Nicholas Fluhart

September 18, 2010

Honda TRX350XX (TRX250X – ATC350X Conversion) TRX350X

Filed under: Bikes, Trikes, ATVs — Nicholas Fluhart @ 12:01 am
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From the ATC era of the mid 80’s up until the end of the following decade Honda and most of the ATV industry was void of any heavy-hitters, performance wise, for trail riding. Of course there were still some thoroughbred race bikes (although none from Honda after 1989), but for off-the-track trail riding you were very limited. The Honda options of the day were the TRX250X and later the TRX300EX, both only middle weight bikes. The 250X was a kick start model with a nice 4-valve single overhead cam engine with a 5-speed transmission. The subsequent 300EX was an electric start model with basically the same engine but with a lengthened stroke making the displacement a little larger. Yamaha had a leg-up on the industry, really in all categories race and trail, and the king of the sport trail bikes was the 350 Warrior which had a stout 2-valve single overhead cam engine with a 6-speed transmission. The paradigm shift occurred in 1999 with Honda’s introduction of the TRX400EX. But up until then, no manufacturer had produced a 4-stroke trail ATV with the bulk power, torque, and speed of the ATC350X last produced in 1986. The industry had decent four-wheeled chassis but no major power plants aside from the Warrior, which was not superior to the 350X. So during that period, what could be done?

The answer: Well, in this case you can have your cake and eat it too. My choice, and the choice of many others before me, was to combine the best of both worlds with the exceptional, time-tested ATC350X power plant within the stability and performance of a four-wheeled chassis, specifically the Honda TRX250X. And thus the infamous Honda TRX350XX is born.

Mine of course has some major upgrades…

My project started in the winter of 2002, and although by that time there were some high-end four-strokes available, i.e. the Honda 400EX, Yamaha Raptor 660R, and the newly released Suzuki Z400, as a 19 year old, full time college student I didn’t have the means to go down and purchase a brand new ATV. Besides, it’s not my style to buy new machines; I like to build my own, and my favorite thing is to combine the past with the present. This was the ideal project for me.

As discussed in my 350X post, I got my hands on some ATC350X crate engines. The next step was to find a suitable ATV chassis, preferably something requiring minimal fabrication yet sporty enough for my riding style. While researching it on the web, my brother and I got some tips at Powroll and thus decided that a TRX250X chassis was the way to go. Fortunately, I knew someone who had a non-running 1991 250X. I easily obtained it for $500. We then ordered the Powroll conversion kit including the Shotgun series Supertrap 4 exhaust which at that time was priced at $565.00 total (the header was not stainless steel).  And the project began.

The engine:

The 350X power plant is a kick start, 4-valve, single overhead cam engine with a 6-speed transmission (no reverse). My goal was to have an ultra-durable and super dependable bike that I could put lots of fun miles on with minimal maintenance. With this in mind, I wanted to keep the engine as close to stock as possible, but I installed some strategic upgrades. Although my engine had never been used, it had been setting on a shelf since 1986 so I wanted to freshen up the cylinder. I bored the sleeve and installed a Wiseco oversize piston kit which not only enlarged the displacement but also increased the compression ratio from 8.5:1 to 10.25:1. The next thing I did was upgrade the clutch with EBC friction plates and heavy duty Barnett springs. Then the engine was ready. These were moderate modifications that would increase performance without sacrificing dependability.

Intake and fuel delivery:

Initially I went with the 250X carburetor and a K&N air filter. I removed the lid from the air box for maximum air flow and jetted the carburetor accordingly. Later I installed a 400EX carburetor which has a larger diameter throat and got a noticeable power gain.

The chassis:

The chassis was pretty much stock when I received it. The only upgrade was a Dura Blue heavy duty rear axle. I left it that way initially but soon found it lacking, specifically in the suspension department. To remedy the issue, I installed front A-Arms from a 1989 TRX250R and front shocks from a 1999 TRX400EX. This widened the front substantially and gave me a lot more suspension travel. On the rear I installed an aftermarket mono shock with a remote reservoir and I added 6 inches to the width by installing Dura Blue polyurethane wheel spacers on the axle. For rear wheels I settled on 20×11-8 Maxxis Razor tires mounted on Douglas polished aluminum 8 inch rear rims. On the front I stayed with the stock rims and mounted a new set of radial directional tires that boasted a smooth ride with great handling. For the operator, I recovered the seat and installed Tusk brand aluminum handlebars with a padded cross-bar and gel hand grips.

At less than $1800.00 total investment, I had a good, competitive bike that easily held its own against the new $6,000 bikes. At that point it looked like this:

In the prime of my riding days, I took many trips and fun rides. We took trips to the mountains…

…and rode many trials and homemade race tracks doing tricks and big jumps.

But alas, as my core group of riding buddies slowly married and moved away our weekly rides had come to an end by early 2005. I rode the bike sparingly until about mid 2006 when it was placed in storage and remained there until a sunny spring morning in 2010. That day I pulled it out of moth balls, not to ride, but to reminisce. It was still fairly clean but it was much in need of cosmetic TLC. It hurt me to see it that way, so I trailered it to my shop and got to work. I removed all the plastics and began cleaning, polishing, and painting on the chassis components. But I didn’t stop there. I ordered all new plastics for it to give it the clean cosmetic finish I always wanted it to have but never had the chance to do it. Plastics for this bike are unusually expensive, more so than most other models.  And at last, after almost 10 years I finally put the final touches on one of my favorite personal projects. I still have this bike today (back in storage) and it looks like this:



  1. How much modification was needed to chamber the ATC350 engine? I just got a 91 250x and once it gets lame if it isn’t a boatload of fabrication this sounds like the perfect conception. I have a Blaster which is light, very light, and once I sat on the 250x I knew it was more trail oriented, heavy (a tank compared to the Blaster), but solid and comfortable. Did the 250r A-arms swap without fabrication? I think you have one sweet machine, which screams, “Why the Hell didn’t Honda make me?!?” Nice work!

    Comment by Nathaniel — April 1, 2011 @ 2:43 am | Reply

    • Thanks for the comment. The upper rear engine mount needs to be shortened about an inch or so (I don’t remember the exact amount) on the sprocket side. I achieved this very carefully with a reciprocating saw and it worked perfectly. It also requires a short spacer on the lowest front mount. Aside from that, it bolts right in like a factory setup. Then there are some minor wiring requirements, but nothing major. You can also use your 250X carburetor, which is more modern than the 350X carb, but you’ll need to jet it. The quad will be even heavier with the 350X engine, but it more than compensates with the additional power. When tuned properly, it will run with the equivalent late-model machines such as the 400EX, Z400, etc. and of course the 6-speed transmission is great.
      I’d definitely recommend the suspension upgrades; it will ride much smoother. The 250R and 400EX A-Arms are a direct bolt-on and you can use your existing spindles. However, you will need the brake lines and tie rods from the donor quad as they will need to be longer. For the shocks it requires a custom upper shock mount to maintain the proper geometry. There are a couple different types of mounts available on eBay.
      Overall, a very fun machine. I agree…Honda made a major error by not building this quad.

      Comment by nfluhart — April 5, 2011 @ 9:37 am | Reply

  2. this story just made my day my buddy picked up a dirtbike off craigslist and trades it for a 300ex 96-97? i didnt get a chance to see it utill he asked me if i wanted to buy it cus it dont start to ez i love old junk so i looked at it and it happend to have an old 350x engine with 400ex a arms and some other mods i looked all over trying to find other atvs with this mix of honda parts and couldnt find shit untill now i run into your story just wanted to say thanks this story helped fill in alot of blanks i got the atv for a steal 500 bucks still have it to this day and will never pass it up bad ass atv honda should have looked into this thanks again -tom

    Comment by tom murray — December 5, 2011 @ 10:16 am | Reply

    • Thanks for the comments. I agree, this is one Honda would have done well with. Unlike the smaller, slower 300EX, this quad would have easily dominated the Yamaha Warrior.

      Comment by nfluhart — December 11, 2011 @ 2:37 pm | Reply

  3. I’m in the middle of this same project. I cant wait to get it done. Do you know what size jet you ended up using to make it run right. Yours looks bad as by the way.

    Comment by Brandon — February 14, 2012 @ 3:02 am | Reply

    • Thanks for the comment. I don’t recall which main jet I used. If you are using a 400EX carburetor, you may try the stock 400EX jet first (I think it’s a 135 or 138 or something), and go from there. You can also do some fine tuning by adjusting the needle if necessary. Good luck!

      Comment by nfluhart — February 14, 2012 @ 12:01 pm | Reply

      • Ok I will be using my 250 carb which may have a big enough jet already I had to run it with no air box lid before. How about the cdi box did you just plug the 350 box in and go or was there more to it?

        Comment by Brandon — February 17, 2012 @ 5:05 am

      • If I remember correctly, there are two wires in the plug for the CDI that have to be switched with each other. Then it’s just a matter of plugging in the 350X CDI. There are no other electrical modifications needed. Off hand, I do not remember which wires they were or their color. You may try to google it and see what you can come up with. If need be, I may be able to supply a pic of the plug or the arrangement of the wires. My machine is in storage as of now, but I’m sure I’ll be pulling it out sometime soon.

        Comment by nfluhart — February 24, 2012 @ 1:36 pm

  4. Will a 400ex axle work on in my 250?

    Comment by Brandon — March 20, 2012 @ 3:11 am | Reply

    • Hmm…If memory serves correctly, I don’t believe it will work. Although they use the same bearings, the distance between the bearings is different. The 400EX carrier is wider and therefore the bearing surfaces and the seal shoulders on the 400EX axle are out wider than what would be needed to work on your 250X.

      Comment by nfluhart — March 20, 2012 @ 12:12 pm | Reply

  5. dude your idea is great and sounds good to mee.. i already have the engine … and i guess im going to have to use my 1987 trx250x for this project i hope it works properly as you said … by the way your quad looks sikkk !!

    Comment by sixto — April 4, 2013 @ 1:01 am | Reply

    • Give it a shot. It’s a fun project and not too expensive if you already have a donor quad.

      Comment by nfluhart — April 6, 2013 @ 7:35 pm | Reply

  6. cant find a exhaust got a 300ex with a atc 350x can u please tell me where i could find something thanks

    Comment by Bryan — January 11, 2014 @ 9:31 pm | Reply

    • Previously, there have been two choices: You can make your own, or buy one from Powroll. However, I don’t know what is going on at Powroll…word on the forums says something about they may have been bought or have gone out of business. Whatever the case, their website is a disaster. You might try calling them on the phone if they are still open. Otherwise you’ll have to fabricate one or wait for a used one on eBay.

      Comment by nfluhart — January 24, 2014 @ 10:39 am | Reply

      • Ok thank you very much for the replay been looking on eBay with no luck looks like iam going to have too make one

        Comment by Bryan — January 24, 2014 @ 10:44 am

  7. Amigo presisamente yo tengo una quad asi me gustaria enseñarte las imagenes

    Comment by JOSE — July 19, 2017 @ 3:51 am | Reply

  8. Just purchased a 2014 250x and I’m interested in the side struts you added. Did you fabricate these or were they an aftermarket purchase?

    Comment by Bruce — August 10, 2017 @ 11:41 am | Reply

    • I’m not familiar with “side struts”. If you’re referring to the nerf bars, those were from DG. I think these have been on the quad since the mid 90’s and the netting was shrunk and faded from the sun. I stretched them while wet and used fabric dye to bring the color back. I used a polishing wheel to shine the aluminum. Newer DG nerf bars have replaceable nets, but these old ones are sewn on. Newer ones can also be purchased in chrome or aluminum. Anyway, you should be able to find them online, probably on eBay.

      Comment by Nicholas Fluhart — August 10, 2017 @ 5:41 pm | Reply

  9. Hey, love the story. I’m thinking of building a trx350x myself. That supertrapp header you installed… Did they make one to fit a 350x motor in a 250x frame?

    Comment by Mat — November 5, 2017 @ 11:05 am | Reply

    • A company called Powroll used to make the header for this conversion. I think they are no longer in business, so you’ll probably have to either find a used one or make one.

      Comment by Nicholas Fluhart — May 17, 2018 @ 11:31 am | Reply

  10. This is so cool, I just recently did this conversion on my 87 250x and have to say this is the best thing that has happened to this old frame!

    Comment by Shawn Graham — May 7, 2019 @ 8:07 pm | Reply

  11. Thank you so much for sharing! Reading this took me in a small adventure! That I too want to take buying a 300ex frame for $75 tomorrow

    Comment by Jesus P — February 15, 2022 @ 1:43 pm | Reply

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