TWO-STROKE TEST: MXA BUILDS A YAMAHA YZ144 BIG-BORE MOTOCROSS BIKE

February 23, 2012
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The plight of the Yamaha YZ125 brings a tear to the eye of every MXA test rider. For 14 out of the last 17 years, the YZ125 has been the best 125cc motocross bike made. Unfortunately, with the advent of the 250cc four-stroke, the YZ125 has been pushed to the brink of extinction. Six years ago, we asked Yamaha to consider building a 150cc version of the YZ125. Yamaha refused on the grounds that the distributors in other countries would not sign off on the YZ150 project.

KTM, which just happens to be in one of those other countries, had no qualms about building a 150cc version of their 125SX. In fact, KTM was so successful with the original 144SX and current 150SX that they sold three times as many 150s and 125s. We think Yamaha dropped the ball and that KTM picked it up and ran with it.

Oh, how the mighty have fallen. The Yamaha YZ125 has been the omnipotent small-bore motocross machines since 1996, but when the AMA changed the rules to allow 250cc four-stroke to compete in the 125 class the writing was on the wall. The GYTR kit doesn’t regain lost glory for the YZ125, but it gives it a fighting chance against the much larger four-strokes.

Surprisingly, Australians have been able to buy a brand-new YZ144 right off the showroom floor. The Australian distributor understood the motocross market better than his worldwide counterparts and built up his own domestic-market big-bore YZ125s. The Aussie YZ144s aren’t produced in Japan, but retro-fitted with a big-bore cylinder and piston once the freighter lands in the Sydney harbor.

The MXA wrecking crew came to the quick realization that if they could do it in Oz, we could do it in America. A quick perusal of Yamaha’s GYTR accessory catalog revealed the secret of the Australian YZ144?Yamaha offers an optional Athena YZ144 kit as part of its accessory division. We called our friendly local Yamaha dealer and ordered a complete Yamaha YZ144 engine kit. It set us back $859.95.

WHAT DO YOU GET FOR THE MONEY?

The YZ144 kit includes a brand-new nickel carbide-plated cylinder (with enlarged water passages), cylinder head, 58mm piston with rings, pins, clips, gaskets, modified powervalves, O-rings and complete instructions. Best of all, it fits all 2005-12 Yamaha YZ125 models.  

In Australia, racers can buy the Yamaha YZ144 right off the showroom floor of their local dealer. In the USA you can get the same Athena kit from Yamaha’s in-house GYTR division. The GYTR?kit comes with a new cylinder, head, power valves, gaskets, piston, rings and clutch cover. The GYTR exhaust system was extra, but the MXA test riders didn’t think much of it.

HOW MUCH WORK WAS INVOLVED?

More than we expected. Initially, we thought that we could just bolt the cylinder on and go riding, but a perusal of the instructions revealed that we needed to grind some clearance from the cases for the oversized piston. No sweat, we thought. We figured that we could tape up the crankcase and grind the clearance needed with a porting tool. Wrong! In the end, we had to split the cases to get access to the area that had to be removed.

Any experienced two-stroke mechanic could do this job in an evening, but if you don’t feel comfortable splitting the cases, you need to budget an additional $300 (on top of the original $860) to have your dealer do it for you.

WERE THERE ANY SPECIAL NEEDS?

The YZ144 big-bore kit does not come with an exhaust pipe, so we ordered a $239.95 GYTR exhaust pipe and $124.95 silencer to enhance the package. This was not money well spent. In comparative dyno tests on a stock YZ125 and the YZ144, the GYTR pipe didn’t add any significant power gains to either the 125 or 144 engine configurations.

After racing the bike for a while, we geared it up (by removing one tooth from the rear). This made the YZ144 much easier to ride and broadened the powerband.

WERE THERE ANY ISSUES?
 
(1)
Jetting was ballpark, but we fiddled with the air screw.

(2) The YZ144 performed much better on the track with taller gearing. It had the power to pull a taller gear.

(3) The cylinder has “Athena” embossed on it, which reveals that you aren’t on a stocker. The Athena kit is made in Europe. The piston is a Vertex.

(4) On a performance level, the Yamaha YZ144 cannot run with the KTM 150SX. It doesn’t come close to the KTM’s horsepower or torque numbers. If you are in the market for a brand-new 150cc two-stroke, go with the KTM. However, that doesn’t mean that there isn’t a market for GYTR’s YZ144 kit. Au contraire. The Yamaha YZ144 isn’t built for riders trying to choose between blue and orange. No! It is for riders who own and love their trusty Yamaha YZ125s and just want to make them more competitive. When it comes time to put a new piston and rings in your venerable YZ125, we recommend going for the one-four-four.

WHAT WAS IT LIKE TO RIDE?

On the dyno, the YZ144 produced close to two more horsepower in the meat of the powerband than the stock YZ125. Best of all, it didn’t lose any top-end power or over-rev. The YZ144 powerband mimicked the stock YZ125 curve, but with more power and torque. Peak horsepower was achieved about 500 rpm higher than on the stock YZ125. From 8000 rpm, the YZ144 was significantly more powerful than the YZ125 and stayed that way until sign-off.

For more information, see your local Yamaha dealer or go to www.yamaha-motor.com.

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