Yamaha XZ550/reviews
Yamaha XZ550S Road Test[edit | edit source]
November 1989
The XZ engine is so full of technical innovation, narrower than the larger, air-cooled versions. A narrower angle, while keeping the overall engine dimensions tight, is prone to producing plenty of primary vibrations. But. clever people that they arc. Yamaha's engine designers have compensated for such bad manners by placing the balance shaft, driven by two spur gears with small shock absorbers, in front of the cases.
If, having digested this serving of the XZ engine's techno pedigree, you're left feeling a touch bewildered, what it means in rider terms is that the XZ550S will pull up the 10,000 rpm blood in each of its five gears, with virtually zero vibration. The finest testimony to the bike's smooth ride arc the well-designed mirrors which not only give excellent rear viewing, but rarely blur at all. whether during hard acceleration or cruising. Indeed, open road motoring for the 553cc engine is effortless, slipping easily up to an indicated 100 mph. Mid-range muscle exertion is taken care of by the famed Y1CS, Yamaha's system of catching the gases in a chamber connected to the inlet tract then, as the valve opens, sending them swirling in the cylinder, resulting in improved mixing combustion and instant
With such an impressive array of attributes, you could be forgiven for wondering if there's a bad apple in the barrel. Unfortunately there is. The culprits are a brace of Mikuni downdraft carburetors. These 36mm items accept fuel via a trio of jets and an accelerator pump, then pass the juice through a highly and somewhat temperamentally-controlled set of bleeds adjacent to a throttle butterfly designed to squirt precise amounts of fuel into the system at pan throttle openings. The basic XZ550 uses a flap in the air box to control air flow to the carbs. By replacing the flap with a diaphragm on the XZ550S. Yamaha has attempted to ration the air to the hungry twin-choke carbs at a steadier rate, to give snappier response when the throttle is open.
Judging by the results, Yamaha would have been better advised to redesign the entire system, 'cos no matter what gear you're in (pulling a passing manoeuvre on the motorway or lane-swapping in town), the response to cracking open the throttle is about as snappy as a month-old packet of crisps. After the initial hesitation, progress is consistently rapid without being in the neck-breaking urgent league, but, as with the original XZ550, there's a flat spot between 4000 and 6000rpm. For a middleweight with over 64 horses on tap, you might expect a little more frenzied activity at medium gallop speeds.
In-keeping with this laid-back power delivery, the gearbox and shaft drive transmission don't react kindly to too much hurry up either. Under hard acceleration the final drive clunks in protest then settles down to a steady whine. Jumping from first to second on the five speed box is as smooth as making love to a roll of sandpaper, yet, by complete contrast, the three higher ratios are decidedly slick in comparison, whether changing up or down. Clutch action is light and responsive, coping admirably with any amount of stick you care to hand out.