The stock air filter needs to be replaced with a Twin Air as fast as possible, since the stock filter has a built in leak (which you should be applying grease to every time you clean it).

Sometimes gas leaks into carb when riding on a trailer, so shut the gas off.

(99 and older) Put rear wheels from a 650 or later 400 on it, so you can use 12" tires all around. The selection is better for 12" tires.

(99 and older w/ composite racks) If the racks com in contact with a tree or another quad (like a 650) the 400 will loose severely since the racks can break easily. If I had an older 400. The first time a rack broke, I would replace it with a steel one from a later 400 (even if I had to modify it to fit). The rear steel racks are a direct fit. The front steel racks may require ingenuity to adapt to the older models.

(99 an older) If you roll the 400, the steel front steering knuckles may bend, causing the brakes to not work as well. Just don't roll it. But if I were to roll an older 400 again, I would get the later model 400 aluminum steering knuckles (both at same time). It's not a direct fit but can be done by swapping calipers left to right, and drilling holes for the larger tie rods or useing later tie rods.

The 400 has more wheelie tendencies than a 650 when loading into ramps and climbing hills. Make sure you are in low range when climbing to better prevent it. Use body language. If you need to lean forward so your stomach is on the handlebars and your helmet is on the front rack, better safe than sorry.

Adjusting valves? Take the front tires off and inner shield out and see if you can get at the head without removing the front fender (cuz it is a lot of work). Take gas tank off if needed. Unscrew fuel switch before messing with cover plastic.

Protect the shifter from being damaged, you can't buy pieces of it, you have to buy the whole thing at $310. You could disassemble it and have it welded a lot cheaper. Get all plastic parts out of the inside of the aluminum parts before welding.