If i were to buy a K and N air filter for my 400 ex would i have to re-jet it. If i had to re-jet it how would i go about doing that or would i have to bring it to the dealer?

 

my friend has a K and N filter on his 400ex. at first he did not rejet the carb. he had no more power than i did and mine is all stock. whoever won off the line was going to win the race. the other day he rejetted the carb. he just went from the stock keihin 148 main jet to a keihin 150 main jet. he did not go out and buy the whole jet kit but instead just bought that one jet for 2 or 3 dollars. now he pulls away from me in fourth and fifth. this is something you will be able to do yourself. the dealer could do it but it is really simple. i always take the carb off so i can work with it. first off i take remove the rear fenders, then the black plastic by your knees when you are sitting on the bike, then the smaller piece of plastic right on top of the gas tank, then i pull off the tank. you can leave the front fenders on and slide the gas tank out. before you take the gas tank off make sure you turn the gas on off and disconnect the hose running from the tank to the carb. now slide the tank off. then there is a clamp infront of the carb and one right in back of the carb. all you need is a fill ups skrewdriver and loosen both clamps. it is a tight fit but you can pull the carb off now. now just the throttle cable will be connected. you can leave that there in place. now that the carb is off you can find the mainjet. on the bottom of the carb there is a bolt that is 17mm. if you unskrew this and look inside, there is a brass piece of metal. this is your mainjet. all you have to do is unskrew this and replace it with the appropriate jet. my friend put in a keihin 150 and seemed to work good. now you just reassemble. just make sure, if you do not go with the brand keihin get the appropriate # jet. a keihin 150 is different from a mikuni 150 which is different from a dynojet 150 etc. hope this is not too confusing

 

Most quads are not properly jetted for the altitudes they are ridden in from the factory (mine was lean from the factory).
Rejetting should help any quad.

 

When you see the K and N really give performance is when you put a pipe on. The K&N commercials say that they give cars 20HP.4 instance if you took a stock Mustang and put just the air filter on you will only get about 1hp.But if you put exhausts and headers on you will see a lot more power from it.Same thing with the EX.