Duncan Racing is at it again

 By the staff of Dirt Wheels, Photos by Pat Carrigan —

The Elka Suspension helps make hard landings a breeze. The Flexx handlebar reduces rider fatigue as well.



Duncan Racing International (DRI) is a company known for building great products and building up ATVs to handle the harshness of racing in all different disciplines. One well-known racing series on the West Coast is WORCS. The Grand Prix-style courses offer rough desert terrain, sand dunes, asphalt, motocross tracks, and more to battle through, which means your quad must be tough to take the abuse and win.

Mike Sloan (#3) runs in the Pro class at WORCS, while Ricardo Gonzalez (#51) races in the Pro-Am class and both are on Duncan-built Yamaha YFZ450Rs. Both of their Yamahas are virtually the same, minus tire choices, so to simplify the situation, we will give you the facts on Mike’s sweet ride.





It seems like half of the upgrades to this powerhouse are nestled inside the frame bolted in or on the four-stroke, single-cylinder, 449cc, liquid-cooled engine.

Duncan Racing ported the head with SERDI valve-seat machining and includes its HP4 valve-train kit. Heavy-duty valve springs, guides, and a big valve kit were included, along with a custom JE piston. The crankshaft was trued by DRI.

On the outside of the engine, performance parts like a Fat Boy 4 complete stainless exhaust system were installed. That, along with a Fuel Customs intake, and a Vortex X10 ECU, was tuned by DRI.

A Zip high-capacity billet oil cover allows another 8.5 ounces of oil to be put in the engine of the Yamaha. DRI also installed an aluminum oil-recovery tank on the Yamaha.

A new clutch system went into the YFZ. Hinson Racing’s inner hub, billet basket, and pressure plate were paired with a Duncan Racing C145R clutch kit to enhance the operation of the machine. A Works Connection clutch lever and perch work well with the new clutch setup. Maxima 10w40 engine oil is used to lube the motor. Race fuel is now required in this engine.


Ricardo’s Maxxis Razr 2 and rear Razr XC tires provide great traction on dry, hard-pack, desert-style terrain yet will rip through slick terrain as well.



Duncan Racing utilizes a setup on their race quads that they know works well and lasts a long time. They use Roll Design A-arms, an anti-vibe steering stem, and big bar mount that is paired with Elka Suspension’s Stage 5 long-travel shocks. The Elka shocks have a triple-rate spring setup with a redesigned reservoir that allows oil to flow quicker in the shock body and reservoir. A Fasst Flexx handlebar is clamped onto the Roll Design bar clamp, and they help soften the ride with elastomers that make it feel like an extra inch of travel was added.


A Duncan Racing Fat Boy 4 full stainless exhaust system helps pull more power out of the Yamaha’s already potent engine.


The rear swingarm is stock on the Yamaha, but an Elka Stage 5 rear shock offers a supple ride that has great bottoming resistance and is fully adjustable through high-/low-speed compression, spring preload, and rebound just like the front shocks. The stock rear axle was replaced by a stronger Redline Performance Machines Dominator axle.


Sunstar sprockets and an RK chain were installed on the Yamahas to help put power to the ground. A Hess motorsports rotor guard and CRF sprocket guard protect important parts.




There are a lot of characteristics to handling, like what type of tires you use, bar position, footpeg choice, and more that affect the performance of your machine. Duncan Racing worked hard to make sure the right parts were utilized for Mike Sloan and Ricardo Gonzalez to go fast.

A GPR steering stabilizer was installed that is adjustable to the type of riding style you have and the terrain you ride on. The stock Yamaha thumb throttle was changed out to a Honda TRX90 thumb throttle, which is more comfortable to use. Pressing on the throttle makes you go faster, and there is a lot more power coming from the YFZs, so a set of Galfer brake rotors were installed for stronger stopping power.



Rath Racing nerf bars, pegs, and heel guards are utilized on these builds for great foot traction and protection from other riders or racetrack and trail debris like rocks and trees. A Fourwerx Carbon seat cover provides a nice surface that is grippy and comfortable for hours of riding and racing.


The right set of tires helps greatly with traction and handling on any machine. Mike Sloan is sponsored by STI tires, while Ricardo Gonzalez is sponsored by Maxxis tires. Sloan runs STI Tech 4 fronts and rears that are sized 21×7-10 upfront and 20×11-9 in the rear.

Gonzalez uses Maxxis Razr 2 front tires sized 21×7-10 while using the Razr XC rear tires sized 20×11-9. Each Yamaha has DWT Beadlock wheels paired with the rider’s tire choice. Both riders and the crew at Duncan Racing International know that flats happen, so they installed Tire Balls into each tire as great flat prevention.

Zip Racing sells a billet-aluminum oil cover that adds an additional 8.5-ounce capacity of oil to the engine. The DRI oil-catch canister is another great addition.




At Dirt Wheels, we are no strangers to a well-set-up machine. Fortunately, the instant we swung a leg over one of the YFZ450Rs, we knew these fit in the well-set-up category.

The power was on tap anywhere you needed it. The low end of the range kicked like a mule, while the mid and top-end screamed. The Elka shocks soaked up everything we pointed the tires at, including jumps, chop, and whoops, and the quad cornered like a dream.

The vibration was relieved by the Flexx bar and the Roll Design anti-vibe steering stem, while the GPR stabilizer kept the front end going in the right direction. The gripper Fourwerx seat cover offered just the right amount of traction to move around on the Yamaha yet still stay put when needed. The Rath pegs may tear up the bottom of your boots easily, but they are one of our favorites to have for keeping your foot in place at all times.


A Hinson clutch kit paired with DRI clutch plates is installed on both Sloan’s and Gonzalez’s YFZ450Rs.


It is no wonder that Mike and Ricardo are as fast as they are when they have the right tools for the job—tools that are built by Duncan Racing International and maintained and wrenched on by their ace mechanic, Kevin Avina. Check out the parts list and get a good idea of what you want for your own play or race quad!





Skid plate $125
Sprocket guard $50

DRI valve-train HP4 kit $1695 (includes: DRI porting, SERDI valve seats machining, billet camshafts, HD valve springs, HP guides & big valve kit)
Aluminum oil recovery tank $139
C145R clutch kit $199
Crown-series brake lines: front, $119; rear, $39.95
Front bumper $199
D’Cor graphics $149
Number plates $59.95
Fat Boy 4 Exhaust $649

DWT:, (800) 722-3746
Four-piece bead-lock rim set $699

ELKA SUSPENSION:, (800) 557-0552
Stage 5 front shocks $1795
Stage 5 rear shock $1295

FASST COMPANY:, (877) 306-1801
Flexx bars $359

FOURWERX CARBON:, (262) 501-9696
Carbon hood scoop $189

FUEL CUSTOMS INTAKE:, (909) 594-6082
Intake (airbox version) $325

GALFER:, (800) 685-6633
Rotors $129 each

GPR:, (619) 661-0101
Stabilizer $525

HESS MOTORSPORTS:, (940) 759-4597
Rear rotor guard $150

Racing billet basket $259
Billet inner hub $309
Billet pressure plate $199

IMS PRODUCTS:, (800) 237-9906
3.2-gallon tank w/ dry brake mount $274
Dry break $225

JE PISTONS:, (714) 898-5114
Custom piston $235

MAXIMA OILS:, (800) 345-8761
ATV Premium SAE 10w40 $7.95 1 liter
Chain Wax $11.50
Dot 4 brake fluid $6.95

Razr 2 front: N/A
Razr XC rear: N/A

POWERMADD:, (651) 462-8465
Handguards $59

RATH RACING:, (320) 234-7223
Nerf/peg/heel guard combo $429

Dominator axle $389

RK EXCEL:, (760) 732-3161
Chain 520 $82.95

ROLL DESIGN:, (760) 731-5920
A-arm kit $1595
Steering stem $299
Big bar clamp $99.95

Tech 4 front tires $64 each
Rear tires $84 each

SUNSTAR:, (937) 743-9049 ext 1334
Steel front sprocket $26.95
Steel rear sprocket $45.95

TIRE BALLS:, (877) 847-3221
Balls $210 each front wheel
Balls $219 each rear wheel

X10 ECU $699

WORKS CONNECTION:, (530) 642-9488
Hour meter $65
Brake master cylinder cover $29
Clutch perch $149

High-capacity billet oil cover (+8.5 ounces) $249

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ATV upgradesDuncan RacingFast quadsYamaha YFZ450R