The Motorbike Archives

This commitment pays off in the form of tradition: Yamaha and Husky have had competitive bikes (and the most championships) year after year while every other manufacturer has had either hit-or-miss victories, offered less-than-competitive machinery or skipped production altogether. Indeed, Honda has just now attained the stage of producing competitive four-strokes, Suzuki has dropped out of the middleweight and open-class picture, Kawasaki has paid more attention to the small-displacement KDXs than the 400-420 class, Can-Am is out of the enduro market completely, and Maico offers only slightly altered E-versions of their motocrossers.

Yamaha has taken an unusual—and conservative—route to develop the IT line. As a rule, their engineers have judiciously applied YZ technology to the ITs a model year after introducing it on the motocross bikes. This has allowed the designers the freedom to carefully choose the technology they want to borrow, but the ITs, good as they've been, always seemed to miss being as good as they could be.

Hang on. This year it's different. The new ITs are not just motocross bikes with lights (nearly a sure bet for mediocrity), but they do feature the latest updates fresh from the drafting board—the same techno-tricks which the YZs are using for the first time in '83. And if you're looking for changes from the knobbies up, you've come to the right bike. Put the IT-K next to last year's J-model and you'll swear it's cut from a different mold.

The big news in the powerplant is the bump in displacement from 465 to 487cc, accomplished simply by increasing the cylinder bore two millimeters. This follows the large, seven-millimeter increase in stroke which boosted the IT's displacement from 425 to 465cc in '81. Aside from the trendiness involved in increasing their open-classers' engine sizes, most manufacturers are nudging 500cc because more displacement is the most straightforward route to better power.

And better power the 490 has. You won't find the increase at the peak of the power curve, but who wrings out an open-class enduro bike to 8000 rpm? You'll find it instead in the basement and mid-range. In fact, if the IT made thumpity-thump sounds, you probably couldn't tell you were on a two-stroke. The dyno backs up the seat-of-thepants impression; the IT is even with or stronger in horsepower and torque than the KTM 504 from 3000 to 6000 rpm, and a lot stronger at the top-end.

In tight, twisty and steep areas that kind of power helps make riding a breeze. Lug the 490 down to a walk and it pulls like a bull with a snap of the throttle. In one of our favorite riding areas we found it was a gas (and a change) using the mid-range to make riding less strenuous, powering up fairly steep hills instead of charging them like madmen.

Part of the credit for the excellent low- and mid-range also goes to this year's addition of Yamaha's Energy Induction System (YEIS). This setup consists of a rubber tube running from the intake tract to a tubular canister. Dispensing with brand names, we point out that the principle of the canister/tube and their effect were discovered by a 19th century acoustical engineer named Helmholtz, hence the generic name of the system—Helmholtz resonator. The canister develops its own resonance (the frequency of which is determined by the dimensions of the system), which offsets the natural resonance developing in the intake tract. The result is smoother fuel delivery at low rpm, helping to prevent surging or flat spots.

As much as we dwell on the midrange, don't be misled—the IT has formidable top-end power too. With about 35 horsepower at its peak, the 490 won't blow by any big-bore motocrossers, but it's certainly strong enough for throttle-to-the-stop Hare Scrambling or the occasional Two Day event. Cross-country running or fireroading is a blast; it's easy to slide the IT or cut through deep sand in practically any gear.

Yamaha uses its proven gearing for the open-classer. The primary gears (63/24) and the final drive sprocket pairing (14/44) have been used for the last few years by the 425 and 465, and the five transmission ratios have served all the big ITs back to the 400. That's fine by us. Each ratio is evenly spaced: second allows you to fade nearly to a standstill without bogging the engine, third and fourth are fine for fast rough trailing and they let you take advantage of the wide powerband, and fifth produces a top speed which should satisfy dry lake fanatics and fireroad crazies alike.

Subtle changes—like the new under-the-seat air cleaner—complement the major eye-catching engine and chassis modifications.

The only blemish with the gearbox is an imprecise feel at the shift lever. The IT rarely catches a false neutral, but sometimes our testers poked at the lever and found they hadn't shifted at all—they were still in second or third or whichever gear they had been in.

It's obvious that refinement and modification of a proven design lead to optimum engine performance. But you can scratch those words right off the page when you want to talk about the IT's chassis and suspension. The 490 may use some parts from last year's running gear, but you'll have to look hard to find them. Breaking precedent, Yamaha designers applied new suspension technology to both the YZ and IT lineups simultaneously. In '82, if you'll remember, the YZs (and the IT175) got a new rising-rate rear suspension system in response to Honda's and Suzuki's progressive-rate affairs. But it was a hurry-up move, and last year's YZs suffered in suspension performance compared to the CRs and RMs. No matter. The IT-K uses both a newly designed fork and the new Monocross system (the 250 and open-class ITs having never used the YZ-J rear setup), and both systems are winners.

Up front the IT has a YZ-style unit, complete with 43mm fork legs and air caps. Though the fork does not have externally adjustable rebound or compression damping, Yamaha has added an internal device they call a proportioning valve to adjust compression damping automatically. The spring-loaded valve has a series of orifices and is located at the bottom of the damping rods; it obstructs the flow of oil through the rod at low and medium fork slider speeds. Thus, the P-valve acts in concert with the orifices in the damping rod to provide resistance to oil flow. On the compression stroke, oil is forced through the normal circuitry in the damping rod—through the unobstructed upper orifices and through the lower ones past the P-valve. As fork slider speed increases (when you hit a bump harder or at higher speed), oil pressure overcomes the P-valve's spring resistance, physically lowering the valve and removing it as an obstruction, at which time oil flow is unobstructed. The intention here is to avoid a hydraulic lock during sudden and extreme compression. If you looked at the damping rate change on a graph, you'd see there's greater damping this year particularly at medium fork slider speeds, and a little less damping at the very fastest speeds.

The latest incarnation of Monocross is a winner. At the heart of the setup is an adjustable shock and pivoting linkage system.

At the rear there's no mistaking the new Monocross suspension. The shock no longer tucks away under the gas tank—Yamaha has long been trying to get away from that. Early Monocross designs had the shock enclosed by the frame's backbone, then it was external but parallel to the backbone, then adjusted rearward and fitted with a remote reservoir. All were halfway measures; heat buildup and consequent damping fade plagued each one. The '83 arrangement has the top of the shock mounting to the base of the backbone and the bottom of the shock connecting to the aluminum swing arm via a pivoting aluminum linkage. The L-shaped arm attaches to the shock at the top of the "L," then to a short frame-mounted strut (which pivots at both of its ends) at the bend of the "L," and finally to tabs welded on to the bottom of the swing arm at the bottom leg of the "L." As the swing arm moves, changes in the angles of the linkage system alter the rate at which the shock compresses—hence mechanically progressive springing.

A new lighter, aluminum-bodied damper complements the '83 Mono-cross system. It features threaded preload collars (allowing about two inches of adjustment), a bump foam to soften impact at bottoming, and adjustable rebound damping. A single heavily knurled knob sits at the bottom of the shock. Unlike the YZ shocks, which have separate knobs for compression and rebound damping, the IT shock has adjustable rebound damping only, a difference warranted by economics. The basic design is that in use since the IT-Gs: a tapered needle moves in or retracts from a damping orifice in the shock, restricting or freeing oil flow and thus altering damping. Actually, the needle position also affects compression damping, but only minutely (about five percent); compression damping is mainly controlled by separate orifices.

If you haven't guessed by now, that updated linkage system required a new frame. The 490 incorporates both YZ-style innovations and traditional IT designs. The IT490, like the YZ490 and the IT250, uses a single downtube frame which splits below the crankcase to cradle the engine. This necessitates an offset exhaust port in the cylinder in contrast to the liquid-cooled YZ250 engine, which uses a centrally located exhaust port. (The choice here is understandable; center-port cylinders theoretically allow maximum efficiency, and peak horsepower across the power spread at all times is the ultimate goal with 250cc motocrossers, not so much the case with enduro bikes or open-classers.)►