The Motorbike Archives

This engine configuration enjoyed universal acceptance, once upon a time. And with good reason, for the vertical twin offered potential simplicity, reasonably light weight in an engine package of just moderate overall size. But in the late '60s, things changed. The motorcycle-buying populace, which for the first time had a large first-time-biker contingent, demanded the flash and pizazz of many little cylinders all in a row. They wanted bikes that go hummm, not rumpedy-rump. And they wanted bikes that would not leak oil. And so, due primarily to the efforts of oriental motorcycle manufacturers, the public got what it wanted. And what about the Brits and their venerable Triumphs, BSAs, Velocettes and so on? Well, you might say they got caught with their spanners down, that and frequent fits of production paralysis brought about by abusive labor union practices. Poor guys, they didn't have a chance.

Ensuing years have seen several Japanese attempts at making viable large-displacement street-touring twins but, with the exception of the Yamaha XS650, response has been less than frenzied, to say the least. It seems that when a multi and a twin share the same showroom, the buyer most often chooses the multi. Presumably, the twin comes off as utilitarian and dull in comparison to the multi.

So much, however, for faulty perceptions. When the Kawasaki 750 twin made its debut a couple of years back, it was greeted with a sort of cautious fascination. Motorcyclists scrutinized the thing in much the same manner as a cat seeing his first mackerel; he is irresistibly drawn to it by something deep inside him, yet his unfamiliarity with this new phenomenon makes him approach it gingerly indeed.

Market response to the KZ750 has been, well, moderate, which is a disappointment. For it is a machine whose construction, price and performance make for an excellent package for bikers preferring dependable, large-displacement, heavyweight mounts. Unluckily, though, it must squat in the shadows of its multi-cylindered brethren.

In terms of power production, there is at least one basic difference between the twin and the multi. The multi implements three, four or even six cylinders as power producing units.

Through using more such units in an engine of, say, 750cc, the smaller reciprocating pieces can be made to twirl, bump, and stroke relatively easier than the two large cylinders of a twin having the same displacement. For example, the inertial problems attendant in controlling a 13-ounce piston at 8000 rpm are much greater than those stemming from nine-ounce piston at the same crank speed. Moreover, each gram (about 1/28 ounce) that can be removed from a valve can affect high-speed valve control dramatically. In turn, comparatively light valves allow use of cam profiles yielding closer-to-optimum performance. What all this leads up to is the fact that the revving multi has inherent high-performance advantages over the twin.

But don't count the twin dead, for it has advantages of its own, including less expensive upkeep, relative simplicity and a potential for excellent performance at moderate crankshaft speeds. And it is within this context that the KZ750 Kawasaki excels. Furthermore — and ironically — the KZ750 powerplant is redlined at 7800 rpm, which is quite high for a showroom stocker, and testimony that the factors limiting the engine speeds of yesteryear's twins do not necessarily apply today.

The powerplant is predicated upon a 360-degree crankshaft supported by four main bearings. This crank layout allows the pistons to ascend and descend in their cylinders simultaneously, firing alternately.

One objection voiced against the vertical twin configuration is that of vibration. In the past, engines of this type have been known to wiggle and shake somewhat. But despite the inherent nature of this glitch, there are ways to get around it, if not eliminate it entirely: The KZ, for example, uses a pair of counterweighted shafts, driven by chains, to dampen undesirable harmonics. One shaft is located in front of the engine, the other behind it. Some purists might object to the use of mechanisms like these to counter vibration, citing unnecessary complexity. On the other hand, however, the counterweighted engine's bits and pieces are not subjected to so much potentially harmful buzzing and shaking, so, theoretically, component life should be lengthened despite added complexity. The KZ750 is not totally free of vibration. The seat transmits a little high-frequency tingling and the mirrors' images fuzz over above 4000 rpm in top gear. There is little perceptible vibration from either handlebar or footpegs. Touring riders anticipating long spells in the saddle can expect minimal vibration from this mount.

Power output for the DOHC engine is claimed to be 55 bhp at 7000 rpm. How accurate this is we can't say for sure. But judging from the motorcycle's quarter-mile performance one would suspect that in reality the motorcycle delivers 40 or so genuine horsepower to the rear wheel. Mind, this is not to put down the KZ's performance or engine design. But it is important to put such things in the proper perspective. Out test bike tipped the scales at a rather heavy 508 pounds, which makes it one of the heaviest 750 twins around. No doubt, were 50 or so pounds shorn from the machine, which it could well afford, all facets of its performance would dramatically benefit.

The engine's power characteristics distinguish the KZ750 from other motorcycles, while still reminding the rider that it is immutably a twin, and damn proud of it. In sluggish city traffic the motorcycle never lost its willing, faithful nature to over-heating, loading up or erratic idling. Low speed punch is excellent, reminiscent of a crisply tuned TR-6 Triumph; one of the finest twins made. And even though there is modest fly-wheel effect to cope with the bike's highish low gear ratio, the bike still can motor away from a stoplight without undue clutch slipping.

Actually, the only true gripe we have concerning the KZ750's performance is traceable to those cursed constant-velocity carburetors that it has. There are two of them: 38-mm Mikunis. These gadgets defy the most concerted efforts to achieve running smoothness at small throttle openings. Say you're bustling around your favorite sweeper. You're trying to maintain an even-throttle mode, wherein you're neither accelerating or decelerating. The motorcycle is well heeled over, and the last thing you need on this Earth is a sudden deviation in power to the rear wheel. But this is just what the CV delivers. Throttle response assumes an on-off nature devoid of smooth transition between power and no-power. One can only suspect that these devices are here on this planet only as testimony to mankind's mortal fallibility. Were these to be replaced with, say, a pair of Dellortos with accelerator pumps, there is little doubt that all-around throttle response would improve.

On the plus side of the ledger, we suspect that these same CV carburetors have a hand in delivering such good gasoline mileage. Our test machine averaged 46 miles per gallon; a figure including much full-throttle acceleration through the gears, plus several top speed runs. It is our suspicion that under ideal conditions — 50-55 mph level ground cruising —the motorcycle would deliver 52, maybe 55 mpg.

Factory literature indicates that our KZ750 powerplant produces 43.4 ft/lb torque at 3000 rpm. Such a comparatively low (for motorcycles) torque peak speed suggests that this machine's torque curve is rather broad, which is just fine by us. When riding the Kawasaki you get the feeling that the powerplant is never really laboring no matter what engine revs the tachometer might indicate. Of course, the CV carburetors are responsible for some of this behavior, but a thoughtful blend of camshaft profile design and not over-large ports contribute a lot as well.

Our test bike displayed a surprising top-end punch. When the tach needle creeps up to 5000 rpm, the rider feels a clearly defined surge of power that lasts up to just this side of 7500 rpm. At freeway speeds the motorcycle might just as well be a spirited sportybike. Indeed, it is at this point the KZ's performance exemplifies the best characteristics of the large-displacement vertical twin. ■