2-STROKE TUESDAY: THREE AMIGOS

 
(10/30/2012)

 
The staff at Dirt Bike magazine is caught in a bit of a conundrum when it comes to Yamaha’s two-strokes. On one hand, we need to applaud Yamaha for continuing to make competition-worthy two-stroke motorcycle for all classes. On the other hand, we’re bummed that the most recent major update to the 125 and 250 happened way back in 2005 and the YZ85 hasn’t really changed since its introduction. We took it upon ourselves to take these three timeless Yamaha two-stroke classics and give them a 2012 update.
 
UN AMIGO
Since its introduction in 2002, the Yamaha YZ85 has remained relatively unchanged—except for small parts and minor styling changes. Ten years later, the bike is almost identical. Justin Hoeft is a top amateur racer with limited support from Yamaha through Thousand Oaks Powersport and also a Dirt Bike test consultant. Here is what was done to Hoeft’s YZ85.
The engine received a complete GYTR 112cc big-bore kit made by Athena. This complete kit includes a new precision-made, stroked crankshaft and nickel-carbide-plated cylinder with enlarged water passages and revised porting designed specifically for improved performance. The cylinder head is also designed with enlarged water passages. The GYTR kit has everything needed for a complete installation—except the required crankcase machining. It includes a crankshaft, connecting rod, cylinder, cylinder head, precision-made piston with rings, clips, wrist pin, studs, nuts, O-rings, and gaskets (different-thickness base gaskets to adjust squish).To help the bike breathe, we installed a GYTR complete exhaust by FMF, a V-force reed valve, and a dual-stage, high-flow air filter. A GYTR complete clutch was also installed, which included a billet clutch basket, billet clutch pressure plate, billet inner clutch hub and a stock Yamaha clutch kit.

 


The chassis got major updates also. GYTR offers a complete Super Mini chassis kit that includes longer front and rear brake lines, a longer rear swingarm, bushings, bearings, collars, oil seals and plugs. We topped it off with front and rear GYTR wheels through Dubya USA utilizing Talon hubs, spline-drive heavy-duty spokes and Excel rims.
With the major updates completed, Hoeft bolted on a GYTR chain and sprocket kit and Tag bars, and Factory Connection handled all the hard work in the suspension department.
The Yamaha ends up compliant with both AMA and NMA rules for their respective Super-Mini divisions. It is, basically, the cream of the class. At the time this magazine was printed, Hoeft was having a very successful year, winning numerous amateur championships at such events as the World Mini Grand Prix, Mammoth Mountain Motocross and, the crown jewel, Loretta Lynn’s AMA Amateur National Championship.
 

DOS AMIGOS
Even though the YZ125 has had a long run without big changes, it’s still considered the best 125 motocross bike. It might not have the sheer power of the KTM, but its suspension and handling are still stellar. When you talk about two-strokes, it’s hard to mention the 125 without cracking a smile—they are just flat out fun to ride! Here is what we did to make the YZ125 even better.
 
The major change that is visible to the eye was the installation of the brand-new white UFO Yamaha two-stroke restyle kit. UFO’s restyle kit bolts right up to all the existing mounting spots, so no drilling or cutting is involved. The kit gave our YZ125 a look similar to the 2012 YZ250F. The front fender, front number plate and rear fenders are almost identical. The radiator shrouds and side number plates have a YZ250F styling hint without going overboard. Dirt Digits Graphics made the kit flow, and the white SDG seat took the style factor to an entirely different level.
We installed an Athena 144 big-bore kit that included a brand-new nickel-carbide-plated cylinder (with enlarged water passages), cylinder head, 58mm Vertex piston with rings, pins, clips, gaskets, modified power valves, O-rings and complete instructions. Best of all, it fits all 2005–’12 Yamaha YZ125 models. The kit was a little more involved to install than we initially thought. After actually reading the instructions first for once, we discovered the need to grind some clearance from the cases for the oversized piston. (Just an FYI) For this you need to split the cases. A Pro Circuit factory-finished pipe and 304 shorty silencer helped our YZ144 with those extra ponies on the mid- to top-end pull. We installed a complete Hinson billet clutch that included a clutch basket with a kickstarter gear, an inner hub, a pressure plate, fiber plates, steel plates and springs.

 


Enzo Racing probably has the most testing time with the KYB components that are stock on the YZ125. Ross Maeda, Wil Decker and Craig Decker have countless hours logged supporting all the factory and privateer race teams of the era when two-strokes reigned supreme. So, it seemed like an easy decision to have them work over the suspension. Applied Racing’s new R/S triple clamp set gave us the option of running rubber- or solid-mounted bars with loads of adjustability.
We used Warp 9’s complete wheel sets with blue hubs, black spokes and black rims. The rear wheel comes equipped with a brake rotor and sprocket, while the front includes the brake rotor. We also installed Ride Engineering’s steel-braded brake lines to give us that extra stopping power.
With all that, did the little YZ become a 250F killer? Not quite. The age when an over-bored 125 can hang with a 250F was over some time ago, but there isn’t a rider on earth who wouldn’t dig the 144 for the sheer fun factor. It loses a few revs on top, but the low-end power is vastly improved. With all these improvements, our YZ125 not only looks updated on the outside, it feels updated on the inside.
TRES AMIGOS
The YZ250 has seen many years come and go without any major updates. 2005 was the last year Yamaha updated their YZ250 by adding an aluminum frame. The formula for building a ripper YZ250 two-stroke has been set in stone for years now, so pay attention, here it is.
 
The engine has had minor massaging to the intake and exhaust ports, never going too crazy, as you don’t want to ruin the existing power characteristics. The head is milled, and a slightly higher compression piston is installed for that nice grunt right off bottom. We also added a GYTR complete exhaust system developed specifically for the YZ250 and manufactured by FMF. V-Force reeds helped feed the powerplant with plenty of VP C-12 racing fuel. The clutch is a complete GYTR billet system consisting of a clutch basket, an inner hub, a pressure plate, fiber plates, steel plates and springs. GYTR’s performance racing chain and anodized sprocket kit transferred all the power to the ground.
Suspension on all two-strokes is important, but especially on a YZ250 putting out the amount of horsepower our GYTR YZ250 did. Factory Connection revalved and resprung our machine. It now makes lots of power with great handling, but sooner or later stopping is going to become very important. A 270mm oversized front-brake rotor kit and steel-braided lines on both ends would take care of all the braking needs. GYTR also makes a host of bolt-on parts that adorned the YZ250 project, such as ARC levers, blue anodized chain-adjuster blocks, blue anodized brake clevis, chain slider and a blue anodized rear sprocket.
We have seen this formula copied with minor changes time and time again. Why you ask? Because it flat out works! Ryan Hughes proved it without a doubt at the Mammoth Mountain motocross event last year on this very motorcycle by winning the Two-Stroke Open class by over 30 seconds.

 
 


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WARNING: Much of the action depict­ed in this magazine is potentially dan­gerous. Virtually all of the riders seen in our photos are experienced ex­perts or professionals. Do not at­tempt to duplicate any stunts that are be­yond your own capabilities. Always wear the appropriate safety gear.

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