Taming the Dirt Bike From Hell

TAMING THE DIRT BIKE FROM HELL

CW PROJECT

Exorcising Yamaha's YZ490

JON F. THOMPSON

IT CAME TO BE KNOWN, SIMPLY, AS THE DBFH, SHORThand for the Dirt Bike From Hell, the title bestowed upon Cycle World's test Yamaha YZ490 thanks to its devilish handling and diabolic power curve. Though air-cooling diehards still relish it to the point where Yamaha sells every one it imports, the YZ490 is, when compared to more-modern, liquid-cooled Openclass motocrossers, as difficult to enjoy as a dirtbike can be. Its problems are basic and relate to the fact that it has been essentially unchanged since introduced in 1982. In fact, a Yamaha spokesman told Cycle World, “Because of its popularity, we haven’t put a lot of money into developing it.” Too bad. The YZ490 would have benefited from development.

In stock form, the engine’s porting and combustionchamber design dictate that no matter how its carburetor is jetted, the YZ will reward the rider with a combination of detonation and four-stroking—a condition in which at some rpm ranges the air-fuel mixture is so rich that oxygen sufficient for combustion exists only on every other power stroke. Jet it leaner to avoid the four-stroking and it’ll seize. Don’t change the jetting and it still might seize. Add anti-seize insurance in the form of an enriched mixture and the four-stroking becomes worse while the very short distances the bike will travel between fuel stops, courtesy its motocross-small fuel tank, get even shorter.

Then there’s the matter of the bike’s power curve, which in stock form is as violent as a step off a canyon wall. From idle to about 3500 rpm, there isn’t much happening, but then all 40 horses come on as though someone had just pulled the trigger lanyard on a howitzer.

The YZ’s problems don’t stop at its engine; they extend deep into its chassis. Its fork, adjustable through oil level, air pressure and compression damping, is harsh, especially in its mid-range, and has excessive high-speed compression damping no matter what the rider does with the considerable adjustment spectrum. Likewise the Monoshock rear end, which has a rebound like the kick of a mule, especially over square-edged obstacles, which lacks highspeed damping and which easily can be bottomed.

Some of the 490’s problems were attacked with the telephone: We ordered a 3.7-gallon fuel tank from Clarke Plastics ($91.50) as a replacement for the 2.64-gallon stock tank, which increased the bike’s trail-riding range. And to knock the edge off the YZ’s power delivery, we ordered one of Cobra’s Hush Puppy silencers ($99.95), which increased low-rpm back pressure and in doing so fattened the lowand mid-range torque curve. And we installed a Yamaha 15-tooth countershaft sprocket ($15.95) in place of the stock 14-tooth unit, further smoothing the YZ’s power delivery and boosting its top speed.

And then we got serious, enlisting Paul Thede, whose firm, Race Tech, specializes in doctoring dirt bikes. Thede started with the engine, removing metal from the head to reshape the combustion chamber and provide for morecomplete combustion and a faster spread of the combustion-chamber burn. This results in greatly reduced preignition.

Thede and his technicians then turned their attention to the YZ’s fork and rear shock. They modified the springs at each end by shortening them, effectively giving a stiffer spring rate. Thede removes from one to five coils from the fork springs, and up to a full coil from the rear spring, the amount to be removed calculated individually depending on the rider’s weight. This change at the rear is especially important because the shock linkage is not progressive. Making the spring stiffer means the unit can be set up with less preload, which, Thede claims, gives it the effect of being more progressive throughout its travel.

Valving at both ends was modified to provide less lowspeed compression and rebound damping and more highspeed compression and rebound damping, resulting in less harshness in the initial stages of suspension compression and a firmer feel at the end of the suspension stroke. It also eliminated the rear suspension’s tendency to bottom.

The cumulative effect of Thede’s work, which cost $450, was to transform the YZ from a bike bypassed by time and technology into a one that’s fast and ridable.

Besides considerably reducing the YZ’s tendency to ping, the cylinder-head work allows for carburetor settings which come much closer to optimal air-fuel mixtures:

Stock jetting for the YZ is a 470 main jet and a 40 pilot, with the needle on its middle notch, but following Race Tech’s manipulation of the engine, we dropped the needle a notch and fitted a 460 main, marginally increasing fuel mileage without any apparent increase in the bike’s tendency to detonate.

And though nothing short of revised geometry can fix the bike’s slow, imprecise steering, the suspension changes yield a dramatic improvement. The bike’s reaction to sharp-edged obstacles is much more plush and though the swingarm still kicks fiercely on rebound, an action which has the effect of dramatically steepening the bike’s steering angle when you least need it, the rebound kick is far less fierce than it was before Thede did his work.

The changes made to the YZ’s combustion chamber don’t exactly transform the bike. It isn’t, for instance, any easier to coax its engine into life; a compression release, or maybe fewer cubic centimeters, is what’s called for there. And even with Race Tech’s tricks—which added to the YZ’s purchase price of $3549, brings the modified bike’s total to $3999—the YZ490 still isn’t on par with the two pure-motocross motorcycles we much prefer over the YZ; The Honda CR500, which lists for $4098, and the Kawasaki KX500, which lists for $3799. But if you’re thinking of a desert sled, a cowtrailer or an all-around playbike, the modified YZ490 begins to look very attractive indeed.

To make the YZ even more attractive, we added, in addition to the larger primary gear, oversized fuel tank and silencer already mentioned. Acerbis handguards ($ 18.95), as well as an MS Racing skidplate ($54.95), exhaust-system bash plate ($34.95) and aluminum kick stand ($61.95). Also added were an Answer Products Alumalite handlebar ($64.95), an EK O-ring chain ($89.65) and a Brake Snake brake-pedal cable ($5.95). All of the above items were ordered from the Malcolm Smith Products catalog and brought the price of our modified YZ to $4537.70. The YZ490 may not be the class of the motocross field, but because of its air-cooled simplicity, some less raceoriented riders still prefer it over its competition, even if the cost of putting one right hits as hard as an unmodified 490’s power curve. And if the YZ490 isn’t exactly transformed by the modifications described here, because of them it no longer is The Dirt Bike From Hell, either. Let’s just call it The Dirt Bike From Heck.

SUPPLIERS

Engine, suspension modifications Race Tech 3227 Producer #127 Pomona, CA 91768 (714) 594-7755

Fuel tank Clark Plastic Products 29032 S. Salo Rd. Mulino, OR 97042 (503) 829-2156

Silencer Cobra Engineering 4760 E. Bryson St. Anaheim, CA 92807 (714) 779-7798