Polaris rejet
 

I have a 2003 sportsman 400 and I'm looking to buy exhausts clutches and I wan to to reject the carb but in not sure what exactly to do for the reject.

All depends on what you end up with.In some cases you may not even have to rejet. Just an exhaust and a clutch kit may not be that extreme. If you add a freer flowing air filter then that might be a different story. One way to tell is is if it hesitates or bogs,another is to kill the engine at a high speed(plug chop) and check the color on the spark plug. Needs to be medium brown. If whitish or light tan in color then increasing at least the main jet will be needed along with a possible clip drop on the jet needle. Look at this carb section pdf from the manual that can show you several things.You have the BST34 carb. https://www.google.com/url?sa=t&rct=...U2gGlaHJWSIlbA

polaris 400 rejet
 

i have a uni air filter idk if thats a better filter than stock is that one where would have to rejet.

Again all ways see how it performs with the stock jetting and do a plug chop. Some machines were jetted on the rich side from the factory to begin with. Old rule of thumb if you have to jump up is 10-15% increase over stock jetting.POLARIS Sportsman 400 main pilot slow jets kit