Yamaha Yz250f
YAMAHA YZ250F
CYCLE WORLD TEST
Almost All the Major Parts are Deceivingly New
Last June (1978) we compared four 250cc motocrossers and after three days of switching machines. came to the conclusion the YZ Yamaha was the least liked. All of the testers voiced the same complaints: slow steering, short on power, pipey, and top heavy.
We're happy to report all of these flaws have been corrected on the 1979 YZ250F.
Except for the new rear-set side number plates, and reshaped fuel tank, the YZ's appearance hasn't changed much. But don't let that put you off. Most of the YZ's pieces have been redesigned or replaced with a better design.
The changes start with the swing arm. It's been lengthened, from IS.5 to 20 in., which transfers weight forward and helps keep the front end down (assuming that's what the rider wants) under power. It's aluminum, like last year, but the various tubes and braces have been moved about and reshaped for added stiffness. The welds are attractive and the arm pivots on needle bearings.
Yet another new monoshock design is used on the F series YZs. It is shorter, smaller in diameter, built from aluminum, and sports full length cooling lins on its main bodv. Damping may be adjusted to one of 20 settings bv simply inserting a flat blade screwdriver into a hole through the backbone frame tube and turning the star wheel a few clicks. Shock spring preload is also adjustable, but shock removal is required.
A full-floating rear brake backing plate is once again furnished. Yamaha was one of the first major manufacturers to utilize this feature, (it was standard on the 1968 Dl l) but through the years it got eliminated. Anyway, it is welcomed back. T he backing plate pivots tin a bushing, the long torque arm runs parallel to the swing arm and anchors to the frame at its front.
The brake pedal is another new item. It is made from an aluminum forcing but has
a steel claw riveted on the front to ensure good grip on boot soles. A nice height adjuster is furnished and a threaded steel rod with wing nut allows quick adjustment. The new pedal has a return spring connected to the back side and the brake rod is protected from boot damage bv a wire guard .welded to the rear frame downtube.
The double down tu be. chrome-moly steel frame looks unchanged, but is all new. Steering head angle has been pulled back and now measures 29.5°. The swing arm pivot points are farther forward and help close the distance between the drive sprocket and swing arm shaft centers to 4.0 in. The engine also sits 1.5 in. lower on the '79F. Otherwise it looks much the same as last year's: A large backbone tube with lightening holes, (the shock lives in it) double front downtubes. no rear fender loop, layers of gusseting around the steering head, poor triangulation under the seat/tank junction area, nice welded loops to hold control cables in place, and a thick black paint job. >
Although the reshaped plastic fuel tank looks smaller, capacity remains the same: 2.2 gal. It has a reasonably sized filler hole and a good plastic cap. The right side petcock’s design makes it impossible to turn off with the rider’s knee, a small but important feature.
The YZF has leading-axle air/oil forks with 38mm stanchion tubes. Travel has increased 0.5 in. to 10.6 in. Accordion type fork boots (yellow, natch) protect the oil seals from mud and dust and add color to the package. Strong, double bolt triple clamps hold the fork legs steady and the top one carries cast-in rear-set mounts for the handlebars. The fork legs extend through the top clamp by 2 in., making maintenance easier and allowing height adjustment.
Most newer machines have a front number plate with straps to keep the front brake cable from hooking the fork tube on compression. The YZ uses a different but equally effective approach; an old style oval plate is employed, and a separate plastic guard keeps the cable out of trou-
ble. As always, superb cable routing is practiced. The front brake cable glides through the some of the best guides available. the clutch cable is routed behind the number plate, next to the right downtube where a wire loop holds a plastic tie at the right location. It then curves around the cylinder to the engine lever. To guard against melting, its lower part has a spring over the outermost housing. All the electrical wiring shows this same care and excellent rubber plugs-ins are used.
Yamaha dirt bikes have some of the best seats around. They are sometimes a little stiff until broken in, but they're always thick and just the right width to provide comfort without interfering with the rider’s legs. The F seat has been thinned slightly and reshaped, but still affords plenty of comfort.
The F's engine has also undergone subtle changes. The radial head, small case powerplant was used in last year's E model, but the power it produced was disappointing. Low end power didn't exist and constant rowing of the shift lever was
necessary to keep the engine on the pipe. The F's engine mods start at the pipe; it is much fatter through its mid-section. Internally, compression ratio has been raised from 7.3:1 to 7.9:1. A two-ring piston is standard and it has its lower half knurled to retain oil and reduce skirt wear. The intake skirt is arched so the six-petal reed valve can start dumping fuel into the lower end as soon as the crankcase pressure signals the need. Basic porting layout is the same except the exhaust and transfer ports have been lowered slightly for increased torque. Port finish is much smoother this year and a 38mm Mikuni carburetor keeps them fed.
The increased torque and horsepower have made the YZ competitive in the engine department. By starting in second gear, we got quite a few hole shots at the track. Gear selection isn't nearly as critical and a fan at the clutch lever will put it back in the powerband if the rider misjudges a downshift. It is super responsive and nonfussy.
Gear ratios on the slick shifting, sixspeed transmission have been juggled around: first, second, fifth, and sixth are slightly higher, taking advantage of the broader powerband and making gear selection less critical.
The 1 has one of the best kick starters made: it folds completelx out of the wax and has an excellent ribbed non-slip boot surface. Primarx kick starting is furnished and eliminates searching for neutral to restart a stalled engine.
A large, multi-plate, wet clutch allows 2nd gear starts w ithout complaint. It has a smooth, progressive engagement and proved trouble free throughout our long test.
Pointless ( DI ignition affords a high voltage, non-fussv. maintenance free spark that extends spark plug life almost indefiriitelv.
The large, side-mounted airbox contains a single foam air cleaner with fine inner and coarse outer hivers. The reasoning here is simple; the coarse foam traps the larger particles, the tine laver gets the minute ones. But the biggest benefit is increased longevitv as it doesn't clog as quicklv as a single densitv foam.
Mov ing the su mg arm pivot closer to the countershaft sprocket has reduced the distance between shaft centers to 4.0 in. The > spring loaded chain tensioner on the E model has been replaced by a constant roller type. A plastic roller below' and slightly behind the swing arm bolt provides tension throughout the rear wheel’s travel. A similar roller is positioned above the pivot bolt to protect the air intake hose. A beautiful swing arm-mounted chain guide is furnished. It has a heavy duty nylon chain block and an aluminum tube curves down to protect it from side blows. An equally thick nylon piece wraps the front part of the arm and a circular one further isolates the aluminum from damage.
We were surprised w hen we started measuring the F. Seat height is 1.4 in. lower than the E’s, footpeg height is 14.6 in. (almost an inch lower than before), ground clearance is down 0.9 in. at 11.3 in., and the swing arm is 1.5 in. longer, but the wheelbase hasn't changed. With a longer swing arm the wheelbase should have grown. But. the swing arm pivot on the frame was moved forward to improve the relationship between the countershaft and swing arm pivot. So. the longer arm had to be used to maintain the same w'heelbase as the '78 model. With 0.5 in. more travel at both wheels, a reduction in seat height and ground clearance wasn’t expected. The lower seat height, smaller rear fender, and reshaped tank give the F a smaller look also.
We tightened all the spokes before the first outing but still had them come loose three times before they finally bedded. Our last few' dirt bikes have had the same problem. Spokes and wheels that were strong enough with eight or nine inches of travel aren’t beefy enough when used on 10.5 in.-plus suspended racers. The extra travel means the rider can go faster through the rough areas, and higher over the jumps, putting tremendous loads on the wheels, spokes and rims, without realizing it. All the new' ultra-long travel motocrossers are experiencing wheel problems when an agressive rider pushes them on a rough, multi-jump, stadium-type motocross course. Big jumps especially, are hard on wheels. The impact isn’t noticed much by the rider, as the long travel soaks it up. but the rims respond by turning egg shaped. Virtually all of the new breed of motocrossers (regardless of brand) require faithful wheel maintenance.
IRC tires are furnished on the YZ. They are a good all around tire, and we left the rear one on until it was worn out. The front, although usable, was replaced with an experimental 3.00-21 Goodyear Eagle II. It gave superior front end control on Corona’s granite soil, and several test riders preferred it to a Metzeler. After the rear tire wore out we also replaced it with an Eagle II which gave excellent performance.
The first corner will surprise anyone who has ridden earlier monoshocks; the new' F corners almost as well as the red machines from Germany. The top-heavy, sluggish feel is gone and a precise, neutral, go-where-pointed feel has taken command. The tucked-in fork rake, lighter shock body, longer swing arm and lower engine placement are responsible for most of these superb handling traits.
Although 10.5 in. of wheel travel is present at both wheels, the suspension can’t be considered plush. It works well, but is stiff compared to a Honda CR or Suzuki N. Both the Honda and Suzuki have substantially more wheel travel and can be set up softer. On a rough course or toward the end of the last moto on a smooth course, the YZ will have trouble maintaining the pace set by the longer suspended CR and N (with equal riders).
YAMAHA YZ250F
$1829
The F is very narrow' in the section below the seat, and makes moving around on the bike easy. This narrowness is made possible by the monoshock frame, because the shock is under the tank, not bolted on each side adding width.
The YZ is a fun bike to ride and a competitive motocross racer. We ran it four different nights at Corona Raceway. We got a first and a second overall in the Intermediate class and our high school rider easily took two first overalls in the Senior class.
Can the YZ give away 1.5 in. of travel and be truly competitive? In the novice and intermediate classes, on smoother courses, yes. In fact, this rider level may benefit more from the reduced seat height and excellent cornering, than from maximum travel and the problems it creates. A Pro will have to add travel or he won’t stand a chance.
Like all ’79s the YZF is going to cost substantially more money, but the buyer in this case is going to receive substantially more.
