Friday, May 8, 2009

Yamaha MX 125c Race Test

The Choice in the 125 class gets one bike harder

Confidence is the key. There is no way that you can really ride a motocross quickly unless yu have absolute confidence in your own abilities and those of your machine.
The true woth of a motocross bike to you is measured by what kind of tune it plays in th espaces of your mind as you click it down through the gears and pitch it screaming into a rutted, loanny turn.
What happens, happens. How you feel about what happens is maybe more to the point.
Up until now, the 125cc YamahaMX series motorcycles have always played a shaky tune. Twitching back ends and bucking fronts kept rider confidence levels lower than basement plumbing, while anemic five-speed engines kept speeds low enough to make falling off safe. As a result, the MX125s were always considered to be good trailbikes ; but as racers the series just didnt make it.
This year Yamaha has redesigned the 125MX, making it into a full-on racing machine that still comes stock with a U.S. Forestry Service approved spark arrestor. Now, the 125MX is ot only a dynamite trailbike, but an excellent racer to boot. There’s something to be said for the dual-purpose concept when it’s executed correctly.
Replacing last year’s standadmount Thermal Flow rear shocks are new gas Kayabas with a single-rate spring and mounted in the laydown position. Its a setup similar to that of the RM Suzukis. Yamaha insists that their gas Kayabas are better than Suzuki sones.
Whatever. The relevant truth of the matte ris that gas Kayabas, wether they’re on a Suzuki or a Yamaha wear out very quickly so it doesnt matter much whose are better.
We detected no significant differences in erms of shock function.
While the Kayabas work, they work very well – as does the entire mechanical package which comprises the MX125C. All the things which made the old MXs no more than just good trailbikes , things like oil injection, spooky suspension and a poky engine, are gone.

In their place the MXC debuts a really powerful, six-speed YZ-like engine that gets you the holeshots and spits you out of turns like a watermelon seed and a suspension that, while no perfect, can be dealt with.
But the MXC is not simply a YZ of a different color. The things that made the old MX a neat bike to own, things like dependability, durability and low maintenance costs are still there. They’ve just been made better. For instance : The engine, though ported to YZ specs, still has an iron cylinder liner that can be rebored if you happen to waste a piston. The YZC’s cylinders, which has a chrome bore, must be rechromed or replaced if damaged.
Have you ever tried to have a cylinder rechromed ? Most people end up having to buy a new one.

Then there’s the suspension. Its not a perfect package, but it can be lived with an dit avoids some of the hassles of the monoshock system like set-up difficulty, weight and air box restriction. The saving grace of the MXC’s rear suspension is that it is understandable ; Motocrossers have been dealing with shock absorbers and springs for a long time. We can get into a shock absorber and get t working just right for our own particular style.
While the monoshock system is something that most people are afraid to screw with, a more conventional system like that of the MXC can be personalized without undue trauma.
That’s something the even relatively serious racer will have to do soon after buying an MXC.

Suspension has never been a strong point on Yamahas and the MXC is no exception. It starts off feeling pretty good but soon the forks start bottoming and you begin to lose your confidence in the shocks. The substitution of 190cc of Bel-Ray 30-weight in each forks tube settles the front end down a little and a couple of clicks on the shock spring adjusters sets the back up just right.
But after a few hot practice sessions, the forks start feeling squirrelly again and the shocks fade. The front wheel seems to want to dance on the bumps, giving you a dreary feeling about what’s going on below.

A front end that jitterbugs through a corner doesnt do much for your confidence. When the shocks fade they begin to return a little too smartly for our tastes, but it’s not too bad.
But as the springs begin to weaken the ride begins to get unmanageable. Yamaha needs to get their hydraulic pressures worked out more completely, and to get turned onto some good springs. Otherwise, as in the case of the MXC, the new owner’s first consideration must be to upgrade the suspension.

This is not to say that you cant wail on this bike in stocks trim. While the suspension isnt quite there, the persevering rider is well-served by the excellent frame, engine and tire package. While the chassis is made from the same light-duty steel that constitues most Yamaha frames, its constructed in short enough sections to make the frame fairly rigid.

You might say that its a YZ frame without the monoshock. Much of the vertical stress that caused last year’s MX swingarm to flex so much is relieved by the laydown shock arrangement this year, so deflection caused by side loads is much less apparent. And the added travel of the laydowns – 5.8 inches- helps to make the rear end more stable.
The result is a machine which handles better than you might at first suspect.
The power produced by the YZ ported six-speed is excellent. We always felt that the reed valve 125cc Yamaha engines lacked the top end power necessary to compete with race-tuned Hondas. The top end shump endemic to the Yamahas was ,we felt,a function of the reed valve induction system.
But a few rides on some factory-prepared Yamaha YZs convinced us that the power was there, it just has to be coaxed out wih the proper work and pipe tuning.
That work has been done. The MXC has all the power you’ll need to be competitive against almost any 125. In stock trim, burning a hefty 20.1 pre-mix and with the spark arrestor still attached, the MX 125C comes on lke a KX125 Kawasaki and keeps delivering all the way to the top. We were very impressed.
For the novice, the reed valve induction still gives the addes advantage of clean running even at impossibly low crank speeds. You can pull it from anywhere in any gear and never fog the spark plug. That gives the bike a decided advantage over many highly tuned competitors, especially when the action hits sand or mud.

Staying on the powerband is no problem with the six-speed gearbox, but keeping the piston and rings is another story. Because of the high power output and somewhat radical porting, Yamaha strongly recommends in the owner’s manual that the piston and ring be replaced after every race. While this may seem a little excessive, considering the pipe wrench durability of past MXs, this is not an unreasonable requirement. If you’re not racing, just riding, you should still pop for a new piston assembly every few weeks. Take our word for it.
Keeping your confidence at the Marty Smith level when your suspension is doing a tap dance through the turns can be a chore if your tires aren’t doing the job.
This year, Yamaha has shod the MW model with a sano set of Dunlop Sports. Between the wraparound 4.10 on the back and the sticky 3.00 on the front, there’s more than enough prime rubber on the ground

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