SUZUKI 500 FANS - THE T500/5 COBRA

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The SUZUKI COBRA THE BIKE WHICH COULDN'T BE BUILT!

The bike which couldn't be built - the Suzuki 500cc Dual Stroke". So went the proud boast of the Suzuki advertisements of the late 60's. For a buying public conditioned to two-stroke BSA Bantams and Villiers engined motorcycles, the 500cc Cobra twin was simply too large to run without seizing. Only Scott had been able to make large capacity two-strokes successfully and then only by using water-cooling, but there had been others who had tried hard to succeed. For example, in the 20's, Dunelt made 500cc single cylinder two-strokes for over seven years. Putting out a mild 5hp the Dunelt had tested the large two-stroke market but had not exactly tested the limits of metallurgy.

cobra

The Cobra from a sales pamphlet

Two-stroke manufacturers however, were anything if not innovative and in 1921 the English Stanger factory built a 538cc two-stroke V-twin which utilised a spring frame. The engine however, not surprisingly, tended to overheat and foul its plugs and the company went under in 1923. The Germans have always been innovative in the field of two-strokes (witness Maico, Zundapp and DKW) and from the early days they tried to build large two-stroke motorcycles. For example the Vis Fabrik built the interesting Vis-Duplex in 1924. This machine was a horizontally opposed two-stroke twin of 496cc built "in line" with the duplex frame.

The Vis-Duplex, much like similarly bikes suffered from overheating of the rear cylinder and was only built in very limited numbers. DKW had more success with a series of 500cc and 600cc air and WATER-COOLED twins in the late 20's and 30's although they also experienced overheating, fouled plugs and abysmal fuel consumption, particularly under racing conditions. Their racing 250cc and 350cc supercharged two-strokes gained much greater success. Such innovation continued with the German Schliha Company building some strange 498cc and 596cc two-stroke singles in the early 30's. The interesting thing about these short-lived two-stroke bikes was that they utilised overhead valves (I'd like to know more about these machines!).

Puch, tried with its split singles to give the 500cc two-stroke an advantage pre-war and Zundapp experimented with a twin in the 50's but no-one succeeded in putting a civilised large capacity two stroke on the road in any large numbers until Suzuki launched the T500 in 1967. Not only did they make a big two-stroke that worked but they built one which was more reliable than many four-strokes of the time, required less maintenance and featured oil-injection direct into the crankcases along with massive and durable main roller bearings. The Cobra was also disgustingly fast for its capacity and put many contemporary 500cc and 650cc four-stroke twins to shame

In Australia the T500 was also exceptionally good value and proved to be what a large number of motorcyclists were looking for - a simple, robust and comfortable machine able to cruise effortlessly, across our vast distances, admittedly with some vibration and a prodigious thirst. Its value was clear, as for example in May 1970 the T500 sold for $775 as compared to a Kawasaki Mach III which sold for $965, a Honda CB450 for $905 and the Triumph Daytona 500 which sold for $895. 1967 first saw the introduction of the Suzuki 500 twin. Initially known as the 500/FIVE (to celebrate its five speed gearbox) the model was possibly the first large capacity Japanese bike to demonstrate some sporting potential. Japanese technology was just waiting to be unleashed on the unsuspecting British motorcycle industry and the Cobra was to herald the arrival of the Superbike era. Sadly for the Suzuki 500 its generous performance and sound engineering was soon to be eclipsed by the release of the Honda 750 and the Kawasaki 500 triple a year later

The 500/FIVE itself was a short-lived model, seemingly only seen in any number in the United States, and was quickly replaced by the Cobra. The most significant change to the original model was given little publicity at the time despite its dramatic effect. The swingarm of the 500 Suzuki was actually changed from 52.7 inches on the 500/FIVE to 57.3 inches on the Cobra. This is a very radical change on any bike but on the Suzuki 500 the longer wheelbase contributed to the bikes reputation for sure-footed and stable handling. For Australian riders it was also very useful as the extra length of the machine made for a comfortable tourer with plenty of room for mounting carriers, saddlebags or panniers.

t500II

The first Cobra from a sales pamphlet - note the short wheelbase.

The Suzuki Cobra was a relatively simple two-stroke which utilised the same 70mm bore and 64mm stroke as its 250 trail-bike cousins were to use (and in turn as the 750cc water-cooled triple would also use). The Cobra had a true capacity of 492cc and a mild compression ratio of 6.6:1 but still managed to produce 46hp at 7000rpm. The Cobra produced this power through a whopping great pair of 34mm Mikuni carburetors - the biggest the company had ever used and only to be seen again on the company's 400cc trail and motocross singles! Despite the large carbies the engine is very tractable and does not require great amounts of throttle to move away from a standstill. This tractability could be attributed to what the Suzuki factory at the t ime described, wait for it, as the "Homo-pressure" type carburetor (I'd like to know what the Japanese PR man's real intention was when he came up with that one)? The Mikuni's actually have probes in their intakes (this is getting worse!) which compensate for changes in altitude and pressure.

In fact one of the pleasant discoveries of riding a 500cc Suzuki twin is to find that theories of buzzing two-strokes do not apply to this engine. The Cobra produces readily usable power from low revs and does not need, nor necessarily like high revs, in order to obtain a good rate of knots. The Cobra puts out 37.5 lb ft of torque at 6000rpm, compare that to the contemporary BSA 650cc Lightning which produced 39 lb ft of torque - at 5750rpm and you'll see what I mean. The big carbies carry two penalties with them, the first and easiest to live with is the intake noise. Despite the large airbox and air filter the engine intake noise is very loud and cannot be avoided. Give the throttle a twist and the resultant howl from the carbies drowns out the rather uneven plip-ploppy exhaust note. If you were pulled up for speeding on a Cobra it was hard to say you didn't know how fast you were going, you certainly didn't need a speedo to tell you, the superb intake howl told you clearly enough.

The less useful attribute of the big carbies was fuel consumption. Thirty-five mpg wasn't all that good at high speed although fourty-five mpg was more common around town. At country cruising speeds fifty mpg was not impossible. The low rev power available from the engine permitted the factory to build a fairly high geared gearbox for a 500cc motorcycle. The Suzuki copes well with touring pulling 3000rpm at 50mph and 4000rpm at 65mph. The Suzuki indicates 80mph at 5000rpm and the engine feels comfortable at these speeds and not at all fussed. The bike will top at least 110mph (theoretical top speed being 117mph). I have seen 116mph on the speedo in 4th gear; but allowing for speedo error this could not have been more than 102mph.

The Suzuki 500 was taken to the Daytona 200 mile race in 1968 and in a very mildly modified form it proved fast and reliable finishing 5th and 9th out of a field of 80 machines. This ability to run fast and strongly without any adverse effects on the motor contributed to the bike's instant popularity with the production racers. In England for example the Suzuki ran basically unchanged and without special cylinders or exhausts and won the Isle of Man Production TT Races twice.

Barry Sheene rode a road racer version with the help of frame maker Colin Seeley and won several significant races on it. Jack Findlay went on to win a world championship race (the 1971 Ulster GP) on a 500 Suzuki that was so similar to the production engine that it is still said to be the closest thing to a championship victory on a road bike. The Cobra evolved into the T500 Titan in 1970 and eventually into the GT500 in 1976.

cobra1

The more common Cobra. One with a longer wheelbase

The model finally faded into obscurity at the end of 1977. A production span of 11 years is nothing to be sniffed at and indicates that the Suzuki was well put together and was able to enjoy a loyal following. the low purchase price maintained throughout its life probably contributed to its popularity. The Cobra was produced for 3 years before the Titan was introduced. The Cobra didn't run for this long without changes and except for the swingarm mentioned previously, the most significant changes took place in the motor. The pistons and cylinders were changed in 1969 and naturally enough early pistons should only be used with early cylinders.

The Cobra pistons can be easily recognised by the transfer port windows in the piston skirt. These windowed pistons not only cost more to manufacture but were reputed to be stronger than notched pistons due their full circumference skirts. In practice if the engine was mightily abused and revved beyond 7 grand for too long, as for example on the race track, there was an off chance that the piston skirt would fracture and fall off with disastrous results.

The windowed pistons were also claimed to run quieter than notched pistons. This is unlikely in practice and significantly the piston to cylinder clearance of a Cobra piston was 0.0071-0.0075 inches. The later pistons ran with a much closer tolerance of 0.0026-0.003 inches. The early Cobra barrels can be recognised by the fact that they have only 10 fins while the later ones have 11 fins and slightly altered ports. Whilst few of the early barrels survive there are heaps of the early windowed pistons around.

cobra engine

The engine has a certain presence

One shop in Perth has boxes of the early pistons sitting around gathering dust. Suzuki 500's are noted for piston longevity and the bike is likely to wear out before the motor requires a rebore, barring bad maintenance or extreme rider abuse, of course. Another change occurred after engine number T500-10659. From then on crankshafts were supplied with larger diameter con-rods (the outer diameter of the big end end of the rod was increased from 41.5mm to 46.5mm. If a rebuilt crank is to be used in an early engine then the crankcase stuffing ribs must be ground down, on both upper and lower cases, by 0.040 inches.

cobra3

A solid looking motorcycle

Most importantly, Cobra gearboxes took a recommended 1200cc of oil whilst later models (from the T500L on) took 1400cc. Prolonged high speed running could starve fourth and fifth gear of oil and overfilling was ineffective as the extra oil merely flowed into the clutch chamber. The engine needs to be split and a rubber partition installed on the primary oil transfer chamber of the lower crankcase half. Suzuki supplies a part for this purpose (part no. 99104-08800).

Riding the Cobra exposes the Cobra's weak points. The suspension, particularly the front, is too soft and under-damped. At speed the front end pogoes up and down although never enough to get the steering out of shape. One way to stiffen the forks up is to add some heavier oil and to place a spacer inside the forks to give the springs some pre-load. A harder nut to crack is the front brake. The front brake should be pretty good and it was by the standards of the day. The front brake was a full width 8" twin leading shoe drum and its fitment received rave reviews from the press at the time. One American magazine said as much, "the brakes are superb with predictable action and high fade resistance. The front unit can be encouraged to completely stop the front wheel at any speed". As I found the front brake absolutely useless, during the test ride, I can only assume the other bikes that particular magazine tester had ridden were not fitted with brakes from new! The Cobra's front drum is not a good brake for a machine of this weight and power, requiring far too much lever pressure to be effective.. It is interesting to note that praise for the Suzuki's front drum diminished markedly after Honda released the disc brake on its new 750 Four. For some reason the Suzuki factory did not release a 500 Suzuki with a disc brake until 1976. Well overdue and too late to save a worthy motorcycle from extinction.

SPECIFICATIONS - SUZUKI COBRA - YEAR: 1968

ENGINE: Air cooled two stroke twin, bore 70mm x stroke 64mm
CAPACITY: 492cc
GEARBOX: 5 speed
BRAKES: Front: 8" twin leading shoe drum Rear: 7" drum
DRY WEIGHT: 412 lbs
TOP SPEED: 11O MPH
ACCELERATION: Standing 1/4 mile - 13.8 sec
POWER: 46 HP at 7000 rpm
TORQUE : 37.5 lb ft at 6000rpm
ENGINE No.s: 12518 - 34218
SUGGESTED READING
SUZUKI TWO STROKES__ROY BACON: OSPREY 1984
SUZUKI 500 TWIN__OWNERS WORKSHOP MANUAL: HAYNES
JAPANESE MOTORCYCLES_C.J. AYTON: MULLER 1981
CHILTON'S REPAIR MANUAL_CHILTON BOOK COMPANY 1981
CYCLE WORLD ANNUAL__CYCLE WORLD 1968

Copyright reserved: M Barnard 1990

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Cobra Photo Gallery

T/500/5 or Cobra

1968 T500

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