Twin cam 4 valver
‘Logistical complications’ meant that the media lolled around a seaside resort, slurping cocktails and listening to Caribbean music when they should have been riding, or at the very least, gazing upon the latest models from Hamamatsu. As the pickled press was poured on board for the return trip, the bikes arrived on the opposite dock. Oh, well, better late than never.
Of course, this unfortunate event had an eerily familiar ring in the very brief history of Yamaha four stroke twins, for scarcely twelve months prior, many of the same press members had been present at the launch of big brother, the ill-starred TX750, which although fundamentally an excellent motorcycle, was beset with more glitches and gremlins than could be imagined. But Yamaha is nothing if not resilient, and soon after the unfortunate Catalina incident, several running examples of the new half-litre model found their way into the hands of road testers. Visually, there were many styling cues to the TX750, and even, vaguely, to the now familiar TX650, nee XS1, but the 500 was in fact new from the tyres up. For a start, it marked the company’s first venture into motorcycle four-valve technology, an area only recently explored by Honda via its XL250 Motorsport road/trail machine. And in the head were twin camshafts,
another first for Yamaha, at least in motorcycling terms, for Yamaha’s twin cam design had been used by Toyota for a number of racing and sports car engines, and went into production in the form of the Celica GT. The most obvious difference between the 500 and 750 was the use of wet sump lubrication on the smaller engine, whereas the 750 used dry sump. Inside the cam box, there was a world of difference. With each cam lobe operating two valves, actuation was via a rocker arm with conventional tappet adjusters. This gave the visual impression of an overhead-valve engine, without the usual circular enclosures on the cam box. The spark plugs were located centrally in the hemispherical combustion chamber, and therefore, difficult to access. Sintered titanium alloy valve seats were press fitted into the cylinder head to reduce wear and allow the use of unleaded and low octane fuel. Down below in the bottom end, the TX500 used a forged one-piece crankshaft with split-shell main bearings, three of them, with connecting rod journals set opposite each other for a 180 degree firing pattern. A pair of straight-cut primary gears took the power to the seven-plate clutch and fivespeed transmission. On the right side of the engine, a double-row chain with slipper tensioners drove the overhead camshafts, with finger-type cam followers, forked to operate the pairs of inlet and exhaust valves in each cylinder. A dual rotor high capacity trochoidal oil pump supplied the vital fluid, while a second pump returned it. One of the quirks of the TX750 had been what Yamaha called the Omni-Phase balancer, which became an out-of-phase balancer as the driving chain wore and stretched– a fault corrected via a revised tensioner in later models. Basically the idea was to cancel the forces which caused vibration, with equal and opposite forces. On the 500, the counterrotating balancer was completely re-engineered and much simplified, now located in the top of the crankcase behind the cylinders, rotating counter to the crankshaft, and driven by yet another chain from the left end of the crankshaft. The shaft ran in ball bearings, whereas all other bearings were plain. By all accounts, the system worked, creating a silky smooth engine, but only once a few revs were applied. The complete alternator had to be removed to permit adjustment of the balancer chain, a task required every 10,000 km. However several testers complained of an annoying whine at around 5,000 rpm in any gear, which appeared to originate from the balancer housing. Yamaha had also responded to criticism of the innermost workings of the TX750 engine, and especially the number of special tools required to work on it. The 500 was much simplified – the only special tool needed for disassembly being a puller for the alternator. And unlike the 750, the rocker box cover could be removed with the engine in the frame, allowing for quick and easy adjustment of the camshaft follower clearance. The ease-ofmaintenance theme carried through other areas too; the air filter could be changed in seconds, as could the oil filter.
Chassis-wise, the TX500 owed little to its brethren, the frame being a bespoke item. Fuel tank, side covers, mufflers, rear mudguard and seat were also created especially for the 500. The instrument
cluster, while reminiscent of the 750, actually came from the RD350, with a few additions such as an oil pressure light. Included in the various light and readouts was, like the 750, a red light that glowed when the front and/or rear brake was applied, and flashed when the stop light globe filament was blown. The instrument cluster contained no fewer than ten 3 watt bulbs! The key – which also operated the steering lock, fuel tank cap and the seat – sat in the middle of the instrument cluster.
Like the 750, the twin-piston front brake calliper came from the RD350, and although the TX500 weighed 50kg more than the 350, the brake still worked quite well. Standard fitment was the familiar 270mm single disc on the right hand side, but the opposite fork leg had the necessary lugs cast in to permit the fitting of a second disc. Not surprisingly, the 750’s massive rear drum brake had been dropped in favour of a smaller 7-inch item originally found on the XS650. The TX500 had some stiff competition in the halflitre class, ranged against the best selling fourcylinder Honda CB500, and the Suzuki and Kawasaki two strokes, (as well as Yamaha’s own excellent RD350 and later RD400) and it’s fair to say that it failed to cover itself in glory. Admittedly, the 500 avoided the built-in foibles of the 750, but invented new ones for itself. A common complaint was that the engine would not perform properly until fully warmed up sufficiently for the choke to be turned off – a tough call if you were running late for work. It also had an annoying habit of stalling for no apparent reason – usually at traffic lights. This was generally attributed to the design of the Keihin CV carbs, which were also found on the Honda CB450 twin and which often displayed similar traits there. At the low speeds encountered in city traffic, the TX500 could be a bit lurchy – a combination of the very light 180 degree crank and cush drive springs fitted to the clutch. There were also reports of leaking head gaskets due to the warping of the head itself. The head gasket issue came to be more than just a quick fix. Like the TX750 problems, disquiet over the smaller model’s appetite for head gaskets spread via the grape vine and it soon gained a reputation as somewhat of a lemon – a reputation not really deserved. Usually, the head gasket, and the cam box gaskets, would leak and when the head was removed, cracks could often be seen between the spark plug hole and one or more of the valves. In the extreme case, the valve seat would actually fall out, with disastrous consequences. Subsequent models beginning with the XS500C, had the lower cam box and cylinder head cast integrally – a mod that was virtually invisible from the outside but which eliminated one of the problem gasket surfaces. This also had the effect of stiffening the head to help prevent it warping. Like the TX750, the TX500, lasted just two
production years, 1973 and 1974. The second ‘B’ model featured upgraded front forks with revised damping and springs which improved front end stability markedly. Slightly higher gearing, and a revised, quieter exhaust system with a balance pipe between the mufflers, which was claimed to improve torque, also appeared on the ‘B’. The B model also received a hefty price rise in Australia, from $1199 to $1399. In 1975, as if to expunge once and for all the miseries of the TX appellation, it became the XS500C, with the aforementioned one-piece head and lower cambox, changes to the carbs from the troublesome Keihins to 38mm Mikunis, heavier flywheels, slightly lower (8.5:1) compression and much attention to reducing the low-speed transmission lash. A slightly larger capacity (15 litre) fuel tank was fitted. Just why this should have taken so long remains a mystery. The 500 twin was finally laid to rest with the XS500E of 1978, by which time the 500cc class itself was seen as fairly old hat among the current crop of big bore multis. The XS500C had a styling makeover that made it reminiscent of the RD 400, with alloy wheels from the two stroke model and a flat-sided angular fuel tank and side panels, as well as a rear disc brake also sourced from the RD400. Showa front forks replaced the previous Kayaba units, with the brake caliper mounted behind the fork legs instead of in front. European and Australian models were fitted with twin front discs, whereas US models had a single disc.
A trio of TX500s
The newest of the three TX500s featured here is actually a 1975 model XS500B (above), which shared its décor and decal treatment with the XS650 of the same year. Other changes included re-jetting the carburettors with a balance tube between the intake manifolds – mods that did little to cure the poor throttle response. Classic Style Australia at Seaforth, Victoria imported the motorcycle from USA and owner Jon Munn says it is a very tidy and original bike. Apart from the removal of the metal tank and side cover badges in favour of decals, there is very little difference to the earlier models. With only 11,600 miles from new, this TX500 is typical of the Eastern US models that were generally locked away for winter and ridden only in the summer months.
An honest original
Peter Dunster, from Canberra, owns the 1973 TX500A shown below; an original, if slightly faded rose. “When I was about 18 I was working at Genge’s (Canberra dealer) and had a Rocket 3 BSA as my everyday ride. Then we traded one of these TX500s in on a new Suzuki and for a while it became the ‘shop’ bike that we used for odd jobs, and the more I rode it the more I liked it. Compared to the BSA it had an electric start, it was lighter, handled nicely, and went quite well provided you kept the revs up to it. It would cruise at 140km/h
all day if you wanted, and had plenty in reserve for easy overtaking. Like all of these, it had a warranty replacement job on the head, but since then it’s given no trouble at all. The CV carbs are still hard to tune properly but the main problem is low speed running, so I just ride it flat out!”
Our featured bike (above) is Ray Nicholls’ 1973 TX500 in the colour catalogued as Oyster, and sports double disc front brakes as well as a home-fitted Perspex screen. It has been the subject of an ongoing program of restoration and refinement by the owner and he says it is a very pleasant motorcycle to ride. The right side muffler is showing signs of the internal baffles swelling due to rust, which causes ripples in the long chrome megaphone itself – a problem that also beset the TX750. But originals of these mufflers are virtually impossible to come by, so there is no immediate cure at hand. Fortunately the left side muffler is in perfect condition.
for the next day’s run. A CB500 Honda was included in the test to give comparative results. The initial testing equipment from Japan was somewhat ineffective. Fortunately, Bob van der Zyden, with his aircraft engineering background, organised European thermocouple temperature sensors which we fitted inside the sump, rocker covers, under spark plugs, oil pump pickup areas, and around the outside of the engines. The sensitive gauges were taped to the top of the tanks, so that riders could monitor temperatures on the high speed runs. This was all very informative, so Bob took it a step forward, and organised, a high tech (for 1973) data acquisition system. Although it was about the size of a large topbox, we managed to mount it onto the rear of the seat. Due to the bad weather, the expensive unit was wrapped with plastic to keep it safe, and we just kept our fingers crossed that the rider didn’t crash and destroy the unit. When the tapes were read, and the oil temperatures, speed, elevation, and rpm were correlated, it was obvious that temperatures were excessive. The CB500 showed a reasonable 250 degrees, whereas the TX500 showed up to 300 degrees. In the limited workshop conditions we adapted several oil cooling systems. These were fitted in various positions, and it reduced the temperatures to an acceptable level. The oil cooler became an add-on fitment in Europe. This was not the only problem, as the numerous top end gaskets were sandwiched together between the long cylinder studs. Promoted by the elevated engine temperatures, all the gaskets gradually settled, which in turn reduced the tension on the studs, and allowed excessive oil and compression leakage. The Yamaha engineers took all this on board and returned to Japan to redesign a trouble free one-piece cylinder head. With this, and other improvements learned from this testing, the later model XS500 was a well sorted bike. However its reputation had been damaged by the early problems, and its future sales were limited. Yamaha learned from the experience, and was now conscious of the importance of doing serious long distance sustained testing before releasing a new model, rather than rushing the model onto the market to gain initial sales. A funny aside was that the Japanese interpreter who came with the team was only fluent in German, and not in English. When we needed to converse with the Japanese engineers on technical matters, the English had to be translated to Dutch, then to German – and finally to Japanese! It was a long-winded way of getting the correct information. Fortunately most mechanics seem to have a logical common thinking, and we ended up working things out ourselves. It made the interpreter a little redundant, but saved a lot of time....”
The TX500 mill laid bare.
TOP LEFT Twin disc front end was an early option, and standard on the XS500C. CENTRE LEFT Metal side cover badges and tank badges on the original model. ABOVE A happy muffler. Twin-cam top end with four valves per cylinder – novel in 1973.
Bulges in the mufflers signify corroded internal baffles. Decals instead of metal side cover badges was one of few visible changes on the TX500B. 11,614 miles in 45 years – not such a tough life!
ABOVE 1975 Yamaha XS500B. TOP LEFT Keihin CV carbs had a reputation for rough running at low revs. LEFT US models had single disc front ends.
ABOVE Ray Nicholls’ 1973 TX500. Revised styling on the XS500D for 1977.
Peter Dunster’s 1973 TX500A.
RIGHT Rod Tingate on one of the test mules in Germany. LEFT Temperature sensors strapped to the TX500. BELOW LEFT Ready to roll in Germany.