Suzuki Gt750

SUZUKI GT750

CYCLE WORLD ROAD TEST

A Posh Tourer With Water Cooling And Authoritative Performance.

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 sneed of 91.06 mph.

Once again Suzuki has introduced a paradox: the GT750. Big, heavy, comfortable, economical and extremely smooth, the GT7SO 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 WW II, was the last large-capacity machine to use it. More recently, the horizontal ly 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 aestheti cally pleasing as any model in Suzuki's lineup. 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 III. 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 16.2 gal./min. at an engine speed of 6000 rpm, which compares favorably 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 oversizes, and if the aircleaner 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 centerand 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 2000 rpm, 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 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 twistgrip 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 kickstarter 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 center line. 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 "undergeared" and think a twoor 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 maintube 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 crossbrace which doubles as the top attachment point for the air cleaner. Two auxiliary toptubes 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 downtubes 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 toptube 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 airscoops. 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 footpegs 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 footpegs.

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. [O]

SUZUKI

GT750