With three exciting new bikes in the 250cc four-stroke category, we must admit that we’ve overlooked the Yamaha YZ250F lately. Not that the familiar YZ is not a great bike, mind you; it’s just that we’ve been a little infatuated with the new red, yellow, and green thumpers. As the second-place finisher in our 2004 250cc four-stroke shootout, the Yamaha possesses the best off-the-showroom-floor powerplant, but was held back in our testers’ overall opinions by its soft stock suspension and overall heavier feel.
When it came time to give our trusty YZ250F test bike a facelift, we decided to forego the weight issue, as bolting on all sorts of titanium nuts and bolts is not only cost prohibitive, but unrealistic for the average reader as well. Instead, we decided to focus on coaxing even more power out of the class-leading engine, and upgrading the bike’s clapped-out suspension to help give the entire package a better all-around feel and better control.
As Chad Reed’s former mechanic at Yamaha of Troy and Team Yamaha, Dave Dye knows his way around a Yamaha YZ250F. These days Dye works with the exhaust pipe gurus at FMF, and we enlisted him to help us build our YZ250F into a great-handling powerhouse that was, at the same time, as reliable as it was in stock condition.
WHERE’S THE BEEF?
To improve the YZ250F’s powerband, the first thing Dye did-naturally-was add a FMF Factory 4 titanium exhaust system to the shopping cart. In addition to weighing in at three pounds less than the stock unit, the system gives the YZF big gains in mid-range and top-end power, with only moderate gains in exhaust noise thanks to FMF’s stepped muffler core design. For the internal motor mods, Dye enlisted renowned motorman Terry Varner to handle the chore. Since the FMF pipe gives the bike more mid and top, Varner concentrated on gaining more low-end snap, as well as even more power right in the middle of the rpm range. This was accomplished through cylinder head porting and the addition of a high-performance cam. Varner finished off the power mods by enhancing the flow of the stock carburetor for improved throttle response. Varner tunes all of his four-stroke engines to run on VP Racing’s Ultimate 4.
To help put the power to the ground better, the stock clutch components were ditched in favor of high-quality billet aluminum components from Hinson Racing. A Hinson clutch basket, pressure plate, and inner hub were installed in the engine and coupled with a new set of stock clutch plates and a set of stiffer Varner Motorsports clutch springs.
Having worked hand-in-hand with Enzo Racing during his tenure at YoT, Dye decided to send our Kayaba fork and shock in for the full works treatment, including Enzo’s patented yet oft-copied fork sub tank system. With instructions to valve our fork and shock for 175 lb. intermediate test rider Jed Herring, the crew at Enzo stiffened up both ends through valving and spring rates.
Up front, .44 kg springs replaced the stock .434 kg units, and a 4.9 kg shock spring replaced the 4.7 stocker. An Enzo Works Bumper kit was also installed on the rear shock, while the sub tank system was fitted onto the front forks.
The rest of the bike was rounded out with a variety of FMF’s in-house 909 brand items, including a quick-adjust AOF clutch perch, a 909 front brake lever, 909 team bend handlebars and grips, and an FMF Racing team graphic and seat cover kit. BRP triple clamps replaced the stock units, as well.
As a front-running intermediate class racer, we found it hard to believe that our test rider Jed Herring has never before competed on or owned a highly-modified motocross bike. “My dad has always believed in leaving them stock and focusing on my riding, not so much the bike,” he said. Imagine his surprise when we presented him with the FMF/Varner/Enzo Racing machine for him to race at the Lake Whitney Spring Classic in Texas! In our initial test sessions with thhe bike, Jed was completely blown away by the improved performance of the machine’s motor and suspension.
In stock condition, the YZ250F comes home on the dyno with about 34.5 horsepower, and in its modified state it gained a full five horsepower at 39.5. On the track, this is readily apparent as the Varner-modified YZ250F boasted more low-end power than any other 250cc four-stroke we’ve tested, stock or modified. Herring commented that it felt like it had twice as much pulling power coming out of tight corners, giving the bike a huge advantage over two-stroke 125s and even other 250cc four-strokes. The mid-range power was also stronger, and it pulled well into the upper rpm range, holding on and signing off a little later than it did in stock form. The engine could be revved out with authority in each gear or short shifted with equal success, which makes it good for riders of all riding abilities. The modified motor revved cleanly and had crisp, instantaneous throttle response, but most importantly did not have the momentary hiccup that it did in stock condition when the throttle was suddenly chopped open.
The Hinson clutch components improve the clutch’s feel and performance, resisting fading under heavy abuse much better than the stock parts. Furthermore, we’ve found that the clutch plates themselves have lasted longer with the high-quality basket, hub, and pressure plate installed. The only downfall of the modified motor is that it is harder to start when cold. Once warmed up, however, it fires up just as easily as a stocker.
All of the newfound power would have been for naught if the bike did not handle better, as well. Thankfully, the Enzo Racing-modified fork and shock were more than up to the task of handling the higher rates of speed that the motor was capable of attaining. The stiffer springs and valving allowed both ends to ride higher in the stroke and thus provide a plusher feel in the small braking and acceleration chop. At the same time, however, hard landings and G-outs were absorbed better thanks to the stiffer overall settings. Our biggest complaint with the stock suspension was that the front end tended to blow though its suspension stroke under heavy braking and cause the bike to bottom in gnarly braking bumps, as well as bottom violently in slap-down landing or G-out situations. Thanks to the sub tank system, the fork maintains plush suspension action in the small stuff, but handles the aforementioned conditions with aplomb. Getting the front end to make that metal-to-metal clank when bottoming out is virtually impossible.
So, you ask, how did young Jed Herring do at the Lake Whitney race? Well, thanks to an ill-timed tip-over while pulling off the track, Jed spent his entire road trip in a figure-eight brace with a broken collarbone. Damn the luck!
PARTS IS PARTS
BRP triple clamp set
Varner cylinder head porting
Varner carburetor mods
Varner Web Cam exchange
Varner clutch springs
FMF Factory 4 titanium pipe
909 team bend handlebars
909 brake lever
909 AOF clutch perch
FMF Racing team graphics
FMF Racing team seat cover
Hinson Racing clutch basket
Hinson Racing inner hub
Hinson Racing pressure plate
Enzo Racing .44 fork springs
Enzo Racing 4.9 shock spring
Enzo Racing fork revalve
Enzo Racing shock revalve
Enzo Racing works bumper kit
Enzo Racing sub tank system