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I have an 03 400ex and its in overall awesome condition except the plastic and it's red right now but I think i'm going to order all new yellow plastic for it soon. But, I was wondering what I could do to beef it up and make it a better overall machine, while not spending a lot of money. It already has an HMF pipe, and i've considered getting an airfilter, but I dont know what to get. I mostly trail ride and occasionally go in the water and mud. Any good upgrades? Or anything in general?
 

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I heard k&N's let water in the engine? or is that wrong. I thought about getting that with outerwear, would that make a little difference?
 

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I have a K&N on my 400 and I haven't had that problem. I guess outer wear would be the same as the dustcover. I have been in some deep water with mine and I haven't had any issues with water. You may have a problem if you take the air box lid off and hit a bunch of water holes.
 

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I'm sure someone on hear will give you way better advice on air filters than I can.
Hear is a pic of some of the water I go into with mine and the last time I rode it was deeper.
 

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K&N is a great filter so is UNI they are good for dusty riding the Uni tends to trap more dust but that's an opinion. If you have a HMF pipe I would suggest a new airfilter and maybe look into jetting the carb. you can get those fairly inexpensive at the dealer or buy the dynojet kit everything you need to know is in the kit if your still not sure you can call them they will tell you exactly what you need.
 

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Welcome, If you still want more. I have heard of people putting a XR400 cam into the 400EX. Curtis Sparks makes a Ignition Advance Key. This advances your timing by 6* resulting in a snappier throttle and more HP.

Do not run the Sparks Key on a built motor. It will cause damage from extensive heat. Only safe on stock motor.
 

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I agree 100%. K&N with a pre-filter is good in my book though.
 

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UNI is a foam filter, and K&N is really proud of their products. I prefer a foam filter myself. No-Toil makes a great foam filter. Honda uses UNI from the factory.
 

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Take the wires that go to your key and hook them up to a toggle switch.
 

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k&n is the best i have not had no probem.that is the kind of riding i do .
I do a lot of riding in the sand and have used k&n for 10yrs with no issues AS long as you use a pre-filter with it. K&N is so expensive cause they are reusable many yrs over just spray on they're cleaner and spray off with water then use they're filter oil and go very painless. The pre-filter actually repel water, but not if you submerse it in mud lol.
 
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