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We remember when the air-cooled sport quad that Honda now calls the TRX400X was a new and very exciting deal. At the time, this quad and its XR400R off-road motorcycle-based  engine formed the backbone of sport quad sales and was a huge factor in cross-country racing. In fact, a Dirt Wheels team won the Baja 1000 on one. Dirt Wheels wisely chose to leave the machine largely stock, but that was hardly the case for other racers. At GNCC races, it was normal to see Honda 400s with hardly a stock part left on it. Full-suspension mods, billet covers and entirely new clutches, in addition to all sorts of engine mods, were absolutely normal. We still believe the 400 is loaded with the X factor and that it is a viable choice as a sport quad for those who want to ride rather than check coolant. With that in mind, we gave the X a hard look and made some common-sense mods that would improve the fun factor. In spite of our confidence in the machine, we are realistic. In no way did we want to build a replica of one of those cash-cow GNCC racers. That would make no sense when Honda has a TRX450 for riders who want to get that serious.


The beauty of the 400X is the air-cooled simplicity and rock-solid reliability. We didn’t want to make any mods that would change that. Mods that required much more maintenance or risked reliability were strictly out of the question. We chose not to modify the airbox or change the stock air filter.

One of our first choices was to find a little more boost through an FMF slip-on Powercore 4 muffler with a spark-arrestor insert. We’ve had great success with these mufflers, and they offer a lot of quiet performance for the money. The pipe was our first stop, and not just because we were looking for more responsive power. Our hope was that the engine would run fine with the FMF pipe and the stock jetting. The pipe was an easy fit. The band-type rear mount is a little fussy until you figure it out, but the inherent adjustment makes mounting a sure thing—even if the quad has had a hard life, you can be sure everything will line up. Starting cold, idle and throttle response were all just fine with the pipe, so we left the carb stock. That was the sum total of the engine mods.


While Honda is continually and rightly praised for making ATVs that just feel right, the cockpit of the X is a little scrunched for our crew. A great answer for that was a Renthal ATV Race bar that has a lot less sweep. That aids the standing riding position, though the bar is a little lower than stock. The trade-off was more than acceptable. Swapping out the bar is a little time-consuming. The stock switch pod on the left side has an internal nub that plugs into a hole in the stock steel bar. That needs to be filed down. We used a small rotary rasp in a cordless drill. The rasp is aggressive for the plastic, but the relatively low rpm of the drill made it quick and easy. The steering tube cover has some lights and the ignition switch, so it must be used, but it bolted to a tab on the stock handlebar that the Renthal does not have. We used the cordless drill again. It quickly popped four holes in the plastic cover, and two long zip-ties mounted it solidly. Room for the key is tight if you use the crossbar pad, but we made it work. Renthal half-waffle medium grips are long-time faves, and we glued a pair in place with no sweat.


One of our priorities was lower gearing to bring the gears closer together, but that proved difficult. Renthal offered larger countershaft sprockets and smaller rear sprockets, the opposite direction from where we wanted to go. In the end, we chose to replace the heavy steel rear sprocket with a lighter Renthal aluminum one. It fit perfectly.

Another priority was to get the machine a little lower and more stable on cambers and while turning. We turned to GBC for some lower-profile race-spec tires. GBC offers an 18-inch and a 20-inch rear tire. We considered the 18s, but they require an 8-inch rim, and we worried they would sacrifice too much ground clearance, so we opted for the 20s. GBC suggested the six-ply-rated XC Racer tires. The stock Ohtsu 20-inch rears actually measured 19.5 inches tall, while the GBC XC Racer 20s measured 19 inches. The fronts measured about the same as stock. Naturally, the tires have a more aggressive tread and overall shape. They also put more tread on the ground in the rear and have a less wallowy feel in terms of handling and suspension.


Much of our test riding was in an area with packed dirt and rock with loose rocks in the trail. We never had an issue with ground clearance, so we may go ahead and try the 18-inch rears at a later date to see if we can get a lower center-of-gravity feel in the turns. Overall, though, we were very pleased with the way Project X turned out. GBC hit a home run with the tire choice. Traction is abundant front and rear, whether the quad is headed in a straight line or not. The handling is crisp with less bounce and wallow to the tires, so steering is more accurate. The rear will still slide but has no tendency to break all the way loose. On steep, loose climbs littered with rocks, the X grabbed hard and accelerated smartly.

The tires allow the quad to put the pipe to good use. It greatly enhanced the response to the throttle, and the engine has more torque everywhere, but especially right off the bottom. In stock trim the engine is soft enough down low to be annoying. We had a Suzuki Z400 along, and it has more high-rpm pull, but even with similar mods, it doesn’t match the X’s initial snap. Add in the fact that the standing riding position is opened up and we have a 400X that is more addicted to fun than ever.

FMF Racing: www.fmfracing.com
Powercore 4    $299.99
GBC Tire: www.gbcmotorsports.com
XC Racer tires
AT20x11/00-9 rear    $97.70
AT21x7/00-10 front    $68.89
Renthal USA: www.renthal.com
ATV 7⁄8-inch bar     $74.95
Half-waffle grips    $13.95
Aluminum rear sprocket    $64.99
Trail Tech: www.trailtech.net
Dual Equinox LED lights w/ handlebar switch    $405

The Honda TRX400X has always been a dead-reliable and fun sport quad. We aimed to increase the fun without affecting the reliability at all.

We chose an FMF Powercore 4 slip-on muffler to help the power and GBC XC Racer tires to get that power to the ground. It turned out to be a great combination.

We already had Trail Tech Equinox LED lights mounted up, and in combination with the stock headlight, we were more than happy to keep riding into the darkness.

We wanted to keep our mods basic to ensure that they offered good value for the money and improved the fun factor of the 400X on the trail.

FMF’s budget slip-on is called the Powercore 4. It comes with a spark arrestor, has a very reasonable sound level, and really helped the 400X have more torque and response down low.

GBC’s XC Racer front tires are six-ply rated, so they are off-road-tough but ride well at the 5 pounds of pressure we ran. The steering was light but crisp.

The GBC XC Racer rear tires were a half-inch lower than the stock rears and have a flatter profile that puts more tire on the ground. Traction was great, and there was hardly any wallow.

The Renthal ATV Race handlebar fit the stock mounts and opened up the cockpit nicely. For feel, the Renthal grips are great, but there are trade-offs for the bar. You feel a little more vibration but fewer bumps.

Topic: Machines


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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 2008 Hi-Torque Publications, Inc. All rights reserved.
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