High Tech 4-Stroke Racers

Four-stroke sport ATVs—like Honda’s popular 300EX, Yamaha’s War­ri­or, and Kawasaki’s Mojave—have not changed much in the last ten years. The ’87 Warrior and Mojave are virtually identical to the ’98 models, except for cosmetic and color chang­es, and although Honda engineers beefed up and stroked the en­gine in today’s 300EX, the chassis has re­mained unchanged since its premiere in the ’87 250X.

For a decade, sport-minded four-stroke ATVers have been forced to re­ly on good old American ingenuity to im­prove upon decade-old Japanese tech­nology. Stock, the Honda TRX250X/300EX can prove to be less than sporty by ’97 standards. Its narrow width, short wheelbase, and short-travel suspension systems can leave even a novice ATVer shaking his head.

The narrow width is easily corrected with extended-length aftermarket A-arm assemblies and axles. The short wheelbase and short-travel sus­pension systems, however, re­quire major fabrication to re-engineer the stock Honda systems or a chas­sis conversion. Racing breeds in­vention. Today’s top-of-the-line per­formance four-strokes can be found competing in the AMA Grand Na­tional Championship series. Here we take a look at four approaches to im­proving the chassis and suspension of the four-stroke quad of choice for rac­ers everywhere: Honda’s 250X/300EX.


A big-bore 366cc, TC Racing engine kit is used on Greg Vucina’s 250X to get it up to speed at the Nationals. Greg mod­ifies a stock 250R frame and a Laeger front end with Works Per­for­mance shocks upfront and an Ohlins on the rear.

High Tech 4-Stroke Racers

Vucina’s Highly Modified Hon­da 350X:

Greg Vucina is the ’97 350cc Four-Stroke A National Cham­pi­on. His racer utilizes a stock 300EX frame modified to accept Honda TRX250R suspension components. Greg’s father, James, started by moving the A-arm mounts forward two inches and fabricating new upper front shock mounts. Laeger +2” 250R A-arms and Works Performance 250R shocks were then bolted up. In the rear, the frame was cut, modified, and re-welded to accept a Laeger stock ’88 250R-length swingarm, Honda 250R rear-suspension linkage, Ohlins 250R shock, and extended-length Lone­star 250R Axcalibar axle.

Special care was taken to ensure that the location of the swingarm piv­ot, suspension linkage frame mount, and upper rear shock mount was in the same relative position in the EX chas­sis so as not to alter suspension geometry.

Check out the massive 38mm Lectron carb and hand-machin­ed aluminum air tube on Greg Vucina’s hot-dog four-stroke. Note the TC National/Su­per­Trapp ex­haust pipe & muffler.

The repositioning of the front shock mounts and addition of the 250R swing­arm (which is two inches long­er than the 300EX swingarm) stretch the 300EX’s wheelbase to a length very close to that of a TRX250R chassis. Additionally, braking action and feel were improved by replacing the stock 300EX’s single-piston brake caliper systems with 250R dual-piston systems.


High Tech 4-Stroke Racers


Tom Miller’s Honda 250RX:

Tom Miller races a TC Racing ported 250X engine at the Nationals. He used a conversion kit from his company—Tom Miller Motorsports—to install the en­gine in an ’89 FourTrax 250R frame.

The racer of ’97 250cc Pro/Am Four-Stroke National #2 Tom Miller is a top-flight example of the popular RX conversion. The ’89 250R chassis is the stan­dard to which all ATVs are compared. Tom Miller Motorsports (TMM) took a full-race TC Racing 250X en­gine and inserted it into an ’89 Honda TRX250R frame. This conversion re­quires cutting and fabricating new en­gine mounts so that the four-stroke en­gine will bolt into the R frame. Mill­er further improved the handling by in­stalling Laeger +2” 250R A-arms, Baldwin Motorsports revalved aftermarket shocks front and rear, and a Lone­star +2” axle.


Byron Goggin’s Standard Laeger 250RX:

Byron Goggin’s Pro/Am Four-Stroke racer represents the next generation of RX conversions. Rather than cutting and welding a stock, mild steel 250R frame, Goggin’s racer utilizes a Chromoly Laeger RX frame. Mark Laeger’s RX frame features 250R geometry with specific modifications to allow the 250X/300EX en­gine to fit properly. The engine mounts are specifically designed to ac­commodate the X motor. Addi­tion­all­y, the lower frame rail is lowered so that the four-stroke engine’s countershaft sprocket is positioned correctly in relation to the swingarm piv­ot; this misalignment in a stock R frame conversion adversely affects suspension performance and erratic chain torque effects. Laeger +2” 250R A-arms, a Laeger stock 250R length swingarm, stock 250R rear suspension linkage, and PEP ZPS shocks are bolted on. An RPM Dominator axle and 250R brake systems, spindles, and hubs round out the improvements.

High Tech 4-Stroke Racers


Jeremy Schell’s Pro-Trax Laeger 250RX:

Tom Carlson was Jeremy Schell’s mechanic and the man behind TC Racing. Jeremy and Tom have one of the cleanest machines on the 4-Stroke National circuit.

Jeremy Schell, ’96 and ’97 250cc Pro/Am Four-Stroke National Champ, has a racer that is the Fer­ra­ri of four-stroke performance quads. The champ’s ride features a complete Laeger Pro-Trax RX chassis. The Pro-Trax chassis features Laeger’s patented T-pin front spindles and A-arm assembly, which eliminates ball joint binding and allows for the use of long­er-travel shocks. The stock 250R rear suspension linkage is replaced with a CR500 rear suspension system. PEP/Baldwin Motorsports long-travel shocks are utilized front and rear. The Pro-Trax chassis represents the cutting edge of ATV suspension systems; it is the chassis of choice for nearly 80% of the GNC Pro class (in­cluding superstars Tim Farr, Shane Hitt, Travis Spader, and Doug Gust).

PEP shocks grace the front end of Schell’s championship machine with Laeger +2” wide +1” moved-forward A-arms and Laeger Pro-Trax spindles.

Furthermore, Schell’s TC Racing Ul­tra-National engine features one of Tom Carlson’s legendary, custom six-speed trannies (a stock 250X/300EX has a five-speed transmission). No ex­pense has been spared on this championship-winning machine!


Racers and recreational riders alike can improve the handling of their four-stroke ATVs by modifying the chassis and suspension. All four of these machines have vastly im­proved on Honda’s shortcomings. Vu­ci­na’s highly modified EX retains the stock frame but requires countless hours of complex welding and fabrication. Miller’s RX requires the purchase of a well-used 250R (which are getting scarce these days).

Thankfully, once you locate a 250R chassis, companies such as Tom Mill­er Motorsports (731)-986-0893 http://www.tommillermotorsorts.com  offer in­stallation kits for this conversion. Both the standard and Pro-Trax Lae­ger RX systems eliminate all fabrication and are specifically engineered for the four-stroke engine. The purchaser is only required to assemble the components.

Which of the four possibilities is best for you? That depends on your fab­ricating ability and wallet size.


Tom Carlson is one of the reign­ing kings of four-stroke engine build­ers. They have won the GNC Pro/Am Four-Stroke National Cham­pionship for seven consecutive seasons (’91–’97) and 95% of the GNC Pro/Am Four-Stroke class utilizes TC Rac­ing-built engines.

Dirt Wheels: What is involved in build­ing a national-caliber four-stroke engine and how much does it cost?

TC: I’ll use Honda’s 250X/300EX for an example since it is the engine of choice for GNC competition. First, the engine is completely disassembled. The reverse gear is removed and the crankshaft, transmission, and clutch are inspected and re-assembled. Then the cylinder head is machined and ported. TC Racing oversized valves with heavy-duty valve springs and titanium retainers are installed. A TC high-rev cam with more lift and long­er duration and a TC heavy-duty cam chain is also utilized. The flywheel is lightened, the ignition is advanced and a TC high-rev CDI box removes the restrictive stock Honda rev-limiter. A Lectron carburetor and a custom TC Racing exhaust complete the package.


A 250cc Pro/Am National engine kit costs $1400–1500. The exhaust system is additional—the cost depends on the application. A 350cc big-bore en­gine for the Four-Stroke A or B class will cost $2200–$2300. For this kit we are manufacturing displacement with quality components. The big-bore requires a stroker crank, heavy-duty connecting rod, big-bore piston, and sleeve.

Most customers, however, require per­sonalized engine building. I’ll ask them what type of riding or racing they do and what their budget is. Then I’ll build them a package that produces the most horsepower for their money.

Dirt Wheels: What kind of horsepower do these race engines produce?

TC: A stock Honda 300EX puts out 18. My Pro/Am National engine produces 40 and the 350cc big-bore cranks out 50–55. For reference, a stock 250R two-stroke produces ap­prox­imately 35–38.

Dirt Wheels: How reliable is a full-race four-stroke engine?

TC: It is very important to keep the air filter clean and the en­gine oil should be changed every week­end of riding. Heat is an air-cooled four-strokes worst enemy. Even good oil breaks down quickly. I recommend 40-wt. petroleum oil for warm weather and a 20-50-wt. for cooler temperatures. Synthetic oils can cause clutch slippage.

Dirt Wheels: Tell us about your six-speed transmission kit for the 250X/300EX.

TC: The complete kit including in­stal­lation is $850. It’s trick.


Curtis Sparks built this engine for Shane Hitt to contest the ’97 GNC TTs in the Pro/Am Four-Stroke class. He nev­er raced it, since he was in the run­ning for the GNC #1 plate in the 250 Pro class. Sparks had won four-stroke titles in the past, but until Hitt won the pro class that year, he had never won a two-stroke pro title.

Curtis Sparks is currently experimenting with liquid-cooling Honda 250/300 EXs.

• Engine features liquid-cooled cyl­in­der (cylinder only—not head). Sparks removed the cylinder sleeve, cut cooling passages into the alumi­num cylinder walls, installed water in­let and outlet fittings, and re-in­stalled the sleeve.

• The liquid-cooling system itself doesn’t directly give the engine more horsepower. Heat is a four-stroke engine’s worst enemy. The li­quid cooling system lowers engine op­erating temperature.

Honda TRX250X

  • Engine Bore & Stroke 74mm x 61.4mm
  • Displacement 264cc
  • Porting/head Sparks National Big-Valve Cylinder Sparks liquid-cooled stock
  • Valves/valve springs 28.5mm intake, 24mm exhaust/Sparks hi-rev springs
  • Cam Sparks #224 grind
  • Piston Sparks 13:1 compression
  • Rod Sparks heavy duty
  • Crank Sparks 4mm stroker
  • Ignition/timing Advanced 6°
  • Flywheel lightened
  • Spark plug NGK D9EA
  • Intake manifold Modified stock
  • Carburetor/mods Sparks-modified 36mm Mikuni
  • CDI box Sparks hi-rev
  • Airbox/airboot/filter none/Sparks hi-flow/K&N
  • Exhaust pipe Sparks
  • Oil cooler/radiator Sparks custom radiator & water pump
  • Clutch/plates/springs Stock Honda
  • Clutch basket Hinson
  • Transmission Stock w/reverse removed
  • Transmission oil Kendall GT-1 40wt
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