ARTICLE: Classic Mechanics (December 1989 / January 1990)
Jun 25, 2014 13:35:25 GMT -5
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Post by Pardee on Jun 25, 2014 13:35:25 GMT -5
YAMAHA TX500/750: A QUESTION OF BALANCE
Brian Crichton investigates a rare and collectable Japanese twin
Classic Mechanics, December 89/January 90
Fifteen years ago a bike had to boast a great looking engine as well as cycle parts if it was going to appeal to an over 250cc enthusiast.
Such a machine was Yamaha's TX500 four stroke vertical twin. The engine was its main feature thanks to double overhead cams and eight valves--really exotic specification in those days. There was more trickery inside too, in the shape of Yamaha's 'Omni-Phase Balancer'.
In its day the TX500 represented a high-tech pinnacle of Yamaha four stroke technology. The Japanese factory had come on strong in the four stroke bike world in a very short time. Known for musical instruments and two stroke machines, they hadn't bothered with four strokes until they announced the single overhead cam XS650 to the world in 1969. But that's another story.
We in Britain didn't see this model until the XS-2 version arrived in 1972. In that same year Yamaha had produced the sohc TX750 and a few months later announced the dohc 500. The 750 was only briefly imported and the 500 was not readily available until 1975.
It appeared that in a comparatively short space of time that Yamaha was making big four stroke strides. In fact, they had more depth of knowledge in that area than many enthusiasts realised because of earlier involvement with car manufacturer Toyota. In that respect Yamaha is in the news again with regard to their five-valve head technology tie-up with Zakspeed's Formula One car team for 1989.
Going back 20 years, Yamaha cooperated with Toyota in producing the two litre, six cylinder, twin-cam Toyota GT, a stunning looker. It went on sale in 1967 and only 500 were built -- plus two open top versions made for the James Bond film 'You Only Live Twice'.
Yamaha was also involved in the work on a single cam 1600cc four cylinder motor which has become one of the most successful car powerplants. It has powered various Toyota saloons including the popular Celica series.
So Yamaha was quite well set up to move into four strokes, and with the TX series they were aiming to beat the traditional vibes of the vertical twin. The Omni-Phase system (no doubt it would be called OPS these days) worked too, but only for a time. It proved to be the Achilles heel and the model quickly earned the reputation for unreliability.
The 750 didn't sell very well and the 500, despite its super spec, was a slow seller too. For this reason the first versions of the 750 and 500 are rare and thus collectable. The TX became the XS500. It was never a very popular machine and production was stopped in the later '70s. Towards the end of its lifespan dealers were offering big discounts to tempt buyers.
Claimed by Yamaha to be the first twin cam eight valve twin in the world, the TX500 was heralded by 'Motor Cycle News' in June, 1973, when it was announced that it would arrive in October with a price tab of £656.18.
By 1975 when tested by myself for 'MCN' the price had gone up to £825. For many years now a 500 has been regarded as quite a small bike, but in those days, though 750s had been with us for a while, a high performance 500 was still regarded as something special. It was a thrill to get behind the bars of this Japanese exotic and experience the cammy nature of the engine. By balancing throttle control and revs it was possible to get the front wheel off the ground in first year through sheer power, impressive considering the machine's bulk.
With the throttle rolled off, engine braking was very pronounced. Add to that the twin front discs and you could slow down pretty quickly. Like the engine, the powerful brakes needed some getting used to.
In an acceleration test against my brother's '68 650 Triumph Bonneville the Yamaha had just the edge -- quite a coup for a 500. Claimed power was 48bhp at 8500rpm and claimed top speed 112mph. The test bike came well short of that, as explained in our extract from the 1975 test.
While Yamaha had already produced a rotary machine for 1972, the RZ201 (there's another story) they had decided not to join Honda and Kawasaki in producing fours, or BSA-Triumph in producing threes. Instead they stuck to the twin and decided to beat vibration with their balancer system which started a new trend. Various versions have since been employed to damp the vibes in all sorts of machines including two stroke mopeds.
The Yamaha system was first used in the dry sump 750 which employed two counter weights. For the later 500, Yamaha employed a single chain driven balancer. Basically the idea was to cancel the forces which produce vibration with equal and opposite forces. Yamaha produced the diagrams and explanations at the time to show how the twin weight system worked in the TX750.
For the 500, Yamaha decided on an oversquare layout to reduce piston speeds and pent roof style heads to accommodate the four valves per cylinder with centrally located spark plug. Two 32mm Keihin constant velocity carbs fed the easy breathing mill which was angled forward in the twin downtube frame.
While British twins usually had 360-degree cranks and long-stroke dimensions Yamaha went for a 180-degree layout and the revvier short-stroke design.
Everyone's talking about unleaded fuel nowadays. It's nothing new -- Yamaha designed the TX500 to run on unleaded by using sintered titanium alloy valve seats press fitted into the head. This and the head design for efficient combustion plus the use of CV carbs were targeted at keeping the anti-pollution lobby happy in Dollar rich California where the main sales potential lay at the time.
While the two valve head 750 was a dry sump engine the completely different 500 was wet sump design. Both were five speeders with points ignition. To assist silencing on the 750 the exhaust ports were interconnected. This can be seen very clearly on the picture of the 750.
Instrumentation was very comprehensive for the time and even included a warning light for when the rear drum brake lining was worn below 2mm plus a warning light to indicate loss of main dip or tail light.
The 743cc model had 80 x 74mm bore x stroke and developed a claimed 63hp at 7500rpm. Both models had electric start plus kickstart lever. Claimed top speed for the 750 was over 120mph (195kph).
A few continental sidecar-cross crews tried this engine to compete against the all conquering Norton twin, but the balance weight mechanism which revolved at crankshaft speed couldn't stand the punishment.
Yamaha justified their decision to stick to a twin and not follow the fours by saying: 1) A twin isn't as wide and so allows a better riding position and easier handling; 2) A four is more complicated to build, has more parts and requires more maintenance; 3) It's more difficult to cool all the cylinders of a four and lubrication would be erratic; 4) A twin is more compact and so allows more space for anti-pollution devices.
Pollution was and still is a significant factor. In the early '70s it was receiving a great deal of publicity in America. Yamaha made a special effort in this respect with a PCV (positive crankcase ventilation) system which routed blowby gas back to the air chamber via a reed valve. Oil which collected at the bottom of the chamber was allowed to drain back into the engine via a pipe (see diagram).
Electrics were 12 volt and the disk brake(s) -- only one for the American market 500 -- featured double pistons. Yes, the TX500 was quite a show stopper when it made its public debut in October 1972 in Tokyo.
EXTRACTS FROM THE 1975 TX500 TEST
"Yamaha's TX500 eight-valver is a hard character to get to know.
"The power in the low gears comes in so suddenly and strongly, and the brakes are so terrific that assumed familiarity during early acquaintance is asking for rejection.
"The TX has to be taken gradually. It is not an easy bike to ride.
"Yamaha's Omni Phase balancing system works a treat. For a vertical twin the TX500 is incredibly smooth.
"A 500 Honda-4 owner tried it and said the smoothness was 'comparable'. Of the two he thought the Honda was the smoothest...well he owns one! Certainly there is very little in it.
"The only drawbacks seem to be in increased engine weight and increased engine noise. Bobweights give off a mechanical chirrup which loses itself under speed. One of the weights can be seen through the oil filler orfice behind the cylinder.
"The sound from the big silencers is very exciting. Reminiscent of a Honda twin the note reaches even more of a frenzied crescendo suitably mute to leave townspeople unoffended.
"Power comes in so suddenly that the rider has to be extremely wary when turning it on out of low gear corners...particularly in the wet!
"The same when turning the power off. With the eight valves popping in and out under the chain driven cams and the balancers whirring away, things slow down pretty quickly.
"Whipping the throttle open in first and snapping shut when the bike is on the power band brings to mind a parabolic curve on a graph to visually portray the bike's characteristics.
"It is possible with the clutch fully engaged to wheelie in first by giving a light tug on the bars to help the tyre off the deck. This demonstrates the impressive acceleration capability of Yamaha's light heavyweight.
"The standing-quarter time of 13.596 secs is further witness to its fleetness off the line, but the exact ton through the speed trap was a disappointment considering the exotic technical specification.
"An explanation was later provided by Mitsui boss Jack Hird who claimed that when it was stripped down after the test three slightly bent valves were found in the left cylinder -- two exhaust and one inlet.
"Also, at no time during the test would the Yamaha start on the button or on the kickstart after a night outdoors.
"By far the simplest method to bring the beast into life was to bump it. An embarrassment on such a well cut and well finished item of transport."
Brian Crichton investigates a rare and collectable Japanese twin
Classic Mechanics, December 89/January 90
Fifteen years ago a bike had to boast a great looking engine as well as cycle parts if it was going to appeal to an over 250cc enthusiast.
Such a machine was Yamaha's TX500 four stroke vertical twin. The engine was its main feature thanks to double overhead cams and eight valves--really exotic specification in those days. There was more trickery inside too, in the shape of Yamaha's 'Omni-Phase Balancer'.
In its day the TX500 represented a high-tech pinnacle of Yamaha four stroke technology. The Japanese factory had come on strong in the four stroke bike world in a very short time. Known for musical instruments and two stroke machines, they hadn't bothered with four strokes until they announced the single overhead cam XS650 to the world in 1969. But that's another story.
We in Britain didn't see this model until the XS-2 version arrived in 1972. In that same year Yamaha had produced the sohc TX750 and a few months later announced the dohc 500. The 750 was only briefly imported and the 500 was not readily available until 1975.
It appeared that in a comparatively short space of time that Yamaha was making big four stroke strides. In fact, they had more depth of knowledge in that area than many enthusiasts realised because of earlier involvement with car manufacturer Toyota. In that respect Yamaha is in the news again with regard to their five-valve head technology tie-up with Zakspeed's Formula One car team for 1989.
Going back 20 years, Yamaha cooperated with Toyota in producing the two litre, six cylinder, twin-cam Toyota GT, a stunning looker. It went on sale in 1967 and only 500 were built -- plus two open top versions made for the James Bond film 'You Only Live Twice'.
Yamaha was also involved in the work on a single cam 1600cc four cylinder motor which has become one of the most successful car powerplants. It has powered various Toyota saloons including the popular Celica series.
So Yamaha was quite well set up to move into four strokes, and with the TX series they were aiming to beat the traditional vibes of the vertical twin. The Omni-Phase system (no doubt it would be called OPS these days) worked too, but only for a time. It proved to be the Achilles heel and the model quickly earned the reputation for unreliability.
The 750 didn't sell very well and the 500, despite its super spec, was a slow seller too. For this reason the first versions of the 750 and 500 are rare and thus collectable. The TX became the XS500. It was never a very popular machine and production was stopped in the later '70s. Towards the end of its lifespan dealers were offering big discounts to tempt buyers.
Claimed by Yamaha to be the first twin cam eight valve twin in the world, the TX500 was heralded by 'Motor Cycle News' in June, 1973, when it was announced that it would arrive in October with a price tab of £656.18.
By 1975 when tested by myself for 'MCN' the price had gone up to £825. For many years now a 500 has been regarded as quite a small bike, but in those days, though 750s had been with us for a while, a high performance 500 was still regarded as something special. It was a thrill to get behind the bars of this Japanese exotic and experience the cammy nature of the engine. By balancing throttle control and revs it was possible to get the front wheel off the ground in first year through sheer power, impressive considering the machine's bulk.
With the throttle rolled off, engine braking was very pronounced. Add to that the twin front discs and you could slow down pretty quickly. Like the engine, the powerful brakes needed some getting used to.
In an acceleration test against my brother's '68 650 Triumph Bonneville the Yamaha had just the edge -- quite a coup for a 500. Claimed power was 48bhp at 8500rpm and claimed top speed 112mph. The test bike came well short of that, as explained in our extract from the 1975 test.
While Yamaha had already produced a rotary machine for 1972, the RZ201 (there's another story) they had decided not to join Honda and Kawasaki in producing fours, or BSA-Triumph in producing threes. Instead they stuck to the twin and decided to beat vibration with their balancer system which started a new trend. Various versions have since been employed to damp the vibes in all sorts of machines including two stroke mopeds.
The Yamaha system was first used in the dry sump 750 which employed two counter weights. For the later 500, Yamaha employed a single chain driven balancer. Basically the idea was to cancel the forces which produce vibration with equal and opposite forces. Yamaha produced the diagrams and explanations at the time to show how the twin weight system worked in the TX750.
For the 500, Yamaha decided on an oversquare layout to reduce piston speeds and pent roof style heads to accommodate the four valves per cylinder with centrally located spark plug. Two 32mm Keihin constant velocity carbs fed the easy breathing mill which was angled forward in the twin downtube frame.
While British twins usually had 360-degree cranks and long-stroke dimensions Yamaha went for a 180-degree layout and the revvier short-stroke design.
Everyone's talking about unleaded fuel nowadays. It's nothing new -- Yamaha designed the TX500 to run on unleaded by using sintered titanium alloy valve seats press fitted into the head. This and the head design for efficient combustion plus the use of CV carbs were targeted at keeping the anti-pollution lobby happy in Dollar rich California where the main sales potential lay at the time.
While the two valve head 750 was a dry sump engine the completely different 500 was wet sump design. Both were five speeders with points ignition. To assist silencing on the 750 the exhaust ports were interconnected. This can be seen very clearly on the picture of the 750.
Instrumentation was very comprehensive for the time and even included a warning light for when the rear drum brake lining was worn below 2mm plus a warning light to indicate loss of main dip or tail light.
The 743cc model had 80 x 74mm bore x stroke and developed a claimed 63hp at 7500rpm. Both models had electric start plus kickstart lever. Claimed top speed for the 750 was over 120mph (195kph).
A few continental sidecar-cross crews tried this engine to compete against the all conquering Norton twin, but the balance weight mechanism which revolved at crankshaft speed couldn't stand the punishment.
Yamaha justified their decision to stick to a twin and not follow the fours by saying: 1) A twin isn't as wide and so allows a better riding position and easier handling; 2) A four is more complicated to build, has more parts and requires more maintenance; 3) It's more difficult to cool all the cylinders of a four and lubrication would be erratic; 4) A twin is more compact and so allows more space for anti-pollution devices.
Pollution was and still is a significant factor. In the early '70s it was receiving a great deal of publicity in America. Yamaha made a special effort in this respect with a PCV (positive crankcase ventilation) system which routed blowby gas back to the air chamber via a reed valve. Oil which collected at the bottom of the chamber was allowed to drain back into the engine via a pipe (see diagram).
Electrics were 12 volt and the disk brake(s) -- only one for the American market 500 -- featured double pistons. Yes, the TX500 was quite a show stopper when it made its public debut in October 1972 in Tokyo.
EXTRACTS FROM THE 1975 TX500 TEST
"Yamaha's TX500 eight-valver is a hard character to get to know.
"The power in the low gears comes in so suddenly and strongly, and the brakes are so terrific that assumed familiarity during early acquaintance is asking for rejection.
"The TX has to be taken gradually. It is not an easy bike to ride.
"Yamaha's Omni Phase balancing system works a treat. For a vertical twin the TX500 is incredibly smooth.
"A 500 Honda-4 owner tried it and said the smoothness was 'comparable'. Of the two he thought the Honda was the smoothest...well he owns one! Certainly there is very little in it.
"The only drawbacks seem to be in increased engine weight and increased engine noise. Bobweights give off a mechanical chirrup which loses itself under speed. One of the weights can be seen through the oil filler orfice behind the cylinder.
"The sound from the big silencers is very exciting. Reminiscent of a Honda twin the note reaches even more of a frenzied crescendo suitably mute to leave townspeople unoffended.
"Power comes in so suddenly that the rider has to be extremely wary when turning it on out of low gear corners...particularly in the wet!
"The same when turning the power off. With the eight valves popping in and out under the chain driven cams and the balancers whirring away, things slow down pretty quickly.
"Whipping the throttle open in first and snapping shut when the bike is on the power band brings to mind a parabolic curve on a graph to visually portray the bike's characteristics.
"It is possible with the clutch fully engaged to wheelie in first by giving a light tug on the bars to help the tyre off the deck. This demonstrates the impressive acceleration capability of Yamaha's light heavyweight.
"The standing-quarter time of 13.596 secs is further witness to its fleetness off the line, but the exact ton through the speed trap was a disappointment considering the exotic technical specification.
"An explanation was later provided by Mitsui boss Jack Hird who claimed that when it was stripped down after the test three slightly bent valves were found in the left cylinder -- two exhaust and one inlet.
"Also, at no time during the test would the Yamaha start on the button or on the kickstart after a night outdoors.
"By far the simplest method to bring the beast into life was to bump it. An embarrassment on such a well cut and well finished item of transport."