ATK 250

 

 

 

Make Model

ATK 250

Year

2003

Engine

Two stroke, Rotax, single cylinder

Capacity

249 cc / 15.1 cu in
Bore x Stroke 67.75 x 69.8 mm
Cooling System Liquid cooled
Compression Ratio 7:1
Exhaust Single, high level

Induction

Mikuni TMS 38 mm carburetor
Starting Kick

Transmission 

6 Speed
Final Drive Chain

Front Suspension

46 mm Paioli forks, adjustable for compression and rebound

Rear Suspension

Swinging arm, Linkage-less Ohlins shock, adjustable for compression and rebound

Front Brakes

Single 260 mm disc, 2-piston caliper

Rear Brakes

Single 220 mm disc, 1-piston caliper
Wheels Aluminium alloy, laced wire spokes

Front Tyre

80/100-21

Rear Tyre

100/100-18
Wheelbase 1486 mm / 58.5 in
Seat Height 945 mm / 37.2 in

Dry Weight

107 kg / 236 lbs

Fuel Capacity 

14 L / 3.7 US gal

The good news is that the ATK LQ models come stock with some of the finest hardware you'll find anywhere, short of a factory works racer. Additionally, ATK will practically custom build the bike for a particular customer, adding or removing options to suit, swapping gearing or tires, etc. This impressive list of trick features includes Talon billet hubs front and rear, DID O-ring chain, Talon/Answer Radialite rear sprocket, Pro Taper handlebars, the latest WP fork with high and low speed compression damping, WP rear shock, shroudless 3.1 gallon fuel cell, machined billet triple clamps, quality Dunlop tires front and rear, and an innovative vacuum-actuated, adjustable power valve.

While the ATK literally reeks of quality custom hardware, it unfortunately has its share of warts too. For the most part, these are features that basically require further "refinement" to improve performance and longevity. Near the top of that list is the thin gage steel used on the frame (for tabs, gussets, etc.) and swing arm. This allows many mountings (fenders, side covers, etc.) to flex or bend out of shape, giving the bike an overall weak and vulnerable look. Older ATKs used to come with trick-looking utilitarian chromoly swing arms--what happened? Speaking of vulnerable, the pipe sticks out like a neglected hernia to the front and right side of the motor, begging to be mashed. Similarly, the chain guide is a disaster waiting to happen, constructed from thin gage aluminum that won't stand up to hard pack ruts, let alone rocks. For the most part, everything can be fixed, its just a matter of deciding where to start.

The Rotax powerplant is slightly under square with a 69.8mm stroke and 67.75mm bore (67.5mm for the 250). Straight from Utah, the motor starts easily in spite of a smallish 125 sized kickstarter and quickly achieves a nice steady idle. The head has an integral thermostat, facilitating motor warm-up. The motor uses a Mikuni TMS 38mm carburetor, which is the pilot-less model that surfaced some years back. Factory jetting uses a 350 main, the double tapered 6DGY4-56 needle with the clip in the #2 position, a "G" slide, and the air screw set 1.5 turns out.

The motor employs a vacuum-actuated adjustable power valve that uses exhaust gas pressure against a variable spring tension to effect valve opening timing. An external plastic cover located just above the head pipe permits the adjustment, using a threaded plastic plug that can increase or decrease the spring tension. Standard position is with the adjuster nut flush with the power valve cover, with two full turns of adjustment possible in either direction from the standard (flush) position. Turning the adjuster out decreases the spring tension holding the valve closed, thus allowing the power valve to open sooner. Conversely, turning the adjuster in increases spring tension and holds the power valve closed longer.

With the above jetting and the power valve in the standard position, the motor is a real sleeper, with practically no grunt off the bottom, a decent midrange and fair top end runout. The lack of bottom end was really frustrating in high-traction sand conditions, as the motor resisted getting into the more powerful midrange, despite repeated clutch feathering. Like lower horse power bikes (read: 125s), proper gear selection was imperative.

For rocks and the slick stuff, this is it; this is the bike. Most agree, however, that South Jersey sand will still demand a little more punch. We're at odds where to proceed next, however, the engine's specs list a seven to one compression ration for the Rotax powerplant, significantly lower than most other dirt two strokes.