On both sides of the Atlantic the '98 YZ250 has proved to be one of the most
competitive quarter-litre motocrossers at every level of the sport. Renowned
for its agile handling and broad powerband, the 250 Yamaha's remarkable performance
has made it a favourite with riders of all abilities, from local club racers
through to the likes of Tallon Vohland and Jeremy McGrath. Now for the '99 season
Yamaha have produced an even more competitive YZ250 powered by an all-new long-stroke
engine housed in an uprated chassis running new front and rear suspension systems.
Engine
Next season sees the arrival of the most radical new 250cc engine to be fitted
to a YZ in recent years. Featuring a new cylinder, completely redesigned YPVS,
new transmission and higher specification carburettor, the '99 YZ250 looks set
to enhance Yamaha's reputation even further in this competitive category.
Long-stroke configuration
In the motocross world many of the leading 250cc powerplants have been using 'square' configuration engines with a bore & stroke of around 68mm x 68mm for a number of years. Now, for the '99 season, the latest Yamaha YZ250 engine runs with all-new bore & stroke dimensions of 66.4mm x 72mm, a 'long-stroke' layout which is designed to make it one of the most useable powerplants available in the 250 class.
Lightweight internals
Cylinder head outer diameter is reduced from 121mm to 111mm, while combustion chamber capacity is also down from 22.9cc to 22cc. A new-design 66.4mm diameter piston features a lighter piston pin, which, together with a new, slimmer con rod and smaller crank pin reduce overall reciprocating weight for superior pick-up at all engine speeds. Matching the new engine dimensions is a new crank which, at 60mm wide, is 2mm slimmer than the '98 model a feature which helps keep overall engine dimensions as narrow as possible. As well as running with all-new bore & stroke dimensions the latest YZ250 cylinder benefits from reshaped porting and revised port timing. Exhaust port width is reduced from 48mm to 43mm, while intake comes down 1mm in width to 44mm, and transfer ports height increases from 13.3mm to 15.5mm.
New 3-valve slide-type YPVS
Since we first introduced the Yamaha Power Valve System our engineers have constantly uprated the design in order to achieve more power over a wider rpm band. Now, for 1999, the YZ250 is to be fitted with a new generation 3-valve dual-action slide-type YPVS - a more efficient system which replaces the rotary design used on previous models. Unlike last year's single valve, this new 3-valve YPVS operates the main exhaust port independently of the two auxiliary ports to give stronger power from even lower rpm. At low engine speeds all three valves remain closed, then as the revs increase the main valve opens while the 2 auxiliary valves stay shut, and as rpm climbs higher the twin auxiliary valves open.
Matching the new cylinder dimensions, the 3-valve YPVS and the uprated porting is a new-design exhaust. Manufactured from 0.9mm thick steel - 10% thinner than on the '98 bike - this slimmer design enhances overall power characteristics and keeps overall weight down. Tailpipe runs tighter against the chassis and is over 10mm shorter for '99, and now features a factory bike style resin tail cap which helps prevent mud build up.
Powerjet carburettor
On the intake side the new long-stroke engine is now equipped with a high-efficiency PWM powerjet carburettor which, together with the revised engine dimensions, reshaped porting, lighter internals and 3-valve YPVS delivers stronger, harder hitting power over a wider rpm range. Operated by an rpm-sensitive solenoid valve, this powerjet system - together with a new 8-bit CDI - ensures instantaneous throttle response to give the '99 YZ250 explosive acceleration off the line and out of corners.
New transmission
Handling the stronger power output is an all-new YZ400F specification transmission which features larger diameter main and drive axles offering greater durability. This new transmission also has three shift fork guide bars as well as a coil spring and ball bearing on the stopper lever - a system which ensures smoother and more positive shifting through the uprated 5-speed gearbox. For '99 the clutch cover is now sealed by an O-ring rather than a gasket to make inspection and maintenance easier, and to ensure smooth operation in muddy conditions the clutch operating arm is now located inside the engine cases.
Lower centre of gravity
Engine position in the chassis has also been changed for the new season to achieve a lower centre of gravity for improved handling performance. The crank centre position on '99 YZ250 sits 13mm lower in the frame than on the previous model, and the forward tilt angle of the motor is reduced from 10.7 degrees to 2.5 degrees
Technical Changes: Highlights
All-new
long-stroke configuration
Please note that many improvements to the 1999 YZ250 and YZ125 apply to both machines. Those changes that are found on BOTH models are described here. Improvements that are exclusive to each model are listed separately.
Improved front and rear suspension
Some of the most significant changes to the 1999 YZ250 and YZ125 chassis are centred on the suspension systems. New design upside down front forks are equipped with a redesigned outer tube featuring a revised taper profile for increased rigidity. New low-friction seals and internals help eliminate stiction, and progressive springs give a softer feeling during the initial stroke, and also help prevent bottoming out. The Monocross rear end has been totally redesigned on both bikes, and features a new swinging arm, linkage and shock. At 487mm the latest shock is 15mm longer than last season's unit, and now utilises a progressively wound 3-rate spring. For improved heat fade resistance the gas capacity is increased from 107cc to 111.7cc, and valving is changed to give a wider range of settings. Like the new forks, the '99 rear suspension unit ensures a softer, more responsive initial action, while the new progressive spring is designed to absorb heavy landings from jumps without bottoming out.
Lighter swinging arm
Unsprung weight is reduced on both the YZ250 and YZ125 by the adoption of a new lightweight swinging arm. For improved bearing durability and reduced friction this new rear end is equipped with redesigned seals at the main pivot. To accommodate the new longer shock absorber the Monocross linkage has also been modified with a new lighter relay arm, although lever ratio remains the same as in '98.
Reduced unsprung weight
Further unsprung weight savings are made with the fitment of new front and rear rims equipped with aluminium spoke nipples sitting in larger 8.5mm diameter holes. At the rear end a lighter, smaller diameter hub with new resin dust covers enhances rear suspension action, while rear axle diameter is increased by 10% to 22mm and changes from a hexagonal head design to a two-plane type for quicker wheel removal.Footrest brackets on the latest YZ250/125 are also modified, and although the actual footrests remain unchanged they are mounted to slimmer, lighter and narrower brackets that ensure smoother progress through deep ruts.
YZ250 chassis only:
In addition to the chassis changes which are common to both the YZ250 and YZ125, the YZ250 also features the following detail modifications for 1999:
To facilitate routine maintenance the rear brake reservoir bracket and exhaust bracket are relocated to new positions on the right side seat pillar. Front, lower and upper engine mounts are reshaped to match the new engine design, and the head-steady brackets are manufactured from stronger alloy. Clutch operation is made lighter by the fitment of a thinner, low-friction cable which is re-routed next to the top tube and down past the cylinder into the new clutch arm which is mounted inside the cases, and CDI unit and ignition coil are also relocated.
Although externally identical to the '98 design, the '99 YZ250 fuel tank features reshaped internal panels to accommodate the cylinder head on the new, more upright long-stroke engine. Capacity remains unchanged at 8 litres.
New-design front forks with progressive springs and higher rigidity outer tubes
Specifications are subject to change without notice, in accordance with national regulations and legislations.
