretty darn restrictive. Just removing the airbox lid and rejetting on a yfm350 will give you close to the power gains you get from a good exhaust. When you let that much more air into the motor you also need to add more fuel to get the biggest power gain.

 

More fuel meaning keep it full at all times? Or as in begger jets? I already ordered GYTR jets.

 

I mean making the fuel mixture richer. A bigger main jet, and you might have to richen the needle and the idle mix too.