Post by Pardee on Jun 25, 2014 13:48:03 GMT -5
FOAMBOAT
Pictoral History of Vintage Japanese Motorcycles by Cornelius Vanderheuvel
Yamaha's successor to the XS650 was intended to meet two requirements in 1972. It had to follow company policy of 'Two cylinders is enough'. It also had to be an answer to the Superbike invasion. The TX750 turned out to be a loser, although it was a valuable exercise in highlighting a technical lesson the entire motorcycle industry was to find of benefit. In 1969 Yamaha had missed a chance to bring out a Superbike. The RZ201 and GL750, described earlier, were going nowhere. Now the stylists had to find a way to create a particular identity for Yamaha. Their problem was that all types of motorcycles were already on the stocks but manufactured by the 'opposition'. The Suzuki GT750 was there, and the Honda CB750, and the Kawasaki Mach 4 -- plus BMW's R75/5. The Triumph bonneville was a 750 by this time, and there was the Trident too. Norton was still in the race with the Commando. Laverda and Moto Guzzi were marketing the highly regarded GT and SF and V7 models. The first Ducati L-twins had been presented to the world. Yamaha's TX750 was meant to combine the best of both worlds. Alloy wheel rims acknowledged an Italian influence. A double disc at the front and a huge drum brake at the rear were state of the art. Slim, chromed mudguards added to the modern look -- as did omission of the usual fork gaiters. Typical Japanese painting and quality of finish were raised to a higher plane on the TX750. The quality of switches, dials and fittings was unapproachable. The engine, it was decided, had to be a twin-cylinder in deference to conservative tastes. The TX was to take the place of the 650 after a while, so nil vibration was a mandatory design requirement. Performance had to be up to the standard of other modern 750's. All the 750s of the other makers were closely analyzed. The appearance of the engine was considered to be crucial; it had to impress; it had to be modern but, also, sturdy and compact. Style rather took over from function, to the extent that no room was left for an oil-filter! In the end, the oil-filter had to be a very special design and was fitted near the front sprocket. The same sort of situation occurred with the contact-breakers which were, finally, hidden, BSA-like, behind a little cover on the left of the engine.
The engine was to be a twin, but was to behave like a four-cylinder. It even had to look like a four! All this constituted a radical departure from the looks and standards of the XS650. Technically, the cylinder and head configuration was half a Honda 750, with a single camshaft carried in separate bearing blocks. Oil was supplied from underneath the camshaft carriers. However, the finish of the Yamaha was even smoother than that of the Honda. The bottom end of the engine had no ball bearings, like the XS650, but shell bearings, like the Honda; the heavy crankshaft was carried in three of these. In order to eliminate the vibration of a parallel twin, the technicians had worked out a system of counter-rotating shafts. Eccentric weights counter compensated for the vibrations of the crankshaft and pistons. Yamaha dubbed the system the Omni Phase Balancer. All this meant that the twin engine was very heavy. The lubrication system was of the dry-sump type -- again like the Honda -- and was deemed necessary to avoid the enlargement that would have been called for in a wet-sump setup.
The Omni Phase Balancer system worked very well. The engine sounded like a twin but felt, to the rider, like a four. There was absolutely no vibration. That was the good news; but, still, there were major design faults. Those balance shafts were way down in the engine, and oil coming back from the cylinder head was beaten into a froth by the weights, which caused operating temperatures to rise. Froth is not a good lubricant! On top of that, the oil circulation system was very lengthy, and complicated, which meant that it took a long time before the oil had passed through the engine. All parts of the engine suffered from a shortage of oil. Another fault was that the main bearings of the crankshaft were not up to their job. There were only three of them, and for a parallel twin ball bearings are more suitable than shell bearings. The engine did not warn when it was over-stressed, apparantely revving happily to the red zone of the rev-counter, at 7,000 rpm. And the truth was that anything over 4,000 brought problems! Another important aspect concerned the times: in the early seventies in many countries there were no speed limits at all. A Superbike was run at top speed then; as was the TX, because the intention was to turn the TX into a superbike 750. But, as indicated, this was not to be. As all the problems became evident, Yamaha tried hard to solve them. One 'remedial' service bulletin followed another. Some time in 1973 an unknown mechanic realized that there were oil-cooler sets on the market that might be of use. All machines in stock were modified to take these devices, and bikes already sold were called back to the dealers. Incorporating a cooler meant that the oil system was now even more complicated. Two pipes to and from the engine passed behind the oil filter and under the fuel tank to the oil-cooler. Often Chausson coolers, intended for Citroen cars, were used, positioned under the steering head. It was a neat modification. But people had lost confidence in the Yamaha and were not happy. Then, in 1974, Yamaha brought out their own oil-cooler, but made the modification very visible. Two chromed hoses went on the right side of the cylinder block towards the oil-cooler, all in view. The modification certainly helped to make the TX more usable, although full-throttle work was still out of the question. If you stayed below 4,000 rpm, the bike was perfectly servicable; 4,000 in top gear equated to 75 mph. Keeping to that sort of performance, the TX would run to 60,000 miles without any trouble. But the bike could not live up to the 'Superbike' description; it was a good tourer. The public was not impressed, and 1975 saw the end of the TX750. It was then the smaller two-strokes and the XS650 that kept Yamaha afloat -- not forgetting the XT500, of course.
Riding the TX
I write in the present tense, as a present day TX rider...There is no decompressor, such as the XS650 had. The 100cc-larger TX engine takes even more effort to start; the starter motor gets the job done, but moans about the hard work it has to do, and I have to say that the kickstarter is the better alternative, even if it takes all one's weight to overcome compression. A good, 'heavy' sound is heard as the engine ticks over at 800 rpm. Power starts at 1,000 and you can shift up at 2,500. Let it come down to 1,000 in top gear and open the throttle, and the bike will take off without hesitation. The seat is rather high, like that of a Honda CB750 K2. And, also like the K2, this Yamaha has a high center of gravity. Operating the clutch takes a lot of effort, recalling memories of Laverda twins. Riding the Yamaha, everything is in the right place and one can ride for a long time without getting tired. Gearshifting has not deteriorated after all these years; and the transmission works perfectly. The TX is a little heavy in the corners; you need to force it a little -- like a Laverda. It never wiggles or weaves. On the straights it runs like a train. As I've indicated, fast road work is not the Yamaha's main strength. It will reach 110 mph, and at that speed it is free of vibration. But low-down power is the chief virtue of the TX. Treat the TX750 like a British twin, and you can be very happy on the Yam. Oddly, the 1990s are far more suitable for the TX than was its own era, for nowadays we can't indulge in full speed all the time. Legislation and 'social acceptance' don't allow it. If you want to go fast, take a modern motorcycle. Modern oil stays up to 30 degrees cooler in the TX and is also more resilient to frothing. The result is that lubrication problems are more or less a thing of the past. The brakes of a TX750 were excellent for its time, and even today we can judge them as being adequate, or even good. And a TX most certainly is a looker
The lessons taught by this model are still being put into practice today; many motorcycles now have balance shafts.
Pictoral History of Vintage Japanese Motorcycles by Cornelius Vanderheuvel
Yamaha's successor to the XS650 was intended to meet two requirements in 1972. It had to follow company policy of 'Two cylinders is enough'. It also had to be an answer to the Superbike invasion. The TX750 turned out to be a loser, although it was a valuable exercise in highlighting a technical lesson the entire motorcycle industry was to find of benefit. In 1969 Yamaha had missed a chance to bring out a Superbike. The RZ201 and GL750, described earlier, were going nowhere. Now the stylists had to find a way to create a particular identity for Yamaha. Their problem was that all types of motorcycles were already on the stocks but manufactured by the 'opposition'. The Suzuki GT750 was there, and the Honda CB750, and the Kawasaki Mach 4 -- plus BMW's R75/5. The Triumph bonneville was a 750 by this time, and there was the Trident too. Norton was still in the race with the Commando. Laverda and Moto Guzzi were marketing the highly regarded GT and SF and V7 models. The first Ducati L-twins had been presented to the world. Yamaha's TX750 was meant to combine the best of both worlds. Alloy wheel rims acknowledged an Italian influence. A double disc at the front and a huge drum brake at the rear were state of the art. Slim, chromed mudguards added to the modern look -- as did omission of the usual fork gaiters. Typical Japanese painting and quality of finish were raised to a higher plane on the TX750. The quality of switches, dials and fittings was unapproachable. The engine, it was decided, had to be a twin-cylinder in deference to conservative tastes. The TX was to take the place of the 650 after a while, so nil vibration was a mandatory design requirement. Performance had to be up to the standard of other modern 750's. All the 750s of the other makers were closely analyzed. The appearance of the engine was considered to be crucial; it had to impress; it had to be modern but, also, sturdy and compact. Style rather took over from function, to the extent that no room was left for an oil-filter! In the end, the oil-filter had to be a very special design and was fitted near the front sprocket. The same sort of situation occurred with the contact-breakers which were, finally, hidden, BSA-like, behind a little cover on the left of the engine.
The engine was to be a twin, but was to behave like a four-cylinder. It even had to look like a four! All this constituted a radical departure from the looks and standards of the XS650. Technically, the cylinder and head configuration was half a Honda 750, with a single camshaft carried in separate bearing blocks. Oil was supplied from underneath the camshaft carriers. However, the finish of the Yamaha was even smoother than that of the Honda. The bottom end of the engine had no ball bearings, like the XS650, but shell bearings, like the Honda; the heavy crankshaft was carried in three of these. In order to eliminate the vibration of a parallel twin, the technicians had worked out a system of counter-rotating shafts. Eccentric weights counter compensated for the vibrations of the crankshaft and pistons. Yamaha dubbed the system the Omni Phase Balancer. All this meant that the twin engine was very heavy. The lubrication system was of the dry-sump type -- again like the Honda -- and was deemed necessary to avoid the enlargement that would have been called for in a wet-sump setup.
The Omni Phase Balancer system worked very well. The engine sounded like a twin but felt, to the rider, like a four. There was absolutely no vibration. That was the good news; but, still, there were major design faults. Those balance shafts were way down in the engine, and oil coming back from the cylinder head was beaten into a froth by the weights, which caused operating temperatures to rise. Froth is not a good lubricant! On top of that, the oil circulation system was very lengthy, and complicated, which meant that it took a long time before the oil had passed through the engine. All parts of the engine suffered from a shortage of oil. Another fault was that the main bearings of the crankshaft were not up to their job. There were only three of them, and for a parallel twin ball bearings are more suitable than shell bearings. The engine did not warn when it was over-stressed, apparantely revving happily to the red zone of the rev-counter, at 7,000 rpm. And the truth was that anything over 4,000 brought problems! Another important aspect concerned the times: in the early seventies in many countries there were no speed limits at all. A Superbike was run at top speed then; as was the TX, because the intention was to turn the TX into a superbike 750. But, as indicated, this was not to be. As all the problems became evident, Yamaha tried hard to solve them. One 'remedial' service bulletin followed another. Some time in 1973 an unknown mechanic realized that there were oil-cooler sets on the market that might be of use. All machines in stock were modified to take these devices, and bikes already sold were called back to the dealers. Incorporating a cooler meant that the oil system was now even more complicated. Two pipes to and from the engine passed behind the oil filter and under the fuel tank to the oil-cooler. Often Chausson coolers, intended for Citroen cars, were used, positioned under the steering head. It was a neat modification. But people had lost confidence in the Yamaha and were not happy. Then, in 1974, Yamaha brought out their own oil-cooler, but made the modification very visible. Two chromed hoses went on the right side of the cylinder block towards the oil-cooler, all in view. The modification certainly helped to make the TX more usable, although full-throttle work was still out of the question. If you stayed below 4,000 rpm, the bike was perfectly servicable; 4,000 in top gear equated to 75 mph. Keeping to that sort of performance, the TX would run to 60,000 miles without any trouble. But the bike could not live up to the 'Superbike' description; it was a good tourer. The public was not impressed, and 1975 saw the end of the TX750. It was then the smaller two-strokes and the XS650 that kept Yamaha afloat -- not forgetting the XT500, of course.
Riding the TX
I write in the present tense, as a present day TX rider...There is no decompressor, such as the XS650 had. The 100cc-larger TX engine takes even more effort to start; the starter motor gets the job done, but moans about the hard work it has to do, and I have to say that the kickstarter is the better alternative, even if it takes all one's weight to overcome compression. A good, 'heavy' sound is heard as the engine ticks over at 800 rpm. Power starts at 1,000 and you can shift up at 2,500. Let it come down to 1,000 in top gear and open the throttle, and the bike will take off without hesitation. The seat is rather high, like that of a Honda CB750 K2. And, also like the K2, this Yamaha has a high center of gravity. Operating the clutch takes a lot of effort, recalling memories of Laverda twins. Riding the Yamaha, everything is in the right place and one can ride for a long time without getting tired. Gearshifting has not deteriorated after all these years; and the transmission works perfectly. The TX is a little heavy in the corners; you need to force it a little -- like a Laverda. It never wiggles or weaves. On the straights it runs like a train. As I've indicated, fast road work is not the Yamaha's main strength. It will reach 110 mph, and at that speed it is free of vibration. But low-down power is the chief virtue of the TX. Treat the TX750 like a British twin, and you can be very happy on the Yam. Oddly, the 1990s are far more suitable for the TX than was its own era, for nowadays we can't indulge in full speed all the time. Legislation and 'social acceptance' don't allow it. If you want to go fast, take a modern motorcycle. Modern oil stays up to 30 degrees cooler in the TX and is also more resilient to frothing. The result is that lubrication problems are more or less a thing of the past. The brakes of a TX750 were excellent for its time, and even today we can judge them as being adequate, or even good. And a TX most certainly is a looker
The lessons taught by this model are still being put into practice today; many motorcycles now have balance shafts.