ATV TEST: Bad big bore 

By the staff of Dirt Wheels

Jay VanValkenberg is the mad scientist behind this TRX-based torque monster. It takes dedication, connections and good friends to make a project like this happen.


Way back around 2004, Jay VanValkenberg had been riding a big-bore TRX250R two-stroke for 15 years. He bought it brand new back in 1989. Then he decided to add a four-stroke quad to his collection. He was sick of waiting for Honda to get in gear and offer a big-bore-thumper, straight-axle quad. Being a diehard Honda fan, he wanted a thumper that had plenty of torque and plush suspension. He decided a fully built TRX450R wouldn’t satisfy him, so he decided to build his own fully custom TRX680R!

We’ve seen a large number of ATV enthusiasts rebuilding the beloved Honda TRX250R to its former glory. The TRX250R is over 30 years old, and Jay and fabricator Bob Paulson (who finally completed this project) believe it is the best-handling sport quad ever made. Both are willing to put their money and time on the line to prove it. (Paulsen has an aftermarket TRX250R chassis with a Honda TRX450R engine in it.)

In street rodding, they call a machine with vintage running gear and modern power a retro-mod. No doubt some of our rabid TRX250R two-stroke fans would call it an abomination and at the minimum waste of a good two-stroke chassis. But, a man has got to do what he has to do, right?

This 680cc four-stroke sport quad has no problem with deep and fluffy sand or steep climbs. It just throws sand and goes where it is pointed as long as the rider hangs on.



Paulson has a company called Paulson Fabrication, and he gave us background on the 650: “I knew Jay when he originally built the 650, but at the time I didn’t know he was building it. He showed up at an Idaho dunes ride a bunch of us had planned, and he had the 650. Here was this wicked-looking quad with a monster XR650R motor in it, and it was in a one-off, hand-built frame. We were all blown away! I think Jay made one trip out in the sand with it, the throttle stuck or something, and he flipped it over backwards. He was pretty upset at the time, although we laugh about it now.”

Paulson continued: “Jay has other quads and eventually bought an RZR. So, he was hitting the dunes with us, but he never had the 650 with him. Every trip we would talk about the 650, and how it needed this and that. It would not run, and could not be kick-started because of a minor miscalculation when the frame was built. Oil lines dripped, and one of them was cross-threaded. There were a number of things that needed attention, but Jay didn’t have time or space. So, finally, one year at Idaho, I made him an offer—bring the 650 with you next year, and I’ll take it home to Wisconsin with me for the year. I’ll bring it back to you completed and ready to ride the following year. Well, it was an offer he could not refuse.”

This quad is a curious mixture of shiny new parts and those with 15 years of patina on them. Together they make an interesting, totally unique package.



The following year Paulson took the quad home and got started. It was September 2018, and he had less than a year to make it better than new. The original plan was for the quad to have electric- and kick-starting options. The XR650R engine needs a full-length kick to start it, meaning a full 180 degrees of rotation from the lever or it won’t start.

The right-side footpeg was based on a 1989 TRX250R, so the foot brake pivots on the rear peg bolt. The start lever would hit the peg mount before completing a full kick. Paulson came up with a custom mount that was offset forward so the starter could swing behind the footpeg. Then he figured out a way to mount the rear brake master cylinder and reservoir backward in front of the peg so it would still function and be out of the way.

The oil tank needed repairs to a welded-in fitting. Paulson cut the oil tank apart and welded in threaded bungs so that fittings could be replaced. He also took an inch off the tank so there was more room for fittings and oil lines where the tank was closest to the motor. The original gas tank sat up super high because of the petcock outlet from the tank. So, he cut it down like the old hybrid guys used to do and moved it further forward in the tank to make room for the tank to come down.

Paulson even came up with a nice mounting plate under the tank for some of the electrical modules. He welded in a better mount for the coil so it sits lower and closer to the spark plug. Paulson remounted the airbox and fabricated and welded new mounting tabs onto the subframe. Jay designed a rear grab bar, and Paulson built that. It’s heavy but super stout and has double flag mounts.

The original frame did not have a relief in the lower frame rail for the larger Baja Designs stator cover, so Paulson notched the frame to make some room. The wiring needed attention, so that was cleaned up.

This quad uses two 12-volt lithium-ion batteries and an original Baja Designs 650R electric-start conversion kit. It actually spins the starter on 24 volts when you hit the starter button. It goes back to 12 volts when you let off the starter.

This clever footpeg mounting and rear-brake master cylinder mounting made it possible to use the kick lever to start this monster.



The front upper shock mounts are adjustable, but the originals needed changes to the design, so Paulson made up some better-looking ones and had them nickel-plated at Oshkosh Plating in Oshkosh, Wisconsin. Paulson had a ton of design and machine-shop help from Rocket Machining and Design: “DJ Spurling and his team are phenomenal to work with, and they have the know-how to complete any machine shop or design work.”

Korr Powder Coating and Blasting in Kaukauna, Wisconsin, ceramic-coated the Big Gun header. And last, the frame was originally powder-coated a nice grey color. Paulson and Jay wanted a little more pop to go with new graphics and the red Fullbore plastic. Korr blasted and powder-coated the frame and subframe a nice-looking red that complements the plastic perfectly. Ian at SSI helped out with custom Paulsen Fab graphics. Quadtech sourced a new seat cover.

A Works Connection lever/perch assembly was a welcome add-on. The bearing that WC uses on their lever creates an even clutch-pull feeling every time, which is super handy on a 650.

A custom fabrication company called Motoquad Chassis Components built the original frame with TRX250R geometry, but the frame is 3 inches higher and wider at the top. They also made the custom +2+1 long-travel A-arms and the +2 250R no-link long-travel swingarm; custom steering stem; and one-off oil tank. Redline Performance Machines (RPM) was the source for the anti-fade brake hub, the MX +1/+4 Dominator II axle designed for standard hubs and a bearing carrier. built the front bumper.

TCS Powersports handled the suspension, including the 19-inch long-travel, triple-rate shocks with 11 inches of travel, and an Elka no-link body they valved and sprung for 12 inches of travel.

Since the engine only kicks to the rear, the seat and rear fenders are quick release to allow it to be started. It always sat and idled while we replaced the seat/fenders.



Jay wasn’t satisfied with a mere 650cc, so Barnum’s Pro Products punched it out to 680cc with a custom 10.25:1 Wiseco piston. He used an HRC crank with a Carillo rod, and the assembly was machine-balanced. Installed is an HRC Stage 2 cam and cam chain interface with Kibblewhite Ti valves and springs with hardened rockers. It breathes through a Keihin PWK 39 carb smooth-bored to 41.5mm.

Motoquad Chassis Components built the custom frame, A-arms, steering stem and swingarm. What a project that was!


It took a lot of guts, time, money, and effort to build a machine like this from scratch, and it is impressive that Jay got it built in the first place. It should have been no surprise that it had some bugs. He probably should have ridden it that first year in bare metal to iron out the issues, then blown it apart to finish it up.

Ultimately, that is exactly what Paulson Fabrication had to do. We are so happy that Jay and Bob didn’t let that false start condemn this machine to a corner in the garage. It would have been a shame, and we are glad it is back in the sand where it belongs. The thing is a monster—a fun monster, but still, a monster that Jay handles well. And, he has riding buddies that don’t allow him to make a mistake like leaving this machine abandoned. 

We aren’t sure that this machine could be built today. Many of the original companies involved no longer exist, or they no longer sell the parts that Jay used in the original build. It was a miracle that he could build it when he did, and surprising that it could be finished and detailed now.

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Dual pass TRX250R racing radiator N/A

Baja Designs: (800) 422-5292,

XR650R E-start kit N/A

200W stator N/A

Barnum’s Pro Products

680cc custom 10.25:1 Wiseco piston

HRC crank with Carillo rod, machine balanced

HRC stage 2 cam, HRC cam chain

Kibblewhite Ti valves and springs with hardened rockers

Keihin PWK 39 carb smooth-bored to 41.5

Battery Stuff:

Scorpion Stinger 12v 675 CCA Extreme High Output battery $351

Big Gun: (714) 970-0423,

EVO R head pipe $280

Blingstar: (951)279-3861,

Dana Creech-style nerfs N/A


Front rotors $101.90 ea.

Rear rotor N/A

Brake pads N/A

D.I.D. Racing Chain: N/A

X-ring endless chain

Fasst Company:, (877) 306-1801

Flexx handlebar $359.99

Hardkor Engineering: NA

Front hubs N/A

Holley Brands: (866) 464-6553,

Earl’s Temp-A-Cure 7-row 

   oil cooler $136.95


Coil N/A

IMS Products: (800) 237-9906,

4.0-gallon fuel tank $274.95

Motoquad Chassis Components N/A

Custom TRX250R geometry frame N/A

A-arms custom +2+1 long travel N/A

+2 250R no-link long travel  swingarm N/A

Custom steering stem N/A

Custom oil tank N/A

Custom air box N/A

Paulsen Fabrication: (208) 999-0848,

Frame modified for kickstarter clearance, custom stainless rear grab bar and various fabrication: Varies

Project 321:

Billet/carbon fiber fuel cap N/A

Redline Performance Machines (RPM): (928) 771-9363,

Anti-fade brake hub $219.45

Honda TRX250R 86-89 MX +1/+4 Dominator II axle/standard hub $416.99

Honda TRX250R bearing carrier $207.96

Skat-Trak: (909) 795-2505,

Extreme 9-paddle rear N/A

TCS Powersports: (951) 373-3035

19-inch long travel triple rate shocks w/11” travel N/A

Elka no-link body, revalved and resprung by TCS/12” travel N/A

Terrycable: (800) 854-4691,

Dual Gasser thumb/twist $168.95

Trail Tech: (844) 378-8143,

Regulator rectifier $59.95

MR-11 HID lights N/A

Fusion Xtreme billet tail lights N/A

Two Brother’s Racing:

M7 XR650R silencer N/A

Works Connection: (530) 642-9488,

Elite clutch perch assembly/Honda lever $155.85

XRP, Inc.:

Oil lines and fittings N/A

XR’s Only: (760) 244-2626,

Hi-Rev CDI ignition $134.95

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