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Presentation of the Yamaha YZM 400 F

The little bear

With a 400cc four-stroke, Yamaha is setting a new highlight in the off-road sky.

He’s actually a horse stealer: small, light, lively and undemanding. But as it is with such ingenious friends, when it comes to manners it’s a mess. And so only a few like the two-stroke with its constant smoking and drunkenness. Least of all the morally conscious Americans, who have banned the bon vivant from the wide steppes of California since the beginning of this year with unattainable exhaust gas limits – and thus deprived the manufacturers of their most important sales market for motocrossers and enduros with a crackling heart.
And so it happened, as it should have happened long ago: Suddenly, sporty four-stroke crossers are also being looked after in the land of the rising sun. Mind you, this field was by no means idle so far, but with the comparatively sluggish and partly antiquated 600 cc four-stroke cars like a Honda XR 600, Kawasaki KLR 650 or even more their weak-chested little sisters like Suzuki DR 350 or the Yamaha TT -Series it was poorly managed – until Yamaha now blows a general attack.
YZM 400 F is the name of the wonder weapon that the brand with the three tuning forks uses as the potential heir to the possibly departing two-stroke dynasty. Tea effort involved in the inauguration is impressive. With the two top World Cup crossers Peter Johansson from Sweden and the Italian Andrea Bartolini as figureheads, a ten-person crew is currently driving the development of the 500 Moto Cross World Cup. In the USA, the newcomer will be pepped up under the current Supercross championship leader Doug Henry in the upcoming outdoor cross championship.
Although the technical concept initially surprised. Instead of using the previously propagated basic advantage of four-stroke technology with as much displacement as possible – the gently starting, easily controllable power development – Yamaha chief technician Hisazumi Takasaki names handiness the top development goal. His way there: firstly, a low overall weight and, secondly, little displacement, in order to avoid the large rotating masses and abrupt engine reactions of large-volume four-stroke engines.
The result: an engine with such a low construction that it could be implanted in the only minimally modified frame of the current 250 series Yamaha two-stroke crosser with the gigantic maneuverability that is well known for this species. The key to this actually lies in the tiny displacement or, more precisely, in the extremely short-stroke design of the YZM motor. The Yamaha technicians, who have been working on this project for a year and a half, cannot get any concrete values ​​from them, but insiders estimate that with a total displacement of 397 cm³, the stroke must be well below 50 millimeters. In comparison: The Honda XR 400 works with 70 millimeters stroke, the 400 Husqvarna with their 60.8.
The whole thing is capped by a five-valve cylinder head, which has been typical of Yamaha since 1985, with three inlet valves to ensure more cylinder filling from the 40s Mikuni flat slide carburetor.
In the case of the six YZM motors produced so far, all of these sits enthroned on a CNC-milled aluminum housing with magnesium side covers. Four gears are easily enough for the current Moto Cross use, even if the declared 57 HP, which is also necessary under the competitive pressure in the 500 World Championship, should not be less than 13,000 rpm.

The Yamaha men also save weight with the lubrication system. As with Husqvarna, there is no oil pump and instead the overpressure in the closed crankcase is used to pump the oil up to the two camshafts via a valve with the help of the timing chain. What remains hidden, however, is the determination of the container on the cylinder head, which at first glance looks like an oil separator, into which the hose for the gearbox housing vent opens, but which is also connected to an exhaust manifold.

All in all, the YZM weighs a meager 102 kilograms – exactly as much as its quarter-liter two-stroke counterpart. For four-stroke crossers, which usually do not fall below 115 kilograms, this is an absolute top value.

The result of Andrea Bartolini at the top-class international Moto Cross season opening in Beaucaire in the south of France proves that the concept of the YZM seems to be working. The Italian placed the white lightning bolt in the sensational fourth place in the third run, directly behind quarter-liter world champion Stefan Everts and in front of Moto Cross superstar Sebastien Tortelli. The impression from the edge of the track: The four-stroke Yamaha cheers up to the top, but after tight bends it can still be driven with astonishing acceleration from very low speeds with a noticeably lazy shift.
D.It is obvious that the long-awaited, very sporty four-stroke enduro will emerge from the Moto Cross prototype. But off-road freaks shouldn’t hope for lively four-stroke technology before 1999. Just as premature is the question of injection, catalytic converter and especially the price. It should be very clear, however, that the handy and feather-light YZM experiment could initiate a trend reversal in off-road motorcycle construction. Because already in Beaucaire, ex-half-liter world champion Joel Smets used a 440 cc Husaberg for the first time.

Four-stroke motocross – everything has been there before

Two decades ago, Yamaha fought with a four-stroke in the World Cross Championship

Four-stroke motocross – a story with ups and downs in which Yamaha also played a role. The steam hammers dominated the off-road sport until the mid-1960s, but the livelier two-stroke engines gradually conquered the terrain. But the hearts of many former cross champions continued to beat in four-stroke. This also applies to the four-time world champion Torsten Hallman, who was a Yamaha importer in Sweden after his career, and his colleague Sten Lundin, who also received two world championship titles. When the then sensational four-stroke enduro XT 500 came to Europe in 1976, the two oldies saw new perspectives. They implanted the central organ of the XT in a chassis made from Husqvarna fragments. As a driver they were able to win old star Bengt Aberg, who contested the 1977 World Championship with the HL 500 built in Sweden. With these arguments, Yamaha could be won over to a modest support of the project. The crowning highlight: the overall victory at the Grand Prix Luxembourg, which until the world title of the Belgian Jacky Martens on Husqvarna in 1993 was to remain the last success of a four-stroke for a long time, as the HL mainly with spare parts from the shelf of the Yamaha importer Hallman had been built, the step to the "real" Yamaha motorcycle was not far. After Hallman had sold a few frame kits in 1977, he built 200 frames each in 1978 and 1979, each of which was sold through the Yamaha dealer network. In its day, the HL, which now looks like an ancient relic, was a thoroughly modern cross machine. The only mildly tuned XT engine acted incredibly smoothly compared to the hard-starting two-stroke engines. The 38-millimeter Mikuni carburettor combined with a sharper camshaft brought a little more power, while the lightweight CDI ignition of a Yamaha two-stroke crosser made the performance characteristics more spontaneous. With barely more than 40 HP – that corresponds to a liter output of 80 HP – the oldie looks rather old compared to the YZM 400 F (150 HP liter output). When central struts with modern lever systems came up, the HL quickly fell behind, and so it happened as it had to: The project, which was only half-heartedly supported by Yamaha, was discontinued after only two years and only experienced in the form of the YZM 400 F after 18 Years of its rebirth. GT

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