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There is little doubt that when Honda introduced the 400EX in ’99, it ushered in a new era of high-performance quads. The capable chassis provides the machine with fast yet predictable handling, and the oil-cooled four-stroke motor is perfectly suited to the nimble chassis. Still, many 400EX owners quickly found that they wanted more power and also wanted to improve upon the machine’s already impressive handling abilities. We approached three popular tuners, CT Racing, Duncan Racing and Four Stroke Tech for setup information regarding engine and chassis tuning. Here are their tips on how to coax some more power from your machine along with additional tips on how to improve its handling.

If you’re out bombing around in the woods trailriding, I’ve found that the average guy doesn’t need a set of reservoir shocks—they’re a little on the expensive side. Their best bet is to get a good front set of shocks. A nice set of triple rate shocks without reservoirs can be found for around $200-$300. Once they’re set up for trail riding, they’ll give you a nice plush ride through the woods.

If you properly set up the rear shock according to your weight, it is pretty good for general trail riding. There’s no need to get an expensive shock if all you do is trail ride.
—Mickey Dunlap, FST

Off the showroom floor, I’d say the Honda 400EX is the second best ATV ever made—that’s my personal opinion. With the ’88 Honda TRX250R being number one. There are two things that put it down on my list into second place. One is the power—it could always use more power, and two, the rear end is a bit harsh and it has more to do with the design of the upper shock location. For the average guy, the suspension isn’t as much an issue as the power is.
The 400EX has two weaknesses. The first thing is that we’ve seen subframes crack, especially with a heavier rider.
The other is when jumping them on hard-pack, with a short-course tire, even with a C- or B-rider, the swingarm will crack prematurely. At least prematurely when compared to an aftermarket swingarm, which people will always compare them to. There have been a fair amount of cracked swingarms that I’ve seen out there over the period of time since the machine has been released.
Anyone seriously riding hard should consider replacing the swingarm. Most of the A-riders on the GNC and GNCC circuit replace the swingarm, as do most of the B-riders, and with C-riders making the switch.

Any time we put on a new rear swingarm, we always put on a new Axis rear shock. There are some other reasonably good shocks out there, but Axis just goes that extra mile to make a better part.
In the desert races, we switch over to a Roll Design swingarm and also upgrade the carrier and swap over to a 250R rear axle. With a couple of little tricks you can go to a 250R axle, put the 250R hubs on there which are stronger, and also at the same time you can go to a 250R rear disc which has a smaller diameter which gives you more ground clearance, which is actually more beneficial in any type riding situation.

Woods riders tend to clip a lot of trees. Two problems happen there. One, the swingarm will break. Secondly, Honda, and I don’t know if this was done to cut weight or cut dollars, uses a rear axle on the machine that is very similar to the 250R model. The only difference between the two is that the 400EX axle has a spline that is very narrow and it uses smaller spline hubs than the 250R model. It turns out it is a lot weaker and will bend a lot easier there.

We use a Roll Design Lobo II front suspension kit—it’s the cream of the crop. It takes that bike to another level. It makes the front end almost as good as the one on the 250R aftermarket chassis. The front end allows you to use longer travel shocks thanks to the new design of the arm (different motion and leverage ratios). It helps the machine steer better, there is less bumpsteer and overall the front end is virtually indestructible.
—Loren Duncan, Duncan Racing

Swingarms are the weakest link on the 400EX. It helps to re-valve the rear shock beforehand, but if you’re a big jumper, you’re going to blow it out regardless. It’s really common to go with +1-inch or +1.5-inch aftermarket chromoly swingarm, and then go with +1 forward front A-arms to get your wheelbase set up correctly. If you go with an extended swingarm, you should go with the +1-inch A-arms. The reason why is that when you alter the length of the swingarm, you need to make sure the weight balance is properly biased from front to rear. It’s not super critical to do both at the same time, but in the long run, the machine won’t be perfect. With all under-suspended machines that are ridden very hard, as it is in the 400EX’s case, it tends to fatigue everything quicker. It hurts the frame, sub-frame and swingarm. That’s why it’s a good idea to get the suspension done first; it makes everything last longer.

The rear shock on the machine is a good shock and can be re-worked. Both PEP and TCS are two companies that I know of that are doing brilliant work with the rear shock. They’re getting about two more inches of travel with a no pre-load mod so the bike sits lower. Now that has to go along with a no pre-load front shock setup as well. Technically, if you’re doing things one thing at a time you can do the front shocks first. I recommend that you go with a no pre-load front end because it helps the bike really bite in the corners. There are four good companies out there that are doing no pre-load shocks-Custom Works Stadium series, TCS, Custom Axis, and PEP.
—Allen Knowles, CT Racing

I like to run a lead additive to help the engine run cooler along with half race gas and half super unleaded.
I would like to see a bigger oil-cooler on the machine to help the machine run a little cooler. I haven’t seen anyone develop one yet, and I haven’t tackled it yet either.

Regarding the airbox, as with most tuners, I put a 2.5-inch hole in the airbox lid and fit a K&N filter with an Outerwear filter.

I use a DP9EA spark plug. It’s a colder plug that helps lower the head temperature by five degrees. The spark plug is very close to the exhaust port, and it can warp the valves where they seat. If you run the cooler plug and the lead additive it will help the engine last longer.
—Mickey Dunlap, FST

Reliability—if you change the oil and adjust the valves they are pretty much indestructible.

The most critical thing people can do to get more power out of the machine is to pick the right pipe. That is where you see a lot of guys go right or wrong.
Another large performance thing that we have found from testing is that dabbling at doing performance work internally is a waste of time. For example, if I take a 400EX and put one of my pipes on it, re-jet the carb and add an air-filter, I’ll get a good 18-percent power increase. It’s noticeable—in a 300-yard drag race we’ll gain 15 to 18 bike lengths. It is a lot stronger. Now if I just put a cam in it alone, I’ll gain one or two percent more power. If I do a cam and a 440 kit it’s also barely noticeable. Now if I do an engine kit with a 440 kit with 11:1 compression, port the head, change the guides, cam it, add heavy-duty valve springs and upgrade the carb and do that entire package, we’ll see tremendous power gains, around a 35-percent power gain.
—Loren Duncan, Duncan Racing

I found that through my testing, the more horsepower your engine puts out the bigger the headpipe needs to be. You really need to plan ahead before you buy a pipe. The kind of pipe you’ll need will vary depending on how much motor work you intend on doing. With a stock motor, the stock headpipe is fine. All you really need is a slip-on pipe, and there are a lot of brands to chose from. We’re not finding any appreciable gains on a stock motor with a larger headpipe. On the contrary, there can be a loss of bottom-end power.
If you do a cam and piston, or do some sort of mid-range motor buildup where you’re not going with really big valves or short guides then you’ll want to go to a bigger headpipe.

A really done up motor will benefit
greatly from using a pipe with an even larger headpipe to let the motor make the maximum amount of horsepower.
The stock guides are fairly long, so you can’t put a super big cam in it or it will end up hitting the rockers.
If you are going to buy heavy-duty valve springs, you absolutely have to hard-face the rockers, which any cam place can do. Hard-facing a rocker arm is when the rockers have been ground down and have been resurfaced with a harder face material (a minimum cost of $200). The added spring pressure will lead to helping burn up the rockers. The mild, or midrange cams can be run without stiffer valve springs.
—Allen Knowles, CT Racing

The tranny works good, and shifts really well. The clutch is a little weak it’s easy for an inexperienced rider to burn the clutch out. You have to be careful not to ride it like a two-stroke (engage and disengage the clutch a lot). We use a combination of Hinson parts and our parts. Stock it’s got seven fiber, and six steel plates. Our setup is eight fiber, seven steel plates in a Hinson 8-plate basket, with a Hinson inner clutch and pressure plate with five springs. With this arrangement, it is very successful. Last year we had the only built 400EX that finished the Nevada 2000, the only machine that finished the Baja 2000, and we ran one clutch for the entire GNC series this past year. We’re pretty confident in its reliability.
—Loren Duncan, Duncan Racing

With the stock carb I take the choke out of the back of them for better flow. Doing that makes it a little harder to start initially. The carb comes with an accelerator pump, so all you have to do is stab the throttle a few extra times and it will fire right up.

I modify the stock rev-limiter which allows the machine to rev a lot higher.
—Mickey Dunlap, FST

The stock carb works well, even on high-horsepower applications. The 400EX is definitely not under-carbureted. The only thing that it needs is to be re-jetted properly.

If you have a modified CDI box, here’s a little tip. If your battery voltage is low, your 400EX may not start. It will crank over and act like it will start, but it won’t. Try charging your battery if the machine won’t fire up.
—Allen Knowles, CT Racing

We’re not big fans of changing the CDI box. If you reach the point where the rev-limiter kicks in where a box would theoretically help you, your engine has already quit making power. In the low 8000 rpm range, it’s done making power. In the mid-8000 the rev-limiter should be kicking in. When you find someone that is revving it that high, they should have shifted 500 rpm earlier. It’s a rare occasion if anyone has ever told us that they felt they gained more power with a modified ignition.
—Loren Duncan, Duncan Racing

A steering damper will help take the jolt out of your handlebars when hitting obstacles in the woods. Some good ones that are relatively inexpensive are made by Denton Racing and PEP. There is one made by GPR that is fully adjustable but it’s a little expensive.

Use standard offset wheels. If you go to wide with your wheels and you ride in the woods, you get a lot of hammering in your arms and it doesn’t turn as well.

As for tires I like the Maxxis Razr. The ITP Holeshots work well also, but I like the Razrs better simply because the front tire has that raised center section that digs in a little more aggressively when going through corners which helps keep the front end more stable. I prefer running those tires in the woods since they are much tougher than the stock tires, and you’re less likely to get a flat.

With handlebars I go with a bar that is more straight across. I like that better because I can stand up on the bike and feel more comfortable. The stock bars are a little swept back for my taste. I personally like to use Renthal grips with Renthal ATV bars.
I like Braking brakes. We use a thicker disc with hard pads and we always use braided lines.
—Mickey Dunlap, FST

We use Braking brake components and pads along with a Dominator axle. We also use steel braided line. The Dominator axles are harder to come by and they are more money, but they are the strongest.
—Loren Duncan, Duncan Racing

Using the right oil is crucial in the 400EX since it runs particularly hot. The two oils we recommend are made by Redline and Kendall.
—Allen Knowles, CT Racing
Saturday, May 29, 2010 5:59:46 PM by Chris trx400ex
My Honda trx is a 2008 and I just gave it it's first oil change. After that I rode it and now I can feel the heat through my pants which are predominantly thick. I don't know what exactly I should do or is that normal? it's brand new and everything so far is stock. Any tips, please help
contact me via cell phone or email please
Chris - (419)902-4389
email :

thank you, Chris
Tuesday, August 03, 2010 2:29:50 PM by Richie
I have a 05 400ex and im looking for a place that has the best deals on rebuilding the engine?
contact by cell or email
Richie 3148528307
Richard Rayfield
Tuesday, October 05, 2010 10:50:49 AM by AJ Fonseca
lookin for a good set of tires for my 400ex/ what should i get?? text me some tips. # 419-343-8413
Wednesday, March 07, 2012 6:45:51 PM by vinnie809
I have a 2000 400ex i am changing the oil what should i use? i only got the bike 3days ago. also when i hit a pudle and it splashes up it kills the bike even if it was a small pudle think i can rap the wires and other stuff in plastic bags when i am on the trail and not the track. also how can i make it more quite i dont like to be heard just seen lol not realy aloud to ride where i do .. any tips would help thankyou
Point of signature?
Friday, March 08, 2013 7:10:55 PM by 2001 400ex
i have a 01 400ex and its not getting spark i have a new spark plug i bought a used coil and still no spark is it the cdi box or is there any thing els it coud be

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WARNING: Much of the action de­pict­­ed in this magazine is potentially dan­gerous. Virtually all of the riders seen in our photos are experienced ex­­perts or professionals. Do not at­tempt to duplicate any stunts that are be­­yond your own capabilities. Always wear the appropriate safety gear.
Copyright 2012 Hi-Torque Publications, Inc. All rights reserved.
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