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Endurance test Yamaha YZ 400 F

Happy end?

At the beginning there was a lot of skepticism: 12,000 revolutions, 50 HP power, and a fragile-looking lightweight construction – can that go well in the long run? The answer is the 100-hour endurance test of the Cross-Yamaha YZ 400 F.

Moto Cross is inexpensive motorsport. The motorcycles are affordable, the effort for maintenance and tuning is limited. But now the new generation of modern four-stroke engines is coming with complex technology. Even the entry price is above the two-stroke level, do you have to add high maintenance costs? MOTORRAD clarified this in a one-year long-term test of the Yamaha YZ 400 F, the purchase price of which is relatively moderate at 12,500 marks. The test program was by no means gentle, the YZ-F was usually used twice a week in races or for training purposes (see box on page 203) – and everything was always demanded of it.
Those who follow the instructions in the manual exactly have to pay for the immense driving pleasure with long evenings in the workshop: dismantle the engine every 1000 kilometers, adjust the valves every 500 kilometers. But don’t panic, such care instructions can be found prophylactically in the manuals of many competitor machines. Perhaps useful for a professional, but rather exaggerated and unrealistic for hobby drivers and amateurs. Therefore MOTORRAD limited itself to the necessary maintenance measures. The Yamaha was pampered with fresh oil every week, and a new oil filter was regularly added. The lubricant, exclusively semi-synthetic Castrol GPS, always made a clean impression, and longer change intervals would probably not have hurt.
A frequently discussed point is adjusting the five valves. Due to the design with bucket tappets and small valves, long setting intervals would be logical, similar to the Yamaha road machines, where 42,000 kilometers are common. This tends to be transferable: At the beginning, three of the five valves were readjusted once after about 13 operating hours, after which they were checked, but not adjusted until the end of the test. In the absence of special tools, MOTORRAD left the adjustment to the dealer, the price including a few minor service work was just under 300 marks.
The carburetor and ignition worked with absolutely no problems. Since the Yamaha really always ran after one or two kicks, the carburetor was never touched. When testing the exhaust systems it turned out that a little fine-tuning enables even better transitions in the lower load range. An adjustable idle air nozzle from Topham (27 Mark, Telephone 05474/9011) is helpful, but the entire rear must be unscrewed for adjustment. The Yamaha is not very sensitive to the spark plug, it contented itself with a total of only two plugs. The second one burned down a lot at the end of the test, but there weren’t any problems with kicking off.
The list of the spare parts required during the season is clear: One set of brake pads at the front and one at the rear was due, a gear lever broke in a crash, a set of cables had to be replaced due to incorrect maintenance ( attempts with lubricants), the guide of the front one Brake line broke. The series chain does not exactly shine with overwhelming quality, an Afam chain even broke at the start and caused the only failure. Overall, the Yamaha worn four chains, more two chain guides. A somewhat clammy O-ring chain that was used for training on wet tracks in the winter season proved to be resistant. Repeated replacement of scratched plastic parts always ensured a clean appearance, these are inexpensive from Acerbis (phone 02471/12690) or Ufo (phone 09383/99006).
The cross-scene also reports almost only positive things, the fantastic driving characteristics and the reliability of the lively four-stroke engine are convincing. In individual cases there are reports of jammed exhaust valves or bucket tappets, presumably because the deco get up was accidentally actuated while the engine was running, for example in the event of a fall. Occasionally we heard of defective connecting rod bearings, and there were also broken kick starters, probably a result of overly insensitive kicks.
D.he balance is therefore extremely positive. The 100-hour Yamaha gets fit again at little cost, so that it can make the driver happy for another year.

Wear and tear in detail – 100 hours and the consequences

So much in advance: It is remarkable that the Yamaha survived a year of Moto Cross absolutely without complaint – hats off. However, the traces of hard use towards the end of the test could neither be overlooked nor overheard. In autumn 1998, with around 80 hours of operation, the Yamaha drive was still fresh, no rattling, no noticeable running noises. In the course of the winter the oldie behaved increasingly rough. However, the performance was still top, with almost 50 hp the endurance test YZ had even increased 2 hp compared to the initial measurement. After dismantling, the inner workings were consistently in top condition, both optically and in terms of measurement technology. There is absolutely nothing wrong with the cylinder head. Valves, seats, camshafts, bearings – everything is almost like new. The clutch steel disks tarnished, but amazingly it is still the first set of disks. The cylinder was also almost unimpressed: no measurable wear, only minimal traces. The piston looks good but needs new rings. The source of the noise was probably the crankshaft in need of renovation, here a new connecting rod along with crank pin and piston pin is urgently needed. Two things come together with the seizure marks on the upper connecting rod bearing: The bearing is heavily loaded because of the extremely short connecting rod, and the bolt runs directly in the steel of the bored out connecting rod; a bronze bearing bushing was omitted. Perhaps a solid bushing would give the engine a breath of eternal life.

Individual tuning instead of mass-produced goods – the right accessories make the YZ 400 F even better

The YZ 400 F seems to be the motorcycle the accessory industry has been waiting for. Shortly after the presentation there were lots of nice, light, expensive and more or less useful parts to buy. The first priority in tuning is adapting to personal requirements, for example the chassis. The standard spring elements are certainly among the good ones but an individual setting is advisable. Specialists for this: Hubert Hofmann, phone 07073/2844, Magic Racing, phone 06894/570278. The long-term test YZ showed that specialists are not omniscient either. It was only after the third attempt that Hubert Hoffmann found a successful coordination of damping and progression of the fork with significantly improved response behavior. The standard rear shock had to give way to an Ohlins shock absorber. Not a huge improvement, but at least the basic set-up of the Ohlins damper is perfect. it works reliably, responds slightly better and, above all, provides constant damping, regardless of temperature and aging. Readjustment was hardly necessary during the entire season. Engine tuning was not an issue, because effort and benefit are hardly related at the amateur level. Therefore, the engine remained untouched during the entire test phase. If you have serious intentions in racing, you can contact Kurz (phone 07967/90080), Hallenberger (phone 06452/912820) or Meckfessel (phone 05402 /8934). The influence on the performance characteristics through accessory exhaust systems is problem-free and relatively inexpensive, with some of the Yamaha becomes even more aggressive and explosive. However, the engine also runs a bit harder in the partial load range, this is especially true for Pro Circuit and DSP. The other side of the coin: Especially with the lively silencers from White Brothers, Pro Circuit and DSP, the YZ-F becomes a no-brainer. MOTORRAD did not try the carbon fiber air filter box from DSP; it is also said to have a positive effect on performance.

Moto Cross is wear and tear in fast motion – 30 million revolutions, 50 thousand gear changes

From April 1998 to January 1999, the Yamaha was used exclusively by the author in regional races and for training purposes. The total mileage was almost 100 hours in around 60 missions. A tough burden when you take into account that a hobby driver can drive hardly more than an hour per training unit, while a weekly training session would only take 30 hours during the season. In addition to the training runs, the YZ-F was used in 13 races, mainly in southern Germany, the range ranged from the DMSB regional cup to the four-stroke championship of the DAMCV and MSR, club championships and the "wild" three -hour cross. The small four-stroke and the advanced age of the driver made it possible to start in different classes: open, 500, four-stroke, seniors and veterans. This also made it possible to start a couple of times in two classes at one event. The comparison with other displacement categories is interesting. The YZ-F competed against competitors from the 250 Honda to the 610 Husky. In all categories the Yamaha was good for podium places, the smooth, energy-saving driving makes the Yamaha always competitive.

Standard material for factory drivers – seven kilograms lighter thanks to carbon and titanium

After Yamaha shipped the hand-carved factory racers of the past two years to the museum after impressive results, but without a title, the official factory team is now starting with modified series machines. These are tuned with many special parts and modifications in a similar way as Doug Henry’s machine, which was victorious in the USA. The accessories reduce the weight to around 105 kilograms: the rear frame, footrests, screws are made of titanium, the air filter box and many attachments are made of carbon. The oil circuit has been converted to a wet sump. This also saves weight due to the smaller amount of oil (less than a liter), and there is no need for hoses, connections or one of the two oil pumps. As with the old factory machines, an Exup system is built into the DSP titanium exhaust. The displacement was increased to 420 cm³ by a 94 millimeter piston, and a balance shaft was dispensed with. Many parts of the factory machine can be bought by everyone, for example from Yamaha USA (information on the Internet at www.yamahausa.com).

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