Honda ATC 350X Rebuild

When it was a new high-performance thumper nearly 30 years ago, the 1985–1986 Honda ATC350X was the biggest performance four-stroke three-wheeler around. It, like nearly all three-wheelers of the time, was more of a serious sport machine for the trail than a racer. At Dirt Wheels, we have seen a resurgence of riders restoring these classic machines, though most are more of what street-rodding calls a “restomod.” In other words, these machines are not returned to showroom condition, but to the level that the owner dreamed of back in the day. Very few restorations make total sense financially. You could probably buy the same machine that someone already completed, or something a lot newer for less money, but many of us like to relive our youth, and for some that is priceless. For others, a project like this is a father/son learning experience.

This build is unusual since it was built with racing in mind. Annual events like Trikefest in Laurel, Indiana, and the Imperial Invasion held in the Imperial Sand Dunes have helped the three-wheeled community stay alive. Thanks to race clubs like the Ohio Trike Crew, New York Trike Racing Outlaws and CALVMX, there are now motocross and flat tracks scattered from coast to coast that allow vintage and modern three-wheeler racing. An ATV Action article published in 1985, featuring a CT Racing, national-caliber ATC250R, inspired us to see how well we could make the 350X perform for vintage motocross racing. This build also serves as a buyer’s guide for any owner looking to restore or upgrade their machine for the track, trail or anywhere in between. All of the mods make the 350 a better performer for trails or the dunes as well.


The 350X wasn’t designed and built for the track, but there were those who chose to race the X at a time when racing four-strokes was quirky, or at least different from the norm. Obviously, starting a project with a complete machine in good condition is the best plan, but we are talking about three-wheelers that are almost three decades old. Most serious fans wouldn’t want to cut up a clean original, so we found a cheap but rough example for $1200. Beginning with a full tear-down, our machine’s rebirth started with having the frame repaired and gusseted by TPC Trikes with its 19-piece gusset kits. Killer Koatings in Independence, Kentucky, powdercoated the finished frame Honda red. All Balls head tube, swingarm, and axle carrier bearings replaced the stock bearings, providing a tight, wobble-free chassis. We installed a Dura Blue heavy-duty +4-inch, adjustable-width axle and rear racing hubs for motocross.


We needed to significantly improve the 350X’s suspension, so we turned to vintage suspension experts at Race Tech. Race Tech installed its Gold Valve Emulator to mimic the function of a cartridge fork’s tenability and installed stiffer .52 kg/mm fork springs. After testing, we switched to a 20-weight fork oil set at 5.5 inches
from the top of the tube.

Race Tech replaced the stock shock piston with its high-flow Gold Valve kit, and the under-sprung stock spring was replaced with a 16.0 kg/mm spring. Both ends received fresh fluid, and any worn items were replaced.


The 350X’s stock head pipe restricts power, and Harkabusic Racing offers a better-performing, cleaned and polished 400EX head pipe modified to fit the 350X. We paired it with the Supertrapp silencer that came on our 350X. CT Racing has been building motors since three-wheeler days, so they know what works for these unique machines. CT’s 350X national motor provides more horsepower and torque across a broader rpm range. CT bored our cylinder 1mm and installed a 12:1 compression LAPC piston, ported the head, cut a five-angle valve job and sent our cam to Megacycle to be hard-faced and re-ground. Megacycle says this cam must use stiffer R/D valve springs and Megacycle hard-faced stock rocker arms.

On eBay we found a new clutch assembly and a set of NOS center engine cases. We acquired a used Honda 400X/400EX carburetor from Power Sports Nation salvage. The 38mm carburetor is not only larger than the stock 32mm unit, it has an accelerator pump.

With increased horsepower and 4-inch-shorter tires, we needed to gear up. Sprocket Specialists is one of the few companies producing sprockets for the X, and they will cut sprockets to fit your needs. We went with 13/34 rather than the stock 13/40. A Renthal R3-2 520 O-ring chain provides great strength and low maintenance.


We wanted our X’s braking performance as crisp as its engine, so we used Rocky Mountain ATV’s Tusk brake pads and HEL USA brake lines. The steel-braided lines enhance brake power and feel, while various line and fitment color options let you customize the look of your machine.



Kenda’s front Max and Scorpion rear tires have been around since three-wheeler days. The 23.5°—8-11 front is an uncommon size due to the 11-inch wheel diameter. These tires feature a harder rubber compound than modern motocross tires, which helps them slide easily and last. We went with 18°—9.5-8s for a motocross-appropriate height and the narrowest possible width, again looking to make breaking the back end loose as easy as possible. OMF’s 8-inch billet center beadlock wheels provide strength, security and sex appeal. They start life as a strong DWT 190 wheel. Being able to swap out the billet centers allows you to change the wheel’s bolt pattern, offset, style and color.

PRM has a line of aluminum hard parts for the 350X and other popular three-wheelers. Their chassis skid plate features drain holes that allow water and mud to wash away. PRM’s Z-plate swingarm skid plate offers lots of coverage and is designed to provide maximum ground clear-ance. We decided to run their Desert grab bar.


Maier Manufacturing offers plastic for many vintage applications. Their body parts are thermoformed from TPO polypropylene with a high-gloss cap sheet. We acquired a set of white plastic, including front and rear fenders; Super Shrouds, which are larger than the stock tank shrouds; and a front number plate. The shine and fender fitment on the 350X is on par with factory plastic. It’s claimed to hold up well to the elements and is proving to be pretty chemical- and scratch-resistant.

We were relieved to find that Clarke Mfg. has red, white, and black plastic tanks for the X and other vintage models. The tank mounted easily. The Clarke plastic tank is lighter and easier on the knees than metal.


Our 350X now has a modern-feeling cockpit. Cosmic Quads used a new Hi-Flite Desert foam wrapped in a custom seat cover it created for us. The foam makes the transition from sitting to standing easier, while providing more room to hang off the side for corners.

Renthal’s Vintage Desert handlebar provides the wide clamping area and tall height needed for the ATC. Compared to our narrow, dull and creaky stock pegs, the HRE’s 2-inchwide chromoly footpegs offer far greater comfort and boot traction. A pair of ASV F3 “unbreakable” levers will bend backwards in the event of a rollover but don’t break. We had to modify our front brake lever a little to work on the old 350X.The part of the brake lever that depresses the plunger on the front-brake master cylinder is a little long, so we had to shorten it.


Everything about riding the 350X turned out better than expected. While it’s not quite a museum piece or show machine, it is a head-turner. You might forget that you are on a machine from 1985. The revamped Cosmic Quads’ seat was huge in modernizing the feeling of the machine. All of the other contact points yielded enhanced comfort and control. Our machine’s brakes now feel as crisp as the brakes on the 2013 Raptor 700 currently sitting next to it in the garage.

There is more power off-idle, bolstered midrange that you need for pulling holeshots, and a substantial boost in top-end power. While the top-end power is much more abundant, this is still a midrange motor, so we never found ourselves just revving its guts out. We suspect that this motor will last a good long time as long as the air filter and oil are kept clean.

Breaking it loose for slides and attacking big jumps comes much more effortlessly with this motor. It’s not only notably faster than stock; it’s easier and more fun to ride. If it weren’t for the chassis and suspension upgrades, the increase in horsepower would make this a scary machine to ride fast on a motocross track. Less frame flex provides a better feeling of how the machine is reacting to the track, better perception of how the suspension is behaving and has a small impact on enhancing control when you are really pushing it.

The Kenda front tire provided precise steering on any track surface from chunky to hardpack. For motocross or trail riding, it’s a one-tire-fitsall solution. Kenda’s Scorpion rear tires provide enough forward bite in everything except mud, and they have the easy-to-slide characteristics needed to go fast on a three-wheeler. On loamy tracks, they work better just inside or outside the race line where the track isn’t packed down. There may be better back tires for rock-hard tracks or muddy trails, but the Scorpions are a great MX tire choice for trikes. In spite of their beadlocks and reinforcing rings, OMF did a great job of keeping our rear wheels light, adding minimally to rotating mass. They look so awesome that we don’t want to scuff them. Thanks to the Race Tech suspension, you can push the X a lot harder in the rough stuff. The rear end performs better on bumps of all sizes. It’s more responsive to small impacts, which lets the rear end hook up well on chunky, disced track surfaces.
The fork enjoys good, small bump absorption as well. On jumps the machine sticks landings, and it takes a considerably bigger hit to bottom out either end. In rollers and widely spaced whoop sections, the suspension is pretty phenomenal. The fork responds better to impacts, and the rear end features a fluid movement, which keeps the rear end tracking and the wheels driving. Overall, suspension balance is biased just a bit toward the front, which helps keep the rear end more planted in braking bumps, where a three-wheeler can easily get in trouble.


You have to acquire and restore a vintage machine to understand how cool it is to complete a project like this. It’s affordable to get into, and you can build up and complete a restoration over the course of a couple years. We’ve found that the discontinued parts are out there if you look a little. Most replacement and all of the performance parts you’ll need to build a vintage racer are still being manufactured today.

Comments are closed, but trackbacks and pingbacks are open.