You can run with the lid off with no problems. A cover just helps to keep you from having to clean your filter so often. I bought a Outwears lid cover, while not perfect it does repel water and keeps about 99% of the big stuff off your filter. You can even use an old pair of Pantie hose and duct tape or velcro to attach it down. Let me save you some money and time when it comes to jetting your 400. Forget about a jet kit and just buy from your local dealer at he parts counter. Bump up the stock pilot from 22 to a 25. This will help with starting and idling. I tried an adjustable neddle and have found stock to be just fine. Then repalce the stock 120 main jet with a 160 and look out. I also added a Whitbrothers E2 exhaust and along with a Proflow intake with a uni filter and jetting this thing runs right with my 450R. I like the ride much better on the z also.
If you have never rejetted a quad before spend a few hours on the internet searching and reading about jetting before you get started. It pretty simple. Dont over tighten the jets, just snug them up. You should also remove the plug to your fuel screw so you can make fine adjustments to it. There is some great how to info on this subjuect as well. Engoy you 400, it's one great quad!

 

<div class="FTQUOTE"><begin quote>Originally posted by: garr79

I ride in dusty areas (dirt, sand, mud). If i run with the airbox lid off, wont that hurt the bike?</end quote></div>

I ride everything from mud to puddles the size of lakes it seems like at times. I have had no issues at all. Just make sure you have the cover for the K&N filter. Those filters are so tight there is nothing getting through and the cover is waterproof.

 

get a Eddie Sanders Racing Outerwears Lid. Its like the stock lid but has a opening in the middle with a filter. Almost the same as having no lid but no dirt gets in.

Now that you have the bike inhaling better, get a full exhaust system to make it exhaul better.

Then buy a Jetkit that comes with the adjustable needle,main jets. I'd start with needle on 3rd clip, 160main and 2 turns out on the fuel screw.