ON THE RECORD: COMPLETE TEST OF THE 2007 YAMAHA YZ125

This Motocross Action Race Test is from the March 2007 issue of MXA. 

There is no need to ask for whom the bell tolls. It rings the death knell of the 125 two-stroke. These marvelous pieces of simplistic engineering are doomed, thanks to the advent of the 250cc four-stroke and its double-the-displacement advantage. But, for those who take the Yamaha YZ125 lightly, there are five important points worth considering before guffawing the blue tiddler onto the ash heap of motocross history.

(1) In the last 11 years, the YZ125 has earned the 125cc Bike of the Year award ten times (losing only once to the KTM 125).
(2) The 2007 Yamaha YZ125 weighs 197 pounds (ten pounds lighter than any 250F four-stroke).
(3) The YZ125 is the best-selling 125cc bike made.
(4) Yamaha two-strokes own 40 percent of the total two-stroke market.
(5) The YZ125 was totally redesigned in 2005—making it the only Japanese 125 bike with new genes. The frame, engine design, tranny and suspension are two generations ahead of what Suzuki and Honda offer (Kawasaki dropped their 125 after the 2005 model year).Yamaha is fighting to keep the venerable 125 two-stroke alive. Have they succeeded? That is what the MXA wrecking crew wanted to know about the 2007 model.

Q: WHAT ARE THE BEST TRAITS OF THE 2007 YZ125?

A: There are four really great attributes of the ’07 YZ125.

Versatility: Thanks to the freshness of the YZ125 design, the bike is one of only two 125s that isn’t hampered by an old design. With its plug-and-play aluminum frame, two-year-old engine and all-new Kayaba SSS suspension, the YZ125 has a reputation as a bike that can be raced for several years before it begins to show its age.

Power: Not the fastest 125 on the track (the KTM 125SX makes the most peak horsepower), the YZ125 has great bottom and a steady pull through the midrange. It has an easy-to-use powerband.

Suspension: When you mix the speed-sensitive damping of the Kayaba suspension components with a titanium shock spring, Kashima-coated internals and jumbo-size shock shaft, you have production suspension parts that are kissing cousins of the works stuff.

Weight: Only the KTM 125SX and Yamaha YZ125 weigh under 200 pounds (and the YZ weighs three pounds less than the KTM).

Q: WHAT ARE THE WORST TRAITS OF THE 2007 YZ125?

A: There is one significant problem area on the ’07 YZ125.

Top-end: If there is anything the YZ125 needs, it is top-end. The YZ125 peaks at 11,300 rpm. That might sound high on the scale, but the KTM 125SX peaks 500 rpm later and with 3-1/2 more horsepower on the top.

Q: HOW DOES THE 2007 YZ125 REALLY RUN?

A: This is a no-holds-barred race engine. To be successful on a 125 of any kind, you have to ride it hard and put it away wet. As long as you keep it on the pipe and catch gears in the midrange, you’ll be flying. Miss a shift or let the motor bog down and you’ll be flicking the clutch and shifting down in a hurry.

Two-stroke riders have to wring it out against thumpers. The YZ125 is decent down low, for a tiddler, and is solid into the middle. Although a little lacking on top-end overrev, the YZ125 engine works so well from low-to-mid that it gets the job done against other 125s. However, when it faces 250cc four-strokes, the YZ125 lacks torque and hookup. It has to make do with its snappier power, quicker throttle response, and a free-revving feel going into turns.

To make a 125 engine beat a four-stroke that has twice the displacement, you have to go for the jugular. That means leaving the power on longer and utilizing the lightweight chassis to bomb into corners. Always attack with the sun at your back.

Q: WHAT ABOUT THE GEARING?

A: Gear it down. Adding one tooth to the rear sprocket will perk up the acceleration in second gear enough to allow a swifter shift to third. Light riders benefit from gearing it down, because with their power-to-weight ratio, they can get full use out of third. Heavy riders need to gear it down to get their lard bodies out of corners with a harder hit.

Q: HOW DOES THE YZ125 HANDLE?

A: A bike that has suspension this good, an effective midrange engine and 197-pound overall weight has no choice but to handle with aplomb. Of all the YZ’s, the YZ125 is the best handling.

Q: WHAT DID WE HATE?

A: The hate list:

(1) Plastic. Yamaha doesn’t offer their yellow or white plastic for the YZ two-strokes, but luckily Acerbis, One Industries and UFO have optional colors.

(2) Footpegs: We don’t hate the YZ footpegs, but the Ti is wasted this low on the frame, and the pegs could be a little wider. We’d save the R&D money for something more important.

(3) Brake cable clamp: DR.D makes a CNC-machined aluminum front brake hose clamp that weighs half as much and is half the size of Yamaha’s humongous stocker.

(4) Grips: Four out of five MXA test riders choose soft Renthal Kevlar grips over the hard YZ grips.

(5) Pipe: Although at close to 34 horsepower the YZ125 isn’t humbled by the power output of 250cc four-strokes, it does need help to offset the 250F’s torque and grip. Aftermarket pipes like Pro Circuit and FMF can pump two horsepower into the YZ125.

Q: WHAT DID WE LIKE?

A: The like list:

(1) Weight. Someday they will build a 250 four-stroke that is under 200 pounds, but until then the YZ125 and KTM 125SX are the only bikes that can make that claim.

(2) Suspension. We love Yamaha’s SSS settings.

(3) Handlebars. The switch to oversized ProTapers is a money saver for riders who favor 1-1/8 inch bars.

(4) Sneakers. The D739 front tire and D756 rear are a mismatch on paper, but for a wide variety of terrain the hard-track front and intermediate rear work very well.

(5) Tranny: It’s a sixer! But don’t think that the YZ125’s six-speed tranny is a five-speed with an extra gear; think of it as having six close-ratio gears that must be used to their fullest. The tight ratios magnify the power and keep the rider’s left foot busy.

Q: WHAT DO WE REALLY THINK?

A: Do you remember 500cc two-strokes? As they fell out of favor with the racing public, they were back-burnered by the factory engineering departments. They saw their last real R&D in the late ‘80s and then went on autopilot until they finally disappeared from the showrooms a decade later. That is what will happen to 125 two-strokes in the future. But, for now, the YZ125 is a modern, fully developed, hard-core racing machine. It is the best all-around 125cc motocross bike made.

MXA’S RECOMMENDED JETTING SPECS
Main Jet: 410
Pilot Jet: 40
Needle: 6BFY42-74
Clip: Third from top
Air screw: 2-1/4 turns out
Note: Yamaha delivers every YZ125 with an optional 6BFY43-79 needle. This is a half-clip leaner, and we use it only when the weather is extremely hot.

MXA’S RECOMMENDED SHOCK SETTINGS
Spring rate: 4.7 kg/mm
Race sag: 98mm
Hi-compression: 1-1/2 turns out
Lo-compression: 13 clicks
Rebound: 10 clicks
Notes: This is a works shock. It has an oversized 18mm shock shaft, Kashima-coated internals, and titanium shock spring. This baby works as well as most modified shocks.

MXA’S RECOMMENDED FORK SETTINGS
Spring rate: 0.41 kg/mm
Oil height: 340cc
Compression: 11 clicks out
Rebound: 6 clicks out
Fork leg height: 5mm up
Notes: Awesome forks. Thanks to the need to manage less of a burden in terms of weight and torque, the 2007 Yamaha YZ125 forks are almost perfect. Yamaha’s reliance on speed-sensitive damping has proven to be a great success when compared to position-sensitive setups.