Carb jetting question

Hi, I am new to 450r's and four strokes. I finally decided to try a four stroke atv. I bought a stock 04 450r.

I removed the stock exhaust baffle and removed the airbox lid and now it will bog and die when hitting the gas.
I put the airbox lid back on and it runs pretty good but bogs every now and then and it always has one soft backfire when I turn it off.

It also has a uni air filter.

Sorry I am a newbie but I am guessing it is running lean, do I only need to change the main jet and what size should I go to with a stock exhaust with no baffle, a uni air filter and no airbox lid?

THanks