Kawasaki Kz750/4

KAWASAKI KZ750/4

CYCLE WORLD TEST

Bigger is better, as a design concept, has a few problems when it comes to motorcycles. Fortunately the major motorcycle manufacturers have begun to figure this out and there are a whole passel of new motorcycles available that offer better performance through less weight. Enter the Kawasaki KZ750, the other part of Kawasaki’s one-two punch against fat motorcycles. First there was the featherweight, superspeedy KZ550 and now there’s the KZ750, like the KZ550, the lightest, fastest machine in its class.

For the record, the KZ750 weighs 491 lb. and has a quarter mile time of 12.26 sec. at 107.78 mph, making it the quickest 750 this magazine has ever tested and the lightest 750 multi available.

Let’s put that weight in better perspective. Kawaski makes a 750 Twin, a simple enough motorcycle with only two cylinders producing moderate power and that motorcycle weighs 506 lb. The Honda CB750F weighs 540 lb. and the Suzuki GS750 weighs 550 lb. Even Kawasaki’s KZ650 Custom, the motorcycle the KZ750 is based on, weighs 20 lb. more than the KZ750. Obviously there’s been some serious lightening going on with the KZ750 because 20 lb. doesn’t just come from the bigger holes for the pistons and lots of Niptonite.

Being based on the KZ650 (A 750 punch in a 500 package, we said in the first test), the KZ750 has a dohc inline four cylinder engine coupled with a five-speed transmission. The whole package is as straightforward as a Japanese multi has ever been. No four valves per cylinder or mysterious flow patterns for the KZ, just good solid engineering.

Once Again,There’s a New Performance King in the 750 Class.

Biggest difference between the KZ650 and the KZ750 is a 4mm bore increase, raising displacement from 652cc to 738cc with the 66 x 54mm bore and stroke. Of course the standard KZ650 couldn’t be bored out 4mm and run with no other changes, though the lack of changes to the bottom end is surprising. Early 650s ran into connecting rod trouble when bored out that far and Kawasaki responded by increasing connecting rod bolt size in later 650s from 7mm to 8mm. So the 750 uses the same connecting rod and con rod bearings as the current 650. Even the crankshaft has the same dimensions, though the main bearings have been revised to fit around the different cam chain sprocket on the 750. Journal diameter and overall size is the same, though.

The cam chain sprocket was changed because the KZ750 uses a silent-type HyVo cam chain instead of the roller chain used on the KZ650. That, of course, means that the cam chain tensioners are different, there being no roller sprocket between the cams such as the 650 uses. The 750 uses slipper-type cam chain tensioners rather than the roller-type tensioners of the smaller model and the automatic tensioner naturally changes.

Besides the cam chain, the cams and valves are significantly different. In the> case of the KZ750 these figures are especially important because they explain much of the KZ’s surprising performance. After all, a 4mm bore increase doesn’t account for an increase in horsepower from 62 bhp to 74 bhp and the higher engine speed at which the greater power is developed. That power increase isn’t just measured at the brochure, it’s measured at the dragstrip. The last 650 tested had a quarter mile time of 13.40 sec. at 96.87 mph and now the 750 trims more than a second off that time and adds more than 10 mph to the trap speed. There really is 12 more horsepower in this engine.

Kawasaki’s 650 had, besides lots of power, good power at all engine speeds. That was accomplished by using mild cam timing. On the 750 the cam timing is more like that of the other 750s. The 650 has 254° of intake cam duration and 260° of exhaust cam duration with 42° of overlap. The 750 has a more normal 276° duration on both cams and has 60° of overlap, a huge increase. Valve lift has increased from 7.5mm to 8mm on all valves. As a result, the 750 has a higher redline. 9500 rpm compared to the 650’s 9000 redline, peak power is produced at 9000 rpm, 1000 rpm higher than the 650.

You’ve seen the ad. The one where you build a giant killer with the big bore kit and the performance cam and the exhaust. Well, that’s what Kawasaki did. Put in a big bore kit and a high lift, long duration cam and then made everything work.

That’s not all. There are larger valves in the 750, 34mm intake and 30mm exhaust compared to 33mm intake and 28mm exhaust valves in the 650. Valve springs are stronger on the 750, though valve stem size is the same and the spring retainer/shim adjusting system is the same on both bikes.

Ordinarily the kind of changes that turn the KZ650 engine into a KZ750 engine would take away power at low engine speeds and make the motorcycle harder to ride, but the 650 had a surplus of power at lower speeds and the 750 makes use of carburetion changes to improve low speed performance. The slide throttle Mikunis on the 650 Kawasaki, when opened fully at low’ speeds, would bog the engine. With the 750’s larger 34mm CV Keihin carbs, total flow' increases so there’s more power and because the CV carbs maintain air velocity the engine responds better to sudden throttle opening at low speeds.

For 1980 all the 750 multis use CV carbs and the Flonda and Suzuki use four valves per cylinder. The Kawasaki 750 doesn’t put out more power than the Suzuki or Honda, but it puts out very close to the same powder and with the much lighter weight it not only has better peak power but better power at lower engine speeds as well.

Transferring all that power isn’t a problem for the KZ750. The clutch is the same size as that of the KZ650, there are the same number of plates and they’re made of the same material. What’s different is the clutch springs. They’re stiffer on the 750 and the clutch didn’t give a moment’s trouble despite extreme use at the dragstrip and hard use on the racetrack. Light weight helps the clutch do its job, but the Kawasaki’s clutch is still superior to that of the Honda and Suzuki 750s when it comes to competitive use.

Transmission pieces, again, are the same for the 650 and 750 except for a couple of needle bearings being different and top gear having fewer teeth. The 650 uses a top gear ratio of 24 teeth on the driving shaft and 27 teeth on the countershaft w hile the 750 uses 21 and 24 teeth. The ratio is almost identical, 0.89:1 for the 650 and 0.88:1 for the 750 and the gears are the same width, so the 750 has thicker teeth for greater strength. Gear material and heat treating are the same on both models.

Overall gear ratios on the 750 are naturally higher than those of the 650 and the difference comes from the final drive ratios. The 750 uses a larger 630 endless chain and the overall top gear ratio has gone from 5.95:1 to 5.70:1 on the 750. The difference makes for 200 rpm less engine speed at 60 mph for the 750. At 4486 rpm at 60 mph the Kawasaki 750 is still spinning faster than the Suzuki 750 but slightly slower than the Honda 750.

What gives the KZ750 its character, and w hat gives the 750 rider the impression of a small motorcycle is the frame. It’s not just like the KZ650 frame, but it’s close enough that most people would have a hard time spotting the differences. Various brackets and attachments on the 750 are positioned differently, but the basic measurements, the swing arm length and shock mounting angle and frame tube diameters and thicknesses are all exactly the same. Along with the bracket changes the generous steering head gusseting looks a little different but is actually as close to identical as possible.

Being built on a 650 frame would ordinarily be enough to make a 750 feel tiny, but remember that the KZ650 isn’t an ordinary 650. It’s smaller than most. The KZ750, for example, ends up with a 55.9 in. wheelbase. Now, wheelbase has a huge effect not just on the overall weight of the motorcycle, but on its handling and stability. Suzuki uses a 60.5 in. wheelbase on its GS750 and Honda uses a 60 in. wheelbase. And they both feel totally different from the KZ750. For comparison, the Suzuki GS425 has a 55 in. wheelbase and Honda’s CX500 has a 57.3 in. wheelbase. A couple of inches of w heelbase difference can have an extraordinary effect on things like steering response and the KZ750. we’ll see, exemplifies this.

Suspension components on the KZ750 are one area in which the larger bike differs markedly from the smaller Kawasaki. While the 650 uses the most conventional suspension with only spring load adjustable, the KZ750 has air caps on the longer forks so front spring rates are variable and the rear shocks have adjustable damping and adjustable preload.

Compared to the 650, the 750’s forks use much softer coil springs. The rate, in fact, is about half as high on the 750 as it is on the 650. But the air pressure in the 750’s forks adds to the coil springs and effective spring rate becomes much closer to that of the 650, depending on the air pressure. Rear spring rate on the KZ750 is 1.5 times as stiff as the rate on the standard 650 and to control the stiffer springs the damping is much more firm. Compression damping of the 750 shocks is fixed at a figure one and a half times as stiff as that on the 650 while rebound damping is adjustable with four settings. The lightest rebound damping on the 750’s shocks is about the same as the damping of the 650’s shocks, while the stiffest damping rate is 1.7 times as stiff. Intermediate settings are equally divided from lowest to highest on the 750, the figures being 60, 72, 84, and 96 kg/0.3m/ sec. from the lightest to the stiffest setting.

The semi-chopper KZ750 LTD, by the way, has softer springs front and rear and softer damping at both ends, plus retaining the four-way adjustable damping on the rear shocks.

Suspension travel on the KZ750 is 6.29 in. front and 3.74 in. rear, about on par with other 750s. Air pressure in the forks can go as high as 35 psi, though normal pressure is between 8 psi and 13 psi. Not having a connecting tube between the air caps, setting and balancing air pressure in the Kawasaki forks is difficult.

What setting works best and, for that matter, how does the motorcycle work? The answers to those questions could be found at the racetrack.

The performance capabilities of the best 750s are beyond normal street riding. Only at a racetrack could we get the KZ750 close enough to its limits to discover those limits. So the KZ750 went to Willow Springs, accompanied by the best handling 750 we knew of, the CB750F Honda, and a KZ650 so we could make valid comparisons.

First off, the CB750F we have available on a long-term test is faster than the last CB750F tested. Its quarter mile time is just slightly behind the Kawasaki at 12.35 sec. and it’s pulling 50 lb. more, so it’s putting out more actual horsepower. Tires on the two bikes were as close as they could be, both original equipment Dunlops, 3.25-19 in front (Fl 1 on the CB, F8 on the KZ) and 4.00-18 K127 in back.

Before direct comparisons could be made, the suspensions had to be dialed in and this brought forth some surprises. Stiffer is not necessarily better. With 35 psi in the forks and maximum damping and spring preload the Kawasaki would dance around on bumpy sweepers, the tires squirming too much for good control. Dropping preload to the second softest position and setting the damping in the second or third position gave the best results, combined with air pressure of 10 psi in the forks. Suspension control was better in bumpy sweepers and the rear end was more controlled under hard precorner braking.

How did it stack up against the Honda?

Very well indeed. Cornering clearance was greater on the Kawasaki 750 than on the Honda or KZ650. With two riders of the same weight going around the track in tandem, the Honda rider would be grounding pegs on both sides and the centerstand on the lefthand side on every corner while the Kawasaki would go around at the same pace without scraping anything. It didn’t matter which rider was on the Kawasaki, it had more cornering clearance than the Honda. Under maximum cornering only the footpegs scraped on the Kawasaki, everything else being well tucked up. Only once did the centerstand scrape and that was when the bike was sideways in a tight turn when the rear tire let go.

Cornering clearance alone doesn’t explain the Kawasaki’s racetrack behavior. The short wheelbase and' steep 27° steering head angle, along with the light weight, make the Kawasaki an extremely quick steering motorcycle. Getting off the Honda and onto the Kawasaki, most riders would turn too sharply going into corners,> the Kawasaki responding faster to their inputs than the riders anticipated. The Honda is long and graceful, requiring lots of set-up going into corners for maximum speed. The Kawasaki was quick and flighty, and it could be tossed into corners or from side to side with little effort. The difference between steering the Honda and Kawasaki was much like the difference between steering a lOOOcc bike and a 500cc bike. The quick steering, combined with the light weight and cornering clearance make the Kawasaki significantly faster than the Honda on tight corners and in areas where the motorcycle needs to respond quickly to the rider’s input such as a succession of direction changes.

But there’s more. Going around fast sweepers the Honda would move around, rear tire squirming from side to side, at speeds that didn’t bother the rock-steady Kawasaki. Despite its short wheelbase and ultra-quick steering, the Kawasaki was more stable than the Honda, particularly at high speeds when the suspension was highly loaded.

On both bikes the tires limited cornering ability, and the hotter the tires got the worse they got. Good tires will improve the cornering ability of both these machines.

Engine performance gives the KZ750 a bigger edge on the racetrack. Not that the Kawasaki has more power than the Honda ... It doesn’t. But it has power spread over a broader rpm range and thaf enables the Kawasaki to accelerate harder, sooner after leaving a corner and getting a better drive off corners, resulting in faster lap times.

Only in braking performance does the Kawasaki not have an advantage. Its braking power is good, but it requires more effort to slow the Kawasaki than the Honda and the difference in effort can make a difference in a long race.

Overall, the Kawasaki is faster around a racetrack because it has greater cornering clearance, more usable power and quicker steering. The difference isn’t night and day. It’s not enough difference, in fact, that a faster rider couldn’t win a race on the Honda. The bikes are close to having the same performance, but in several critical areas the Kawasaki has just a little more performance and that makes it the performance king of the 750 class.

Consider some other measurements of performance. In high gear roll-on tests the Kawasaki beats the Honda or Suzuki 750s. In a 40 to 60 mph high gear roll-on the Kawasaki is a tenth of a second faster than the next-fastest Suzuki and it beats the Suzuki by eight-tenths of a second in the 60 to 80 mph roll-on.

The roll-on performance explains why the Kawasaki isn’t just a racetrack terror. It’s not peaky and it’s not hard to ride. It doesn’t explode in a burst of power, it’s just fast.

On the street the Kawasaki is equally capable. The broad powerband makes downshifting more an option than a requirement. There’s enough power at even the lowest engine speeds that the Kawasaki rider is always ready for challenges from the next lane.

How the Kawasaki runs is as important as how fast it runs and the KZ750 is especially impressive for the way in which the engine works. Instantly it fires when it’s cold and the choke is full on. Only the engine doesn’t rev up to 4000 rpm like most of the other new motorcycles. The KZ750 idles fine with full choke at 1500 rpm. It can be ridden away immediately if the choke is left on and there are no driveability troubles. After a minute the 750’s choke can be turned off and the engine will idle smoothly at 700 rpm, not missing a beat. Of course most of the Kawasaki’s excellent cold running performance comes from Kawasaki’s air suction emission system that enables the bike to have proper jetting, the air fed into the exhaust port taking care of emissions instead of using excessively lean carburetion.

Like other motorcycles equipped with CV carbs, the KZ750 has a sudden throttle response, the engine turning on or off instantly with the throttle. Fortunately the Kawasaki can still be ridden smoothly with careful throttle control.

Being a built-up 650, the KZ750 vibrates some. Inline Fours always have torsional vibrations and unless the engine is rubbei mounted a la XS1100 or CB900, there’s vibration. Somehow the Honda CB750 damps out the vibration better than the Kawasaki, though the KZ750 is worlds better than the KZ1000 when it comes to vibration control.

Noise control is also better on the KZ750 than it is on the KZ1000, mostly due to the Hy-Vo primary chain being quieter than the gear primary of the larger machine. The exhaust note of the 750 is different, though. It’s not like the KZ650 or the larger Kawasakis. It’s more of a howl, or a scream, with a tiny bit of a roar. It's not loud or offensive, just noticeable.

Working with a smaller machine, it was harder for Kawasaki to make the KZ750 comfortable, though it obviously isn’t impossible. For normal riding the 750 has a softer, more compliant suspension than the 650 or just about any previous Kawasaki for that matter. The double DU bushings in the KYB forks reduce seal drag and the spring rate is soft while the highly progressive air springing keeps the front end from bottoming, eliminating any sudden jolts through the forks. Rear Kayaba suspension works just as well, the preload and damping controls effectively tuning the KZ750 for heavy or light loads. When the KZ750 is loaded with two people the suspension adjustments can be understood. The stiffest preload and damping settings aren’t on the Kawasaki for the best handling, they’re on the bike so it can handle heavier loads without wallowing or becoming overloaded. Even twoup the Kawasaki doesn't lose its handling aplomb. It’s still steady, controlled and very fun.

Still, the KZ750 rider never forgets it’s a small 750. Not even when he brings it home and his wife asks if that’s the new' KZ440. Yes, it happens. And sitting on the bike the bars are closer than you’d expect and the ground is closer, too, because the Kawasaki has a lower-than-average 31.3 in. seat height.

You notice the KZ750 is smaller than other 750s when you’re riding between lanes of stopped cars and other 750 riders won’t go through the narrow spots you ride the KZ750 through. And always there’s that quick, instantaneous steering, the kind of steering that has the front wheel turned before the thought has gotten out of your mind.

The KZ750 doesn’t suffer from light weight, it revels in it. The lightweight wheels and plastic rear fender and slimmer, lighter fork sliders and aluminum rear footpeg brackets don’t keep the Kawasaki from doing anything. The lightweight headlight mounting bracket even makes the bike look light and clean. The plastic bodied rear view mirrors not only save weight, they don't vibrate any more than the heavy, weighted mirrors on other 750s. The tubular brake caliper stay on the rear disc is lighter than the solid cast arm used on the 650, but works just as well. The single throttle cable used with the CV carbs is simple and effective. Having the fork lock in the ignition is certainly more convenient, though that is one convenience that saves little weight. And there’s even a voltmeter, probably the lightest auxilliary instrument available, fitted between the new speedometer and tachometer.

There are sacrifices, certainly, but not problems. If the tiny 12 AH battery goes dead (and it eventually will) there’s no kick starter on the KZ750 because that’s “unnecessary” weight. And the seat, while not being particularly offensive, doesn't help comfort either because it's thin and light and isn’t as comfortable as heavier, higher seats would be.

What the KZ750 is, is the most sporting motorcycle in the 750 class. It’s not the cushy riding freeway machine the GS750 is or the sleek and smooth future machine the CB750F is. But don’t think the KZ750 has sacrificed versatility for its performance. It hasn’t. The only thing the KZ750 has sacrificed is weight and in so doing it may have gained more than any number of valves or adjustments can make up.

KAWASAKI KZ750/4

$2749