Suzuki GT 750

An American Perspective

December 1971

SUZUKI'S T500 "TITAN", largest of the twin-cylinder two-strokes in general production, aroused the interest of the motorcycling press several years ago. Critics of the design argued that because of cooling problems, it wasn't feasible to put two large cylinders so close together. Spacing the cylinders farther apart would add to the width of the engine, thus spoiling the lines of the machine, besides adding to the bulk of the engine transmission package. But with a sound basic design and Japanese technology, the T500 soon became a machine to be reckoned with in all categories. As a touring bike it is comfortable, reliable, economical, and it comes within a hairsbreadth of being a true Superbike in terms of performance. The last T500 we tested turned in a standing quarter-mile of 14.31 sec. with a terminal speed of 91.06 mph.

Once again Suzuki has introduced a paradox the GT750. Big, heavy, comfortable, economical and extremely smooth, the GT750 is capable of high 13-sec. standing start quarter-miles, effortless high speed cruising, hairline steering and tremendous braking. It's a Superbike in every sense of the word. The most interesting feature of the GT750 is the engine, and more particularly the method used to cool it. Water cooling for two-strokes is not new, the British Scott, which went out of general production just before WWII, was the last large-capacity machine to use it. More recently, the horizontally opposed Velocette "LE" 200-cc Twin employed water cooling, but it too is defunct. Several advantages accrue to water cooling a two-stroke, where heat is an inherent problem because a two-stroke fires twice as often as a four-stroke. Water cooling more efficiently disperses this heat, which can cause piston and cylinder distortion, with a consequent reduction of obtainable power. If clearances between moving parts must be made large enough to avoid excessive friction and possible seizures, this also leads to loss of power from blow-by past the piston rings, and contributes to the mechanical noise of the engine. A properly designed water cooling system can reduce these problems.

Among the negative aspects of water cooling is the need for a radiator, thermostat, water pump (which can sometimes be dispensed with if a thermo-syphon system is used), and a cooling fan, items which Suzuki has managed to skillfully blend into a most attractive package. With the exception of the radiator, the motorcycle is as cleanly styled and aesthetically pleasing as any model in Suzuki's line-up. Moderately valanced chrome plated fenders blend well with the wide dual seat, large instrument cluster and large, slightly bulbous gas tank. Even the radiator, which is protected by a chrome-plated safety bar, finally takes its niche in the design, although it looks somewhat out of place at first. A transversely mounted two-stroke three is a design made popular by Kawasaki, with their frighteningly fast 500-cc Mach 111. The GT750 engine features horizontally split crankcase halves, which support the massive, four-main-bearing crankshaft. The crank has throws spaced 120 deg. apart, giving the same number of firing impulses as an in-line six-cylinder four-stroke. Looking at the engine from the top, we find three sets of contact breaker points located at the extreme left end of the crankshaft which are driven by a flexible coupling.

Just inboard is an idler bearing and to the right is a gear, which drives the tachometer and water pump. The water pump is a vane-type unit driven by a horizontal shaft and located in the bottom of the crankcase. It has an output of l6.2 gal./min. at an engine speed of 6000 rpm, which compares favourably with many small cars. At the right is a one-way clutch for the electric starter, a main bearing, the left cylinder's flywheel assembly, another main bearing and the central cylinder's flywheel assembly. Still another main bearing follows, which is adjacent to the helical gear for the primary drive. Grafted onto the other side of the primary gear is the third cylinder's flywheel assembly, another main bearing and the alternator. The main reason for taking the primary drive from between the second and third cylinders was to keep the engine's width behind the crankshaft to a minimum. This arrangement leaves more space between the numbers 2 and 3 than numbers 1 and 2 cylinders, but with water cooling it doesn't make any difference. The one-piece cylinder casting is fitted with cast-in iron liners which are not removable. However, pistons are available in two over sizes, and if the air cleaner remains intact and properly serviced, piston wear should be negligible. Cast-in aluminum cooling fins don't help cooling the cylinder and the head much, but are much nicer looking than a smooth exterior.

All aluminum crankcase, cylinder and cylinder head castings are polished to a high luster, adding a touch of class to this unique motorcycle. A single cylinder head casting with relatively shallow combustion chambers is used, and the outlet pipe from the engine's cooling system emerges from between the center and right-hand cylinders. Conventional two-ring pistons are virtually identical to those of the T500 (the bore/stroke dimensions for both machines are the same), and, like the T500, needle bearings support the pistons, and roller bearings support the connecting rods. It is interesting to note that while the pistons and connecting rods are almost identical to those of the T500, the flywheels are smaller in diameter and roughly approximate the size of those used in the TR500 road racer. Water for cooling the engine enters the crankcase from the radiator at the front of the engine. The pump picks up the water and pushes it up through the water jackets around the cylinders and into the cylinder head jackets. Until a temperature of 82 deg. C (180 deg. F) is reached, the thermostat remains closed and the heated water is redirected to the pump in the bottom of the crankcase. Complete opening of the thermostat occurs at 95 deg. C (203 deg. F), and if the water temperature reaches 105 deg. C (221 deg. F), the tiny cooling fan behind the radiator cuts in and keeps running until the water temperature falls to 100 deg. C (212 deg. F).

During a series of tests carried out in the Los Angeles area by a team of Suzuki engineers during the hottest part of the year, it was practically impossible to get the water temperature high enough to activate the fan, so there's a good possibility it will be dispensed with altogether. Total coolant capacity is a generous 5.4 U.S. qt., and the system is pressurized as are those on automobiles. Also included in the system is a header tank, which further precludes boiling over. Drawing heavily from their water-cooled motorcycle grand prix experience, and from their work with small water-cooled cars and trucks, Suzuki engineers claim a temperature reduction of some 30 percent over the machine's air-cooled counterpart, a six percent increase in torque and the ability to reduce piston clearance from the 0.0026-0.0030 in. of the T500 to a very close 0.0019 in. An added benefit of the close tolerances is reduced piston noise, which is further lessened by the deadening effect of the water in the jackets. Silence and smoothness are key words for the GT750. Rubber mountings inside the crankcase attachment points to the frame allow the engine to shake just slightly around 2000rpm, but above that figure the motor is completely smooth up to maximum revs, Both the handlebars and the footrests are solidly mounted, indicating no need to damp out spurious vibrations at those points of rider contact with the machine. In a further effort to make the GT750 as quiet as possible, Suzuki incorporated an air filter intake system to minimize the intake roar characteristic of two-cycle engines. At the rear of the air cleaner box are two rubber tubes with slightly flared ends through which air is admitted to the filter.

These tubes do a remarkable job of silencing, and because they are pointed downwards, it is more difficult for water to enter the filter. Wanting to maintain as much symmetry as possible, but faced with the large physical dimensions of each of the super quiet mufflers, Suzuki opted to split the center cylinder's exhaust system into two smaller mufflers containing the same volume as one large one. A connector (balance) pipe running between the three header pipes not only aids noise reduction, but also increases the amount of torque at 2000 rpm by about 20 percent. Viewed from the rear, the GT750 looks like a four-cylinder machine with its four pipes. The two large outer pipes are tucked up out of the way, while the two smaller inner pipes are somewhat lower but tucked in far enough to preclude dragging on sharp corners. Although the machine's actual ground clearance is only a shade over 5 in., this lowness is near the center line of the bike; the only thing we were able to drag during vigorous cornering was the side stand. A particularly appealing feature is a gadget to reduce the output of exhaust smoke under certain conditions. When a two-cycle engine is operated at low speeds for a time, unburned oil collects in the bottom of the crankcase where it stays until the machine is accelerated hard, as when going on the freeway, at which time the bike takes on the qualities of a crop duster! This device is a one-way valve in the bottom of each crankcase, which is connected by a tube to an adjacent cylinder's transfer port. Instead of collecting, this unburned oil is directed to and burned by the other cylinder and is ejected from the engine at a more constant rate. The same amount of oil is used, of course, but not so noticeably.

Three 32-mm Mikuni carburetors are connected to the twist grip through a junction box with a single cable, and operate surprisingly easily. A starting enrichment lever (choke) located on the left handlebar lever mount must be used to start the machine on cool mornings. It makes starting so easy, we can't see a reason for retaining the kick starter. Depressing the starter button instantly fires the engine, indicating that less than one revolution of the crankshaft has taken place. A negative feature of the starting system is that, as is, the crank lever bumps the rider's right calf, transmitting the engine's vibrations to his body. With the kick starter assembly removed, the entire engine unit could be made smaller and lighter, with a subsequent reduction in the area occupied by the transmission. The transmission design follows previous Suzuki practice with constant mesh gears and one shaft located behind the other along the crankcase horizontal centerline. All indirect, the ratios are ideally suited to the power characteristics of the engine; they are not especially close, but making them close would be foolish because of the broad, flat torque range of the engine. Shifting action is very smooth and positive, but we feel that the GT750 is "under geared" and think a two- or three-tooth-smaller rear sprocket would be more appropriate. At 70 mph on the highway the engine is spinning at 4500 rpm, many more than are needed to keep it up to speed with a low throttle opening. In fact, the torque curve is so flat that pickup is possible from 2000 rpm in top gear with almost no change in apparent acceleration. We were amazed at the small amount of throttle needed to keep the GT750 moving, and at the large engine's willingness to rev. Also impressive, if not outstanding, was the fuel economy. Dividing our test riding as we do (70 percent highway, 30 percent in town), we got gas mileage ranging from 41 to 47 mpg, outstanding for a four-stroke of comparable size, and nothing short of amazing for a two-stroke.

Oil consumption was also low. The oiling system on the GT750 is virtually identical to that of the T500 which employs a variable displacement plunger unit that increases its delivery according to the engine's rpm and the amount of throttle opening. A cam, connected to the throttle cable junction box, opens the pump in accordance with the throttle opening. At cruising speeds, the pump is almost shut off, but as soon as the throttle is opened to climb a hill, the oil supply is increased. Conversely, when the throttle is closed, the pump’s output is reduced to help lower the chances of the engine's loading up with raw oil. Two additional outlets at the pump supply oil to the right-hand cylinder, but the system works the same. Oil is injected to all the main bearings except the one located next to the primary gear, which receives its lubrication from the transmission's oil supply. Oil used to lubricate the other mains then travels through the crankpin centers and lubricates the big-end bearings before being flung upwards to lube the wrist pin needle bearings. It is then burned and expelled out the exhaust. There is also an oil feed point in each inlet port to supply oil to the wrist pins and cylinder walls.

At 524.lb. with a half-tank of fuel, the GT750 is just slightly heavier than the Honda CB750 Four, but it feels deceptively light at any speed. In slow traffic the only clues you have about the machine's bulk are the width of the gas tank (which holds a moderate 4.5 gal. Because of a cutaway at the front for the radiator header tank and filler), the broad handlebars and the size of the instrument panel. At the top, in between the speedometer and tachometer, is the water temperature gauge. Neutral, high beam and turn signal indicator lights are located on the face of the tachometer, but there is no charging light. Our test machine had a confusing array of thumb-operated switches on the left side, which will be modified for production. From top to bottom were the headlight switch/dipper assembly, turn signal switch, horn button and a headlight flasher. The right side houses an on/off switch and the electric starter button. Later models will not have a headlight flasher, and the headlight switch/dipper assembly will be moved to the right side.Like the T500, the GT750 gets top marks in the handling department.

A massive double cradle frame features a huge main tube with the radiator filler going through it, running from the steering head back to the rear of the gas tank. It butts into a transverse cross brace which doubles as the top attachment point for the air cleaner. Two auxiliary top tubes extend from the steering head rearward. They finally terminate at a point some 9 in. behind the top rear suspension mounts to serve as mounting points for the rear fender. The down tubes act as a mounting point for the radiator and continue under the engine rearward to just behind the transmission, where they bend upward and attach with the top tubes under the seat. Diagonal tubes branch from the bottom cradle and extend back to the rear suspension top mounts. Strengthening triangulation is evident here as in the steering head/main top tube area. The relatively short swinging arm is supported at the pivot point by large bushings, and the tubes are a healthy 1 5/16 in. in diameter. Thick steel gussets, which, provide mounting points for the bottom of the rear suspension units and the rear axle are uncommonly long and strong. Even under hard acceleration in high-speed corners, the GT750 tracked as though on rails. In fact, the only part of the suspension package which isn't quite up to spirited cornering are the front forks. In keeping with the comfort-oriented idea behind the machine, the forks are softly sprung, but they nonetheless provide excellent control on all but the roughest surfaces. Notable also are the De Carbon-type sealed gas rear shock absorbers featuring a five-position spring rate adjustment. The Suzuki's shocks are very sensitive to small road irregularities, and have sufficient damping to prevent rear wheel hop under heavy braking, or yawing in fast corners. At first we had reservations about the brakes. With many of today's high performance motorcycles going to disc brakes on the front, it seemed logical that Suzuki would follow suit. They haven't, but the GT750's brakes are just short of fantastic, capable of hauling the heavy machine down from 60 mph in 118 ft.

The front brake is little more than two T500 brakes in a single housing with cooling air scoops. Twin cables fitted into a balance bar at the brake lever are adjustable for length both at the lever and at the brake operating arm end. Although the front brake heats up under hard usage, fade is minimal. The rear brake is identical to the rear unit on the T500, but it, too, is reluctant to fade. Riding the GT750 is a joy, which must be experienced to be fully appreciated. The wide, ultra-soft dual seat is very comfortable, although we had a heavy friend who complained that the padding was too soft and allowed his buttocks to hit on the bottom of the seat. Footrests are slightly on the long side, and do not fold rearward. This makes it easy to bump one's calves while "walking" the machine from a seated position. The relationship between handlebars, seat and foot pegs was deemed very comfortable by all our testers. The main complaint stemmed from the upright seating position, which doesn't allow the rider to lean forward into the wind at high speeds. The almost complete lack of vibration and unnerving mechanical, intake and exhaust silence give one the feeling of riding a steam turbine. The loudest noise, by no means unpleasant, was the gear whine of the all-indirect ratio transmission and the rear chain. Our duty pillion passengers also rated the seat comfortable and were pleased with the lack of vibration through their foot pegs. Detailing is exquisite. Heavily applied chrome and lavish use of the buffing wheel on aluminum parts slightly outdid the paint, which was moderately less perfect than on other Suzuki models. Welding on the frame is very good, and all engine parts fit neatly together. Lighting is excellent, with what is possibly the largest taillight on a motorcycle. All electrical components are first-rate, including the neatly routed wiring. We also liked the location of the ignition switch: right in the center of the instrument panel. Suzuki's most pleasing combination, the GT750 is the most refined, and yet most awesome, two-stroke ever.

Dan Hunt tests the GT750

REAL COOL-THE WHISPERING GIANT

SUZUKI'S GT750 really moves out. And it handles quite well. But the fancy new water cooled 750 Three will not be the favorite of many Café racers, because its conception doesn't lend itself to the street racing mentality. It is a big machine and at 524 lb wet, perhaps the heaviest two-stroke ever produced. As such, it loses out if considered only in terms of the other Superbikes that have preceded it.

You must look elsewhere to find out why Suzuki has taken the inherently lightweight two-stroke engine design and come up with a bike that is heavier than the 750s of BSA, Triumph, Norton, Honda, BMW and Kawasaki. The reason the GT750 is heavy? All that extra plumbing for the water-cooling system. No more the simple cooling fins. In place of that we have water jackets, hoses, radiator, thermostat, temperature gauge, and even an auxiliary cooling fan. But for this extra weight and complexity, the compensations are considerable: Two strokes have greater cooling problems than four-strokes. Firing twice as often they produce more heat. Lacking the elaborate oil circulation system of a four stoke , which handles as much as 60 percent of the cooling functions, they must depend more upon fining to dissipate heat. Thus, in an in-line three-cylinder designs, the centre cylinder will invariably run hotter than the outer two. Kawasaki gets around the problem by wide cylinder spacing and careful aerodynamics; modify the Kawasaki front fork and fender, wrongly and you may have a heating problem. Suzuki, in using water-cooling, over comes the in-line Three problem entirely. Cooling all three cylinders evenly with water jackets reduces piston and cylinder distortion and therefore prevents loss of power. Clearances may be tighter, preserving yet more power, reducing noise, and squelching mechanical noise. And, the silencing effect of the water-jacket itself is extraordinary.

The GT750 is quiet!

Fuel economy is improved as a function of better heat dissipation and engine efficiency, so it is no surprise that the GT75O gets good mileage. The figure we obtained in a combination of town and motorway riding was 51 miles per gallon (Imp.), comparable to a few 750 four-strokes. Now we begin to see what the new Suzuki is all about. It is a big, high performance machine for long distance touring-a new role for a large displacement two-stroke.

The innards of the GT750 are somewhat similar in design to the in-line Three pioneered by Kawasaki: horizontally split crankcase halves; massive four main bearing crankshaft with throws 120 degrees apart; five-speed transmission. Then the other differences; a vane type water pump located in the bottom of the crankcase, one-way clutch for the electric starter at the right. The helical gear primary drive is taken from between the second and third cylinders to keep the engine's width behind the crankshaft to a minimum. This leaves more space between cylinders two and three than between one and two, but, with water cooling it makes no difference. Upstairs you'll find a one-piece cylinder block with cast-in iron liners, and a single cylinder-head casting with shallow combustion chambers. The outlet pipe for the cooling system emerges between the centre and right-hand cylinders. Conventional two-ring pistons are supported on needle bearings, while the connecting rods ride on roller bearings.

The cooling system, pumping 19.2 gallons per minute at an engine speed of 6,000 rpm compares favorably with that of a small car engine. Water enters the crankcase from the radiator at the front of the engine. The pump takes the water, pushes it through the water jackets and up into the cylinder head jackets. The thermostat remains closed until the water temperature reaches 180F and the water redirected to the crankcase pump. At 203 degrees F the thermostat is opened completely to fully utilize the power of the radiator. The little auxiliary fan cuts in only when the water temperature reaches 221 F. In boiling Southern California I never encountered a condition hot enough to make this fan operate. The basic cooling system is so effective that Suzuki may dispense with the , fan in subsequent models. Suzuki engineer's claim that the water-cooling about 30 Per cent more effective than air-cooling, resulting in a six percent increase in torque. Piston clearance on the GT750 is .0019 in. compared with the .0028 in. clearance used on the air cooled 500 twin. Now to the riding. You are straddling a hefty machine, but the 32.0 in. seat height is not to imposing to an average size rider. Open the fuel taps, pull the choke lever and hit the start button. The machine starts burbling in that smooth muffled idle reminiscent of its ancient predecessor, the Scott. If you have time, wait for the temperature needle to rise to the bottom of its operating range.

The gear lever, operating a one-down four-up pattern, is on the left. As soon as you're under way, the machine begins to lose the initial impression of weight and mass. It steers easily in tight going and U-turns easily on a narrow one-lane road. Above 30 mph the feeling is one of relative lightness and quickness. In sweeping turns over 65 mph, the bike "sets" well into a turn with little threat of under-steer. In spite of the four balanced silencers, you get the impression that they will not ground easily. The ride is typically Japanese, that is, stiff, and I'd rate damping action excellent at the front and good enough at the rear.

The bike is a solid, stable motor-way runner-wiggle free and placid. At 80 mph in fifth gear, the engine is turning hardly more than 5,000 rpm. At 110 mph it is geared to turn 7,000, rpm, which is quite slow in two-stroke terms. Acceleration is scorching, as it turns the quarter mile in the high 13s. But the effect is mild, because the power band is fat. In fifth gear with a solo rider, the bike dawdles along easily at 2,000 to 3,000 rpm and pulls away with no snatching or flat spots. The only objectionable traits the machine exhibits are centred around the transmission. With so little noise coming from the engine itself, and from the mufflers, you can hear the gearbox whine noticeably in the higher ratios. And the act of shifting is' reward with a loose, clunky action, after the manner of, but not quite as pronounced as the BMW or Moto-Guzzi. But unlike these Continental tourer's, the inertia in the Suzuki gearbox is low. The GT750 may be up shifted or downshifted quite rapidly.

I was surprised that Suzuki had not chosen a disc brake for the front, as all its Japanese competitors have done for their big bore machines. The inside line here is that the company would have nothing to update the machine in coming years if the disc brake was featured this year. But you can't object, as the big drum brake is superb and hauled the GT750 from 60 to zero in only 118 feet. It is the equivalent of two T500 brakes in a single housing with air scoops . Both it and the T500 sized rear brake, are reluctant to fade.

Seating and peg comfort is excellent for both rider and passenger. The handle bars sweep back a bit too far which means a riding position that is too upright.

Finish of the machine is good - with clean welds and neatly fitting engine parts. The electrical wiring, too, is quite tidy. The general styling and colour- cool purple and white, or aqua blue and white-is bright but tasteful.

Balancing complexity against the benefits-smoothness, power and reasonable fuel economy-the Suzuki GT750 may be the first big-bore two-stroke to win converts from the firmly entrenched long distance four-stroke clan.