Yamaha It490k
YAMAHA IT490K
CYCLE WORLD TEST
Yamaha introduces the do-it-yourself bullet.
With all the variety in big bore dirt bikes, there have been some surprising gaps in the class. Sure, there are twostrokes and fourstrokes, motocrossers and enduro bikes and even a couple of special desert models with long-travel suspension and big gas tanks, but something was missing, too.
To satisfy their needs, lots of desert and enduro riders have bought motocross bikes for their performance, then added big gas tanks, changed the gearing, and maybe rigged some lighting. Motocross bikes haven't been equipped as big-bore play bikes, enduro bikes haven't had the suspension of the motocrossers, and they've been heavy. Now that more of the big motocrossers are being produced with four-speed transmissions, that crack in the open-class wall is getting wider.
That's where Yamaha's 1T490 comes in. Ride it in the brochure and it's per fect. With a big 487cc reed-valve Single, it should have enough power to roost the Chinese army. And it comes with the five-speed transmission that the YZ490 motocrosser doesn't have. This year the IT doesn't have a left-over frame and suspension. It gets the same frame and most of the same suspension as the motocrossers use, with the progressive linkage-controlled rear suspension. Sus pension travel is 11.8 in. at both ends, right up there with the motocross bikes. Add a 3.6 gal. gas tank, enduro lighting and the 1T490 looks ideal.
Take it off the brochure and ride it where big dirt bikes are so much fun, and it loses something. The idea is right, the execution is not.
Fortunately, it isn't difficult to make the IT into the kind of bike it should be.
Foundation for the 1T490 is the moto cross-style frame, with thin-walled large diameter chrome-moly tubing used for the large single front downtube and other tubes. Yamaha has previously mounted a single shock high on the frame, with a braced swing arm pushing directly on the shock on early models, or via linkage last year. Now the shock has been lowered in the frame, with its top mounted directly at the end of the frame backbone. The lower end of the shock is attached to one of a pair of aluminum struts that arc through their travel in re sponse to the swing arm movement. This provides a progressive resistance to the rear suspension. Having the shock lower in the frame also reduces that top-heavy feeling that has been a part of Yamahas as long as single rear shocks.
Frame tubes are strengthened with plenty of gusseting and braces around the 28° steering head and above the swing arm. Small tubes are welded below the engine's magnesium cases, bolting to each side of the downtube. These light weight case savers don't pack with mud as easily as skid plates do, yet provide excellent protection for the engine. In back there's a rear frame loop more than able to hold a mud-covered rear fender and the tool bag on top of the fender. Besides the usual light-duty tool bag filler, there's a spanner to adjust rear spring preload.
Rear spring preload can be easily ad justed without pulling the bike apart. The adjusting nuts are on the bottom of the shock, just above the knurled knob that sets rebound damping to any of 25 positions. From the top of the shock body a short hydraulic hose leads to the reser voir mounted beside the shock, on the right frame tube. The motocross bikes have a compression damping adjustment knob at the top of the shock, the IT doesn't. This is the biggest difference in rear suspension between the enduro and the MX machine.
A pair of aluminum struts connects the shock to the forward end of the swing arm, much like a Honda Pro-Link sus pension. The main pivot between the two links is fitted with a grease nipple and a lightweight plastic shield is mounted just behind the lower shock mount to keep the damping adjustment knob free from mud. The swing arm is a lightweight alu minum piece made from two extrusions welded together. At the back end there are solid aluminum blocks holding snailtype chain adjusters. The axle slots ex tend to the end of the swing arm, with easily removed pins keeping the axles from sliding out unintentionally. With the pins pulled and the quick-detach brake rod moved, a rider only needs to loosen the axle nut to slide the rear wheel off the bike. The brake backing plate slides into a protrusion on the inside of the swing arm, eliminating a separate static arm. This is an excellent quick de tach wheel, everything working just as it should for one minute wheel removal.
Lightweight hubs and wheels are used on the IT. Like the motocross bikes, the IT's wheels use a hollow rim for the bead, keeping unsprung weight down. But instead of using the machined alumi num spoke nipples, the IT uses conven tional steel parts here. A double leading shoe brake is used in front, a single shoe is used in back.
Forks are more motocross hardware. The Kayaba 43mm forks have 11.8 in. of travel, are held by double pinch bolts in the triple clamps and have only air caps for external adjustment. Normal air pressure is zero. The excellent owners manual goes into detail on the effects of changing oil volume and weight.
Covering both wheels are wide, wellmade plastic fenders. They protect the rider and so do the small plastic handguards over the levers. The large capacity 3.6 gal. gas tank is pleasantly slim, has a large filler hole and a reserve position on the petcock. A useful, bright headlight is mounted with the front numberplate. All the plastic parts of the IT show' careful consideration of its use. The airbox, for example. The duct between the airbox and the carburetor runs around the left side of the rear shock. On the right side of the airbox, covered by the right sidecover, is the opening for the air filter. Remove one wing nut and the sidecover is off, then pull a rubber strap and the side of the airbox is off. The air intake is at the very top of the high airbox, and it includes a water drain as the air is ducted into the airbox. Only by putting the seat of the IT490 below' water level should this bike be drowned.
Okay. The suspension sounds good and the hardware is excellent, but it’s the motor that counts most in an open class anything. People buy bikes with giant motors because they want horsepower, lots of horsepower. That horsepower ought to be spread throughout the powerband of an enduro bike. A 490 anything ought to be able to dig a new Panama Canal.
Unfortunately, the IT490 isn’t like that. The parts are there, the performance isn’t. It has a 3mm larger bore than last year’s IT465, making for an 87 x 82mm bore and stroke. It breathes through a 38mm Mikuni and a giant reed valve, with one of Yamaha's YEIS boost bottles connecting to the intake tube between the carburetor and reed valve.
Behind this enormous motor is a wideratio five-speed transmission.
The IT490 can be difficult to start, requiring two to six kicks. Vibration is minimal when the transmission is in neutral and virtually undetectable when the bike is in gear. It's one of the smoothest running big bore enduro bikes made. Move the folding tip gear lever just a little and it's in gear. Shifts are short, but take more than average effort.
Let out the clutch and shift through the gears and the IT490 feels as though it’s powered by an electric motor. A weak electric motor. It pulls well from idle to top speed, it just doesn’t pull hard. Sandwashes are especially revealing. Big bore bikes love fast, uphill sandwashes where their wonderful power can be used. This one pulls sandwashes faster in fourth than fifth, and it’s not because of the gearing. Gear ratios are spread well, with the top four ratios much like the gear ratios in a four-speed YZ490. Eirst gear is noticeably lower.
Engine difficulties were particularly noticeable on a ride through Mexico. The engine pinged excessively, even at 5000 ft. elevation. Before the ride the engine had been broken in and the carburetor jetted according to a supplementary carburetion sheetvn the owner’s manual. That factory-issued note recommended a 370 main jet, not the original 390, and it also advised lowering the needle one notch. When the pinging continued at high elevations the original 390 main jet and needle positions were installed. This eliminated the pinging and increased power slightly.
Even with a hard-riding desert expert aboard, the IT490 could go 100 mi. on the big tank. It held up well, too, not even requiring a chain adjustment in 400 mi. O-ring chains work great for extended off-road trips and the Yamaha’s is no exception.
The IT490's suspension is also less able than it should be. The front tire skates if the rider doesn’t climb on top of the gas tank in every corner. Both ends of the suspension bottomed easily in every dip. Adding preload to the rear spring reduced the bottoming in back. In front we had to add one inch of fork oil in each leg. Suspension action with these adjustments is good, but the front tire still skates.
Back at the shop the IT invited modification. There’s just too much promise in this well-equipped enduro mount. A look at the cylinder ports showed the IT to have a barrel much like the YZ490, but with a 3mm lower and 2mm narrower exhaust port. Additionally, the YZ ports are more square shaped, with larger area for flow just as the port is uncovered. The IT cylinder could be modified to YZ configuration, but we installed a YZ cylinder. It's the fast, easy and expensive way. The stock IT piston was used.
Another trip to the desert showed power to be better, but still not what we hoped for. The bike would now pull fifth in an uphill sandwash, but it was still inferior to a powerful open-class motocross bike. Next it was time to try shortening the exhaust. This is a common modification on pro-raced YZ490s, and it’s easier than it sounds. A half inch was removed from the headpipe by cutting the pipe at the weld between the headpipe and the first cone. Cut off a half inch from the headpipe and weld the pipe back together. A gas welding outfit works fine. With this change the engine began to come alive. Now it felt like a 490, but it felt like a 490 with a potato up the exhaust pipe.
YAMAHA
IT490K
List price .........$2249
The final change was to the silencer, which turned out to be the potato, in this case. Replacing the IT's silencer with a modified RM465 silencer turned the engine loose. The engine is now responsive and the power enormous. The heavy flywheel helps keep the rear tire from spinning wildly, but the IT can wheelie at will. It pulls from idle and runs like a tractor on tight trails in the mountains. It makes better, more useable power at every engine speed. Gas mileage didn't change. For riders without leftover Suzuki silencers handy, Answer Products is producing aluminum replacement silencers for the IT490.
Getting the front tire to turn the bike was as hard as finding horsepower. Moving the fork tubes up in the triple clamps helped some. Two different 3.00-21 front tires were tried, both of them good tires when used on other bikes, but they didn’t help the IT490, particularly in sand. When a 3.25-21 Metzeier was installed, the skating went away.
Going back to Square One with this collected knowledge, it would be easy to make the IT490 into the kind of bike experienced riders would like. We'd start with the silencer, replacing the stock part with an aftermarket model. If that didn’t provide enough power, then we'd trim the head pipe. As a last resort, the cylinder could be carved for a higher exhaust port. None of these changes required a change in carburetor jetting.
As modified, the Yamaha is a bullet. The engine is smooth running, and the new-found power makes the motorcycle easier to pitch around. It feels lighter because it can be controlled better with the throttle, now. Fire roads and sandwashes are special fun. The rider can pitch the IT into full-lock slides, controlling the bike with the motor. It stays stable and predictable. Even tight going is more fun now that the front end can be lofted with a little blip of the throttle. Add brakes that can stop it quickly from 90 mph and the modifications become worthwhile.
Throughout the riding and hop-up efforts, the IT has been a model of reliability. In 1200 mi. of hard riding only the lightbulbs have needed replacement. Even the chain is still in good shape.
Maybe the big IT isn't a perfect big bore dirt bike, and it needs more work than it should to make it run right. But when all that’s done and that monster motor is turned loose, it’s all worthwhile.
