I am the proud new owner of a 400ex. I am looking for an easy cheap way to add some power. I have read about the pipes and what they can do, what about the k&n power kit or uni filters? What kind of results can someone expect to get with just one of these? I realize that a combo of the two would be better, I will have to add them one at a time due to funds.
thanks for any replies.

400ex 2002 bone stock.

 

I suggest the K&N power kit first.
What this will do is clean up any flat spots in the power range, and give it a little more through out the range. It also will let you tune to your mods, what ever comes next and thier after.
I see most poeple on this site say don't waist your money just bump up the main when you get mods.
Well I heavily disagree, and I would guess that most wo have a jet kit would agree with me.
The mid range gets the most, with the replacement of the stock needle to a more agressive needle and then it's basicly jetting around that.
The kit gives very good instructions a 800 number to assist you and if any thing it gets you in toon with your carb.
A basic hint, almost all kits instructions will leave you slightly rich, don't get complacent jet down to get it rite it will improve with every change.

 

here's some REAL cheap mods you can do

1.) remove the airbox lid - free
2.) up the main jet a size or two - $5
3.) change the front sprocket (down for acceleration, up for top speed) - $15

TOTAL = $20, and about a half-an-hours worth of work

and you WILL notice a difference![img]i/expressions/face-icon-small-wink.gif[/img]

and if you want to do a little more work and spend a little more money, you can make an even bigger difference you can modify the stock exhaust as illustrated here:
http://home.earthlink.net/~gellett/exhaust/index.htm

and you can change the rear sprocket also (larger for acceleration, smaller for top speed) - $30

I think if you remove the airbox lid, change the main jet, needle jet positions, do exhaust modifications, and change both sprockets to 16/36 you could be running 70+mph with no loss in acceleration or low end power... if you did all of the same mods but kept the gearing stock or even went with a 14 front, you might still be able to hit the 65mph stock top speed, but you would have incredible acceleration and low end

 

thanks for the replies. I'm concerned about removing the airbox lid entirely, we ride in some really dusty conditions in the summer. Will a K&N keep out the fine powdery dust well, even with an outerwear? Is a Uni filter better? We ride mostly in the mountains, but some at the Pismo dunes as well.

 

I have been checking that out my self, I have a K&N and ride in some fine dust, as we ride in an area in neworleans wich is sooty fine sandy dust.
I have found thus far the K&N works much better over the stock foam filter, although at this time I don't have an outerwares but will soon and suggest the same to you.
I have found in wet conditions none are 100 percent although the foam are worse than the K&N. I checked this through I have a foam in my sons honda and I have the K&N.
You don't have to run a K&N and I believe they are all fine after market filter, I just used K&N for many years and found it to be my prefference.
Pulling the top of the air box does give benefits to power concerns and thier is a water proof outerwares, just a thought.

 

I have owned stock, K&N, and a uni filter. So far the UNI has been the best overall, doesn't flow quite as good as the K&N but with my experiences it keeps out a lot more crap. ON the K&N after a day of dusty riding there was dust stuck all over the inside of the intake and in the Carb. Never had that problem with the stock or uni. I've also put a jet kit in one of my older quads and it showed little to no advantage over just buying a $3-5 jet! I would do what DSAPP said and you will have more money in you wallet to save for things that would actually help!

 

I would buy the Unifilter and the jet separately to save money for the same result. Then do a silencer (slip-on) rather than replace the perfectly good 400EX stock headpipe. You won't feel as compelled to gear down once you have a silencer and rejet properly done, because it will be pulling that stock 15 tooth gear hard enough to be exiting at last.

For the filter discussion: The K&N in my experience will work fine with an outerwear with no lid on in dusty conditions, but the uni will be better for filtration, and you lose all the better flow from the K&N once you put the prefilter on. Go with a foam filter and be safe, you don't lose anything by doing so. I run a foam filter with no airbox lid on my CR125R, and its way better than the K&N setup with a prefilter and no lid on my 400EX .

 

Not to start a argument, and not to confuse the guy who is asking the question.
But look I do carberation for more than twenty years and do some what they refer to on here as stage two and stage three kits, but here is my question.
The first thing I think of when I have a 400EX is the huge dip in the mid range that shows up on the dynos, now no pilot, main, or air filter corrects it, now the air box lid does alot for the quad, and you have to jet all hondas even stock, because they are so rich.
But look, the dynojet needle works wonders on the mid range and lets you dial it in better, way better. So why do all you honda guys shy away from something that works so good.
Like I said I don't want to argue, just wondering, why this one quad has so many poeple who would talk you out of a $45 kit, but I see the same poeple flush hundreds down the toilet with mods that won't even work with thier set up.
Now my other point about dynojet kits, and I should mention thier suggestions of mains are rich in quads don't know why, but they give you plenty of jets, well any way I like to suggest to guys expecialy the newer to machanics and bikes to get this kit and it actualy teaches them a graet deal about carbs and how to tune them.
I also have had many come in and say these damn kits don't do any thing, go for a ride drop the main down a couple, the guy swears buy them next.
So not for arguement, but for my curiosity, let me know why.

 

thanks for all the input. Now I have some questions about the jet kit. Until now, I haven't had that suggested. Can you tell me how this works and why it is more "tunable" than just replacing jets? I ride at both the coast and at about 6500 ft elevation in the mountains and it may be a concern to me how easy it would be to make jetting changes. Also, how much difference can you feel just by changing jetting and the filter? Does the jet kit come with detailed instructions so I can learn how to adjust for certain altitudes?

 

1) For tunability for going to 6500 feet and back to sea level, this is a tough one, but I have hierd some guys who live so high above sea level that go to sea level what they do is just put thier air box lid back on to richen up for sea level, well your going the other way, so this is a delema you need to address.
2) The kit comes with detailed instructions on jetting requirements per say stock, with pipe,pipe and filter and all of the above at different altitudes. You will recieve about four or five different jets with each having an aplication. You also get a trouble shooting chart and a 800 # with graet technical support from dynojet them selves.
3) The big one, Why I say it's more tunable, let me start from the beginning.
0-25% throttle runs off your pilot or low speed circut, 25-75% runs off your needle, or midrange circut, 75-100% runs off your main, or high speed circut.
Ok knowing this, you will recieve a more agressive needle over the stock one, you will be instructed on changing low speed adjustments and instructed on changing mains to your aplication.
The dynojet jets are different in size and numbering than stock jets (so don't mix them) when you say your at full throttle and running off the main, this is actualy a combination of all three circuts, and the dynojet jets are actualy smaller but because of the taper can flow just as much fuel as the stock larger jets and due to a bike carb working off of differential pressure between the float bowl and venturi, this requires more pressure for fuel to be pushed out a smaller hole.
And were this comes into play is thier is an overlap between circuts, the dynojet jets better controll the overlap so you are able to more procisely tune each level of carberation.
Now this doesn't sound like a physics class I don't know what does[img]i/expressions/face-icon-small-happy.gif[/img]
What I have found to be the graetest gain from my experience with dynojet is not top end horse power but cleaning up the acceleration ranges from bottom to top and at a better pace, now any top end ponnies gained with any mods are more than welcome.
I have found expecialy with the honda XR's, is the carberation once fixed, makes a screaming package.
Now the best part of any type jetting packages is the pure knowledge you will gain while using them.
Now I'll give you fair warning, if you don't get the jetting correct, you won't feel much differents. I found the quad packages suggestions still leave you a little rich, so keep jetting the main down untill you get a good plug read.
For plug read info, go to the ngk web site and you will find in thier a full view of pictures that show you how it should look and what bad ones look like and the solutions on how to fix it.