Would I have to re-jet my carb when applying an after-market filter? This may be a stupid question so flamers welcome.

 

You dont have to but you should and you get a litte more power.

 

if you keep your lid on and just replace your filter you wont, but if you take your lid off you will.

 

Depends on the bike, and depends on the filter (foam vs K&N). You don't say what bike you have and what kind of filter, so we are at a loss to be of much help.

Like he said, in a lot of cases the limiting factor is the snorkle size, so venting the airbox lid will make more difference than any filter.

One thing to consider is that if you don't have to rejet, you didn't realize any increase in performance!

 

I'm sorry bout that, my fault. My bike is a KFX400, and the filter i had in mind was K&N. Normal airbox, with lid.

 

Ok, that helps. That engine loves to breathe! Just taking your airbox lid off will net you some great HP gains. I prefer the EHS Racing lid (http://www.ehsracing.com/) with an Outerwears across the opening. On the other hand, you may live where it is wet, and not be able to take the lid off. Something you will need to decide.

I would say that with a K&N, you will probably need to rejet, even with the lid on. But, it isn't worth a rejet just for that, so you might as well get an aftermarket exhaust at the same time. Otherwise, you are going to be rejetting again later when you get one.

Personally, I just don't trust K&N! I would encourage you to check out Pro Designs foam filter (http://www.prodesignracing.com/). It comes with an adapter that bolts into the airbox, and the filter is huge, and flows plenty of air. If you insist on a K&N, absolutely run an Outerwears on it.

If you don't rejet and you needed to, you will end up running LEAN, and burn up your engine!

 

When I was looking at air filters for my z400, I looked at the k&n and it says when you click on it that a jetting change is not required and that the air filter is designed to work with stock jets. You would get alittle more power with a jetting change but Im just letting you know its not required and it will still work fine