MIDDLEWEIGHT MARVEL!

Suzuki’s GT550 confounded perceptions about two-stroke triples being classy and comfortable high-speed tourers. John Nutting tested three versions of the model in the 1970s.

While the top-of-the-range GT750 is rightly regarded these days as the epitome of the Hamamatsu factory’s stroker technology it wasn’t always that way.

Back when two-strokes ruled the roost at Suzuki a midrange model was snapping at the heels of the ‘Kettle’.

Suzuki’s range included three triples; the GT380, GT550 and the GT750, but the GT550 was my favourite. The two smaller models were air-cooled, and while the smallest was more nimble, the GT550 offered usefully potent performance without the weight or bulk of its larger sibling.

The Suzukis offered refinement and luxury with a keen price tag. When the GT750 and Honda’s CB750 could be bought for £720, the GT550 came in at £569, a fifth less.

Fact was there was no sacrifice to be made in opting to buy the GT550. Having tested three different versions between 1972 and 1976 I think it is one of the great unsung classics of the period.

Like its siblings, the GT550s styling featured acres of chrome plate, candy paint and polished light alloy, yet it was peerless as a grand touring machine offering up to l00mph cruising in relative comfort.

Some will say that as a two-stroke, the GT550 and its ilk were doomed as petrol prices rocketed during 1973’s fuel crisis but the test data suggests otherwise: this ring-ding could be frugal with the fuel when ridden with care.

The GT550J followed the GT750’s launch, with a test bike being provided for testing at Motor Cycle in July 1972. Ordinarily, a three-cylinder engine with an across-the-frame 120° crank could be buzzy but the GT550 feit silky at an easy 85mph, the engine’s rubber mounts provided utterly smooth running while the raised handlebar and the slightly rear-set and roomy footrests offered a relaxed perch, as the huge silencing system reduced the exhaust note to a purr. Adding to its relaxed nature, the GT550 was also surprisingly flexible, pulling strongly from less than 3000rpm and making town traffic as easy to cope with as motorways.

There was nothing unusual about the engine’s design, except for the cowling enveloping the cylinder heads that Suzuki called ‘Ram Air System’. Within the horizontally split aluminium-alloy cases, the crankshaft was a pressed-up assembly using ball main bearings and needle-roller big ends. Each of the three pistons had its light alloy cylinder with generous finning and a cast-iron liner, fed by three 28mm choke Mikuni carburettors. While the outer cylinders had their own silencer, the central pot exhausted into a bifurcated system below each of the outer pipes to give the machine a symmetrical appearance.

Engine lubrication was provided by Suzuki’s CCI system in which oil was drawn from a tank under the seat and pumped to the inlet ports, the rate being controlled by the engine speed and throttle opening. To minimise oil build up and the risk of leaving a smoke trail under hard acceleration, connecting pipes linked the base of each crank chamber with the adjacent cylinder’s transfer port.

Some say that the slightly undersquare 61mm bore and 62mm stroke, giving 543cc, contributed to the GT550s broad spread of power. But this is largely immaterial in a two-stroke: more likely the lower bore-to-stroke ratio than the 739cc triple {70 x 64mm) enabled more room for the cooling fins without adding to the overall width.

Unlike the 750’s inboard primary drive gears, the clutch on the 550 was driven by helical gears from the right-hand end of the crank and a five-speed gearbox sufficed, given the spread of power which peaked at a modest 48.5bhp at 6500rpm. The torque also peaked at just 8000rpm with 44lb-ft.

The effect of this was that throttle response in top gear was strongest at around 70mph, when a crack of the twistgrip produced what I described in the test report at the time as a lightning’ reaction. Overall gearing was quite low, although it never felt that way thanks to the smoothness of the engine.

On the MIRA proving ground’s test strip it also resulted in the GT550J being as fast as the 750, which had been tested six months earlier. With the mirrors removed and rider flat on the tank, the GT550 clocked a two-way average of 107.27mph, rewing to 7700rpm in top, a figure that bettered the 750 by 1mph. With a slight tail wind it hit 8000rpm in top at 112.15mph.

While the GT550’s gearing made the best of its power delivery, it didn’t do much for its flat-out acceleration, which with an elapsed time for the standing quarter-mile of 15 seconds could be matched by later versions of the smaller GT380, although the terminal speed of 90mph was much higher for the 550.

A wet weight of 4551b with a gallon of fuel (206.8kg) may have played a role in this, but in fact the GT550 was much lighter than the 750 — all of 601b (27.2kg) — while being roughly the same overall size.

With a 58.5in wheelbase (1486mm) that was almost identical to the 750’s the GT550 didn’t have scalpel-like handling but then that was never likely to be the objective. It did feel more accomplished than the Kettle on twisty roads with satisfyingly neutral steering and firm suspension.

Again, this would be inviting the GT550 to explore the edges its decidedly unsporting design envelope. This was never more obvious with its limited cornering clearance, which was dictated by the width of the exhaust system and centrestand. Much fun could be had creating showers of sparks on the GT550.

At the front was the impressive looking four-leading-shoe drum brake, which was also used on the first GT750. I’ve criticised it in the past, and it’s true that careful set up of the double cable system is necessary to extract its best, but it performed admirably under test, hauling up the bike from 30mph in 29ft 6in compared with the heavier 750’s 32ft.

The brake was sensitive, grab-free and showed no signs of fade even after repeated use from maximum speed.

In sum, that first GT550 was an utterly civilised motorcycle: with a starter motor under the gearbox, it started faultlessly on the button and warmed up quickly using the cold-start jet lever adjacent to the clutch lever. Instruments and controls were a delight to use. Lighting was strong, fed by a powerful excited-field alternator, and my only criticism was of the weediness of the horn.

For 1973, Suzuki introduced disc front brakes across its three-cylinder machines, including the GT550K, which like the GT380 used a single rotor. This version wasn’t tested by Motor Cycle but the following year’s for 1974 was, when the GT550L received a significant number of changes, including additional chrome-plated items like the rear chain guard, headlamp shell and air-filter side covers. The front fork’s rubber gaiters were discarded in favour of exposed legs, bringing the machine fashionably up to date, white the Ram Air shroud’s styling was tidied up.

A notable feature that also appeared on other Suzukis was a gear-position indicator located between the instrument faces. As each gear was selected its number was illuminated in a display window, though the green neutral lamp was retained.

A reai advance was the revision of the carburettor slide opening arrangement in which push-pull cable operation was adopted along with lifting arms, replacing the awkward junction box and making adjustment much easier. The cold-start lever was moved down to the carbs.

Overall performance and feel of the GT550L felt much like the original model from two years earlier except that the handling seemed heavier, even though there had been no changes mentioned to us by Suzuki, A welcome move was that the exhaust system had been redesigned to provide more cornering clearance.

At the test strip the GT550L was at 108.16mph slightly faster than the J but quicker through the standing quarter-mile with a time of 14.4 seconds, in the fuel consumption tests, carried out at constant speeds, the more precise metering of the carbs was reflected in lower consumption through the range: even at 70mph it was recording 43mpg.

With such a light twistgrip action and such a willingly smooth delivery it was all too tempting to wind up the GT550L, especially when it was used for the National Rally, in which over a weekend dozens of checkpoints across England had to be reached before a final average speed check. At times, the range dipped to 90 miles and the three-pint reserve at night-time when garages were closed was barely adequate. But, equally, with care the bike could be used economically, as reflected by the overall consumption of 54mpg.

At first, the new disc brake, operated with a floating caliper, felt as potent and sensitive as the dual drum it had replaced, until used in the rain! To my surprise at the test strip, the stopping distance in the dry — 31ft 6in — also proved to be worse than the drum-braked model.

Apart from the usual changes in graphics, the GT550M for 1975 was mostly unchanged, but towards the end of the year Suzuki introduced into some markets a more highly tuned version of the bike.

This was formalised with the GT550A for 1976, the year that Suzuki’s four-stroke machines were launched. With chrome-plate replacing the iron liners in the cylinders, porting changes and a revised exhaust system devoid of the linking tubes between the pipes, power was upped to 53bhp at 7500rpm but with a sacrifice in torque, which dropped to 38.7lb-ft but higher up the rev range at 6000rpm.

Along with higher overall gearing from the use of different sprockets, the changes had the immediate effect of increasing top speed by 5mph to more than 113mph. Acceleration also improved with a cut in the standing quarter to 14.3 seconds at 94.3mph.

And while the bike could now hold a cruising speed of more than 100mph it was at a price, with the engine’s low-end lunge weakened and its fuel consumption plummeting to less than 34mpg at 70mph. Overall, the test average was 35.5mpg.

The 550 triple’s final year was in 1977 as the GT550B is was sold alongside the then-new GS550 DOHC, with which the two stroke’s styling shared many features, such as the black side panels, turn signals, instruments and headlamp shell.

As exhaust emissions regulations were being tightened up and fuel prices increased the future of road-going bikes no longer included two-stroke engines.

Contrary to the popular image of multicylinder two-strokes, the GT550 was a uniquely sophisticated crossover that combined the best features of the GT380 and GT750. Those values are still just as valid in classic motorcycling circles.

GT550J OWNER IAN HOLMES

The Suzuki GT550J owned by VJMC member Ian Holmes is an example of the earliest drum-braked version of the triple that dates from 1972. It’s well known in classic motorcycling circles as it was restored 20 years ago and has been seen at shows and in CMM since.

Detailing is remarkably faithful to the original, right down to the unusual gunmetal finish of the engine cases and the code numbers on the control cables. Already a Suzuki rider with a GS550 four-stroke four, Ian bought the GT550 in the mid-1980s as a collection of parts in a box for £20 and a crate of beer. He assembled it, painted it in olive drab and used it as a daily ride for many years until it was abandoned under a tarpaulin in his father’s garden.

He started to restore it in 1992 and by 2000 it was almost complete, including the complex three-into-four exhaust system. Parts were bought and exchanged as used and new-old-stock. The only departure from the factory spec that will offend the purists is the use of stainless-steel wheel spokes and nipples. For display, a perfect seat is kept in reserve.

Ian’s GT550J hasn’t been without its problems. Typically, the starter-motor clutch has failed — this can be corrected with a more robust later version. Most owners have the clutch welded up.

It is the one bike in a collection of rare two-stroke Suzukls that Ian will never part with.

SPECIFICATION Suzuki GT550

Engine: Air-cooled 2-stroke triple

Capacity: S43cc (61 x 62mm)

Valve operation: Piston ports

Compression ratio: 6.7:1 (from ex port closure)

Lubrication: CCI pump

Ignition: Coils and contact breakers

Carburation: Three VM28SC Mikuni

Peak power: J: 50bhp @ 6500rpm K: 48.5bhp @ 6500rpm A: 53bhp <§> 7500rpm

Peak torque: J: 441b ft 5000rpm K: 42.81b ft @ 8000rpm A: 38.7tb-ft@>6000rpm

Primary drive: Gear

Primary ratio: 2.242:1(74/33)

Clutch: Wet multi plate

Gearbox: Five speed

Internal ratios: 2.846, 1.736, 1.363, 1.125 & 0.923: 1

Final drive: 50HDS chain

Final drive ratio: 2.5:1 (40/16) A: 2.4:1 (36/15)

Overall ratios: J: 16.7, 10.17, 7.99, 6.59 & 5.4:1 K: 15.95, 9.73, 7.64. 6.30 & 5.17:1 A: 15.3, 9.34, 7.33, 6.05 & 4.97:1

Frame: Duplex tubular steef cradle

Front suspension: Telescopic oil-damped fork

Rear suspension: Pivoted fork, 2-spring damper units with 5 position adjustable preload.

Front wheel: Laced spoke, steel rim

Rear wheel: Laced spoke, steel rim

Front tyre: 325-H19 Bridgestone

Rear tyre: 400-H18 Bridgestone

Front brake: Four-leading-shoe 200mm drum (J model) Single 275mm hydraulic disc (K to B)

Rear brake: 200mm drum

Electrical system: Alternator, 50/40W headlamp, electric starter motor

Battery: 12v-llAh

Fuel tank: 15 litres (3.3 gallons)

Wheelbase: 1460mm (57.7in)

Seat height: 800mm (31.5in)

Castor angle: 61°

Trail: 118mm (4.6in)

Dry weight: 200kg (4401b)

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