Post by Pardee on Jun 25, 2014 13:30:30 GMT -5
YAMAHA TX750
Motorcyclist Test by Dave Holeman
Motorcyclist, December 1972
Yamaha has innovated, from the ground up, the most novel vertical twin ever to be offered to the consumer. It's an engineering masterpiece that only the Japanese could consider feasible and affordable to manufacture in mass and sell over the counter. It has a most novel counterbalance system to smooth out inherent vertical twin vibration, electric gadgetry that will dazzle IBM engineers, a new and mostly appealing sound, and other design wizardry from both the new and old schools. Performance, comfort and handling are mostly elementary; so let's take a trip through the innovative design approaches that the Hamamatsu engineers have chosen to use.
The all aluminum engine uses a chain-driven single overhead camshaft to actuate the valve system. Driven at half crankshaft speed, the camshaft rides in a matching pair of cam blocks fitted with plain bearings. As we go through the engine, you will notice virtually a complete lack of needle or roller bearings so prevalent with Japanese engines. Rather Yamaha has chosen to use plain bearings for this low-revving engine. Cast into the cylinder head are right- and left-side vent tunnels that run from the front to rear and over each spark plug for maximum cooling. These are located behind the cast crossover tube joining the exhaust ports. Each cylinder uses one each exhaust and intake valve that, incidentally, are identical to the XS2 650cc items except for longer stem length. Each valve is returned to seating position by an inner and outer pair of springs. Adjustment of the valves is accomplished quickly by removing each of the four inspection caps and setting them at .05mm (.002") with the feeler gauge included in the tool kit.
Under the head is the single-piece cylinder casting with cam chain tunnel and cast-in steel sleeves. We would venture to say that Yamaha is thinking further in the future than we might suspect in reference to a bigger engine. Visual inspection of the cylinder shows more than enough meat there to enlarge the bore alone enough to get 900cc. By both boring and stroking, we could easily envision a 1000cc engine in the distant future...or sooner for you hot rodders? Of course the cast-in steel liners give maximum heat transfer to the aluminum finning for optimum cooling.
Now we're going to get to the real meat of what makes Yamaha's 750 the most interesting and innovative of all the '73 machines so far. The crankshaft is forged steel with the two rod throws positioned inline. That is, both pistons rise and fall at the same time. The huge 360-degree crank rests in three plain automotive type bearings. Each forged steel connecting rod uses a plain big end bearing with the caps retained by hardened allen bolts. Interestingly, they thread into the caps rather than having locking nuts. Since this reduces the free length of the bolt, torquing stretch is minimized. The small ends of the rods are coated with special copper plating to eliminate the need for bearings. In this manner the rod rides directly on the piston wrist pin.
Novelty supreme comes from the use of vibration to eliminate vibration. Like fire to fight fire. To make sure we had all the facts straight, a day was spent studying the TX 750 engine, chassis and suspension units with Yamaha's top three resident engineers--Nick Shiokawa, Tak Takahashi and Sunny Nagashima. Development of the TX 750 started only two years ago. At the outset the Yamaha engineers knew of the vibration problem inherent with a vertical twin. Therefore, they searched for a method, other than rubber mounting, that could be used to not just reduce but eliminate vibration. Accordingly a Japanese college professor came up with the idea of using opposing vibration to offset or override the shake of an unbalanced engine. With this in mind, it must be noted that the TX 750 engine was designed from the outset with the counterbalance vibration-proof system included. It was not an afterthought.
The idea of using a counterbalance system is not new or original as Saab has used one for years on their 1500cc and late 1700cc V-four automobile engines. Actual development of the counterbalance system was undertaken and completed by Ford of Germany in the early Fifties for their V-four-powered Taurus. Yamaha's system uses the same principle of having two counterweights revolving at the same speed as the crankshaft. The TX 750 has two flywheel counterbalancers located at the lower end of the crank. Next to the cam chain drive sprocket, on the center of the crankshaft, are the teeth used to drive the counterbalancers. The chain runs such that the two counterbalancers spin in opposite directions. The larger forward counterbalancer sets up a vertical vibration as it spins opposite of the crank. Behind it, the smaller counterbalancer sets up a horizontal vibration while spinning with the crank. The simultaneous and opposing vibrations set up a 360-degree counter-vibration action that overcomes much of the shaking caused by the engine.
Power is transmitted to the gearbox from the right side of the crankshaft via straight-cut primary gears. The clutch is a large 14-plate, oil-bathed unit. Transmission gears are appropriately large to match the power generated by the 45-cubic-inch engine. Naturally a five-speed transmission is used. Shifting chores are handled with three shift forks riding on and actuated by a shift cam drum. This large assembly is located directly above the gearbox unit. Unfortunately, the TX 750 has succumbed to the irritable transmission gear clunk and driveline whiplash that are inherent with many Japanese big-inch road machines. When shifting from neutral to low, there is a tremendous clunk from the gearbox, then a sharp lurch forward. It's almost mystifying why the Japanese don't use the same shifting system that the British twins and Ducati incorporate to eliminate this irritable clunk.
The clutch must be used religiously to smooth out shifting up and down between gears. Here also was an area of irritation as the clutch adjustment varies greatly with engine temperature and usage. If adjusted properly when cool or even warm, the free play would grow rapidly and in proportion with operating temperature. When hot (under normal use) the excessive lever free play minimizes the disengagement plate tolerance and makes the clutch drag. If adjusted properly (at the lever), the clutch slips when the engine is cool. On the other hand the oil drag on the clutch plates when cold inhibits smooth shifting. Upon asking why the factory hadn't decided to use a more predictable dry clutch, we found that they had tested one but found it made too much plate drag noise when disengaged. Thinner oil such as ATF can't be used to control dragging as the engine, gearbox, and clutch use the same oil supply.
Speaking of oil brings to mind the super-complex lubrication system incorporated by the Yamaha. The TX 750 uses a dry sump system where oil is forced through the engine to critical parts and a separate container is used as a central reservoir for the lubricant. In this system oil is not retained in the sump. Starting out with the reservoir tank (hidden behind the right side panel cover), lubricant flows down the middle inlet on the top of the crankcases and into a central passageway that leads to the trichoid oil pump. The trichoid force pump and return (to oil tank) pump are driven at half crankshaft speed from the clutch, but also on the same drive shaft is a centrifugal magnetic filter and finally the contact points. Meanwhile, the lubricant is forced from the trichoid pump through more passageways to a special check valve, then into a multi-disc filter. From here oil goes back to the drive shaft and the centrifugal magnetic filter. At this point the oil split up with 80% pressure fed through the crankshaft and big end rod bearings with the small end and cylinder walls splash fed. The other 20% runs up the oil line behind the cylinder to the valve train and cam chain. After performing its lubricating chores here the oil drops into the sump and through a screen filter. Now the return pump picks up the oil and splits it up again with 80% going back to the tank and 20% forced through the shift mechanism, then dropping onto the gears for lubrication. Complex? You bet.
With a claimed output of 63 horsepower at 6500 rpm we found the performance pleasant, though not shattering by any means. Even if the power claim is correct it's just not sufficient to overcome the very fat weight figure of 518 pounds wet. Comparing weight figures, we note that this is seven pounds lighter than the Honda 750 Four, but five more than the XLH Sportster, 51 greater than the XS2 650 and a whopping 60 above the BMW 750. The total effect of this fatness is moderate acceleration times and speeds and excessive stopping distances. We spent a day at Irwindale Raceway with Yamaha engineer Sunny Nagashima observing acceleration, braking and speedometer correction tests on 350, 650 and 750 Yamahas. Acceleration wise the TX 750 is a moderate performer. The best Tech Editor Tony Murphy could record was 13.87 e.t. and 95.84 mph, both on the same run. This would compare as slower than the Honda 750 and Moto Guzzi but slightly quicker than a BMW or full dress Harley 74.
The performance isn't all bad, just disappointing. Fortunately the engine has good low-speed pulling power from 2500 up to 6500. It doesn't surge or bog, just pulls at a moderate, constant rate. Everyone who rode the machine found it easy to ride, predictable and smooth. We spent 400 of our 1200 miles riding two-up and found other interesting information. Riding through winding mountain roads taxes the engine efficiency and there is a noticable increase in operating temperature and resulting decrease in performance. Sunny informed us that he had recorded oil temperature readings of 240 degrees during testing in 115-degree desert heat. This was with a solo rider. We found the engine was happy to hold 80 mph for miles and miles across the desert floor with two up, but again with a sharp increase in operating temperature over riding solo. We are sure that an engine oil cooler would be of benefit for the touring rider. The enclosed oil tank, small fins and polished castings just are not complementary to good heat dissipation.
We did find that the engine runs clean as a whistle and didn't lose a drop of oil during the test. However the engine and oil tank breathers spewed a constant mist over the tail section of the machine and left small puddles in our otherwise clean garage. Enough oil accumulated on the centerstand to cause this writer to drop the machine once when a foot slipped off it while trying to raise the heavyweight to center. It left a nasty gash in a friend's truck side. More breather restriction and rerouting of tubes is in order.
Fuel consumption was also disappointing for a touring mount. Our overall average came out to 34.4 mpg, identical to a XLH Sportster. Our worst was 29.1 and best was 39.6 mpg. With the small 14-liter (3.7-gallon) gas tank we once ran it bone dry at an embarrassing 108 miles. With this as a minimum it would make it impossible to travel cross country in many areas without a backpack of extra fuel. Oddly enough the Japanese domestic model comes standard with a 19-liter (5.0-gallon) tank. Don't get your hopes up yet about this as an option. The Japanese tank isn't available here and would be an extra if it were. Considering it would involve having to install a shorter seat with the bigger tank, the cost would be more than prohibitive.
The TX 750 comes with a superb hydraulic disc front brake and a good drum rear binder. Both are identical to the XS2 650. The front disc uses a dual puck actuating system rather than one forcing the disc against a fixed pad. This permits side flotation of the pucks (in the case of an irregular or warped surface) and therefore equal pressure and friction on the disc. Our braking tests gave us a best stop from 60 mph of 118' 6" and from 30 mph, 29' 9". Average for a 3000-ound sedan is 130 feet at 60 mph and 30 at 30. It must be noted that the TX 750 is quite difficult to haul down at 60 mph due to the tremendous weight transfer on the front. The XS2 650 is easier and more controllable to stop and at 60 mph will come to a halt in 100 feet flat.
Probably the real pride and joy of the Yamaha is the instrumentation and electrics. The very handsome 140 mph speedometer comes with a tripmeter and the super-easy to read 10,000 rpm tachometer has steady needle movement. Both would fit nicely in a Ferrari. The clever warning lights are in preparation for U.S.A. and Japanese safety standard requirements. There's a cluster of three red warning lights at the bottom of the instrument panel. The left light warns of insufficient oil pressure. Center is a light that comes on when the rear brake lining wear exceeds 2mm (.080") and is actuated by a sensor resting on the shoe cam. But for some odd reason there is not a similar warning light for the front brake which takes 70-80% of the stopping work load. Right is a light that comes on and off with the brake lamp. going a step further, this light will flash on and off when the brake(s) is applied and the bulb is burned out, to indicate need for replacement. Below the two (right and left) turn indicator lights is an interesting white lamp. This one comes on in the event that the high or low headlamp beam fails, but at the same time it transfers the unused current to the remaining good light. For daytime riding it ensures one headlight beam will burn at all times.
The soft fork springs and poor shock damping doubtlessly minimize what feels to be a fairly good handling chassis. The front forks use a marginal double damping system that allows the units to constantly clack when topping. The straight-wound springs are rated at 35.7 inch/pounds and give a soft, cushy ride. Unfortunately they sacked out prematurely from lack of sufficient pre-load and allowed the machine to rest 2 inches below its 5 1/4 inches of travel. When completely collapsed the wheelbase shortened almost two inches and also lessened the trail to cause quick handling under load. The rear shock springs are progressive-wound with a minimum tension of 118 inch/pounds and a maximum of 138 for the three inches of travel. The springs felt right for the machine, but damping was absent, allowing that pogo stick action to occur. These springs would probably work well over Koni dampers.
High-speed cruising with the TX 750 leaves something to be desired in the comfort department. The wide handlebars are swept back too much and place the rider's wrist at an awkward angle. riding two-up accentuates this and causes premature rider fatigue plus leading to high-speed squirming as the pilot holds on to the grips too hard. The saddle uses foam that is far too soft. You can push to the steel base of the seat with one finger. This lets the rider's and passenger's tail bones rest on the hard bottom and suffer that electric-type vibration. Yamaha might think of studying the saddles of the BMW and Moto Guzzi for both depth and length.
For the first time Yamaha has used alloy rims on one of its street mounts. The D.I.D. aluminum rims are replicas of the famous Akront hoops from Italy. No tire bead locks are used. The rear 4.00x18-inch tire works very well for all road conditions. The front 3.50x19-inch rib-style rubber is tremendously overworked with two up or during hard braking. A K87 Dunop would be an improvement.
Starting chores are handled by a chain-driven electric motor mounted in the forward part of the engine. We sometimes had to crank it considerably to get the fires burning, but it always did the trick. Carburetion is the same as the XS2 650, a matching pair of 38mm Mikuni Solex vacuum slide (SU type) units. Performance is smooth under most conditions, though they do spit erratically, like the slide diaphrams are vapor locking when the engine gets hot.
After completing our lengthy test on the TX 750 we tried to evaluate whether all the engine complexity, counterbalance system and machine fatness were necessary. Engine complexity such as Yamaha has used might be justified if it performed in the top ten percent of its class, but this isn't the case. Servicing is indeed going to be an intricate task for any mechanic. The counterbalance system is novel, but also costly, heavy and involved for its level of efficiency. The old Matchless 650 twin was a very smooth engine, every bit as much as the TX 750, yet a compact 40-inch vertical twin free of extra weight, counterbalance flywheels and complexity. The heaviness of the machine is certainly not appealing or advantageous. Weighing in at more than the electric start Sportster seems hard to justify.
The official price of $1554 has just been established. This will drop the Yamaha under the hornet's nest comprised of Honda's 750 Four, Norton's Commando twin, Moto Guzzi's twin, Triumph's triple, Ducati's twin and Kawasaki's 900 four. The void that Yamaha's TX 750 was intended to fill was narrow at the outset; a super-smooth twin that would make a multi or rubber mounting of engine unnecessary. Did they pull it off? Mechanically, it hardly seems likely. For the TX is not the fastest, quickest, lightest or smoothest tourer. Neither is it the most comfortable, most economical or best handling roadster. Test evaluations place it down the road, in varying degrees, in all categories except electrical safety equipment. Price-wise, The TX is meeting head on with Triumph's venerable 650 and even their new 750 vertical twins. Tough Competition. Let's just say that Yamaha is still king of the two-strokes.
Motorcyclist Test by Dave Holeman
Motorcyclist, December 1972
Yamaha has innovated, from the ground up, the most novel vertical twin ever to be offered to the consumer. It's an engineering masterpiece that only the Japanese could consider feasible and affordable to manufacture in mass and sell over the counter. It has a most novel counterbalance system to smooth out inherent vertical twin vibration, electric gadgetry that will dazzle IBM engineers, a new and mostly appealing sound, and other design wizardry from both the new and old schools. Performance, comfort and handling are mostly elementary; so let's take a trip through the innovative design approaches that the Hamamatsu engineers have chosen to use.
The all aluminum engine uses a chain-driven single overhead camshaft to actuate the valve system. Driven at half crankshaft speed, the camshaft rides in a matching pair of cam blocks fitted with plain bearings. As we go through the engine, you will notice virtually a complete lack of needle or roller bearings so prevalent with Japanese engines. Rather Yamaha has chosen to use plain bearings for this low-revving engine. Cast into the cylinder head are right- and left-side vent tunnels that run from the front to rear and over each spark plug for maximum cooling. These are located behind the cast crossover tube joining the exhaust ports. Each cylinder uses one each exhaust and intake valve that, incidentally, are identical to the XS2 650cc items except for longer stem length. Each valve is returned to seating position by an inner and outer pair of springs. Adjustment of the valves is accomplished quickly by removing each of the four inspection caps and setting them at .05mm (.002") with the feeler gauge included in the tool kit.
Under the head is the single-piece cylinder casting with cam chain tunnel and cast-in steel sleeves. We would venture to say that Yamaha is thinking further in the future than we might suspect in reference to a bigger engine. Visual inspection of the cylinder shows more than enough meat there to enlarge the bore alone enough to get 900cc. By both boring and stroking, we could easily envision a 1000cc engine in the distant future...or sooner for you hot rodders? Of course the cast-in steel liners give maximum heat transfer to the aluminum finning for optimum cooling.
Now we're going to get to the real meat of what makes Yamaha's 750 the most interesting and innovative of all the '73 machines so far. The crankshaft is forged steel with the two rod throws positioned inline. That is, both pistons rise and fall at the same time. The huge 360-degree crank rests in three plain automotive type bearings. Each forged steel connecting rod uses a plain big end bearing with the caps retained by hardened allen bolts. Interestingly, they thread into the caps rather than having locking nuts. Since this reduces the free length of the bolt, torquing stretch is minimized. The small ends of the rods are coated with special copper plating to eliminate the need for bearings. In this manner the rod rides directly on the piston wrist pin.
Novelty supreme comes from the use of vibration to eliminate vibration. Like fire to fight fire. To make sure we had all the facts straight, a day was spent studying the TX 750 engine, chassis and suspension units with Yamaha's top three resident engineers--Nick Shiokawa, Tak Takahashi and Sunny Nagashima. Development of the TX 750 started only two years ago. At the outset the Yamaha engineers knew of the vibration problem inherent with a vertical twin. Therefore, they searched for a method, other than rubber mounting, that could be used to not just reduce but eliminate vibration. Accordingly a Japanese college professor came up with the idea of using opposing vibration to offset or override the shake of an unbalanced engine. With this in mind, it must be noted that the TX 750 engine was designed from the outset with the counterbalance vibration-proof system included. It was not an afterthought.
The idea of using a counterbalance system is not new or original as Saab has used one for years on their 1500cc and late 1700cc V-four automobile engines. Actual development of the counterbalance system was undertaken and completed by Ford of Germany in the early Fifties for their V-four-powered Taurus. Yamaha's system uses the same principle of having two counterweights revolving at the same speed as the crankshaft. The TX 750 has two flywheel counterbalancers located at the lower end of the crank. Next to the cam chain drive sprocket, on the center of the crankshaft, are the teeth used to drive the counterbalancers. The chain runs such that the two counterbalancers spin in opposite directions. The larger forward counterbalancer sets up a vertical vibration as it spins opposite of the crank. Behind it, the smaller counterbalancer sets up a horizontal vibration while spinning with the crank. The simultaneous and opposing vibrations set up a 360-degree counter-vibration action that overcomes much of the shaking caused by the engine.
 | Novel counterbalance flywheels (top) spin at crank speed. Forged steel crank rides in three plain main bearings. The offset sprocket centers eliminate chain tensioner. Cast aluminum cylinder (middle left) is smooth with thin finning. Big bore possible. Multi-disc, wafer oil cleaner (middle right) along with other filters clean lubricant. This assembly (bottom) drives at half crank speed from clutch. On shaft are contact points, seal, magnetic centrifugal filter trichoid and force oil pumps. Complex. During the early production stages (a domestic version has been on sale in Japan since July) a spring-loaded chain tensioner was used to reduce lash in the counterbalance drive. This system didn't fare well as chain life was short. Later versions come without the tensioner as the Yamaha engineers again proved their innovativeness. Racing bicycles commonly use offset or eccentric drive sprockets to eliminate chain lash during pedal power strokes of the rider. Using this same principle, Yamaha has offset the center one millimeter (1/2 millimeter radius) for each of the three counterbalance sprockets. In this manner, the chain lash or play is taken up on the larger radius of the sprockets during the power pulsations of the crankshaft. Thus the elimination of the tensioner, quietening of chain noise and longer life. The TX 750 was designed exclusively for touring; therefore, the great emphasis is on smoothness. Obviously more emphasis was placed on eliminating vibration than optimum performance; interesting in that this is the fourth basic method of reducing engine vibration. The Ducati 90-degree V-twin uses perfect engine balancing, the four-cylinder machines (particularly the 900cc Kawasaki) use opposing cylinder pulsations, the Norton Commando and Suzuki triples have automotive-oriented, rubber-mounted engines and now Yamaha comes with a counterbalance system. The amount of vibration getting to the rider with the TX 750 is about equal to the Honda 750-Four, is definitely less than the XS2 650 or Kawasaki 750 but is more than produced by the Ducati 750 or Kawasaki 900. Compromise? Yes, but the TX 750 does provide a good deal more comfort than many present road machines and is head and shoulders above all but one of the older 650cc British twins. |
Power is transmitted to the gearbox from the right side of the crankshaft via straight-cut primary gears. The clutch is a large 14-plate, oil-bathed unit. Transmission gears are appropriately large to match the power generated by the 45-cubic-inch engine. Naturally a five-speed transmission is used. Shifting chores are handled with three shift forks riding on and actuated by a shift cam drum. This large assembly is located directly above the gearbox unit. Unfortunately, the TX 750 has succumbed to the irritable transmission gear clunk and driveline whiplash that are inherent with many Japanese big-inch road machines. When shifting from neutral to low, there is a tremendous clunk from the gearbox, then a sharp lurch forward. It's almost mystifying why the Japanese don't use the same shifting system that the British twins and Ducati incorporate to eliminate this irritable clunk.
The clutch must be used religiously to smooth out shifting up and down between gears. Here also was an area of irritation as the clutch adjustment varies greatly with engine temperature and usage. If adjusted properly when cool or even warm, the free play would grow rapidly and in proportion with operating temperature. When hot (under normal use) the excessive lever free play minimizes the disengagement plate tolerance and makes the clutch drag. If adjusted properly (at the lever), the clutch slips when the engine is cool. On the other hand the oil drag on the clutch plates when cold inhibits smooth shifting. Upon asking why the factory hadn't decided to use a more predictable dry clutch, we found that they had tested one but found it made too much plate drag noise when disengaged. Thinner oil such as ATF can't be used to control dragging as the engine, gearbox, and clutch use the same oil supply.
Speaking of oil brings to mind the super-complex lubrication system incorporated by the Yamaha. The TX 750 uses a dry sump system where oil is forced through the engine to critical parts and a separate container is used as a central reservoir for the lubricant. In this system oil is not retained in the sump. Starting out with the reservoir tank (hidden behind the right side panel cover), lubricant flows down the middle inlet on the top of the crankcases and into a central passageway that leads to the trichoid oil pump. The trichoid force pump and return (to oil tank) pump are driven at half crankshaft speed from the clutch, but also on the same drive shaft is a centrifugal magnetic filter and finally the contact points. Meanwhile, the lubricant is forced from the trichoid pump through more passageways to a special check valve, then into a multi-disc filter. From here oil goes back to the drive shaft and the centrifugal magnetic filter. At this point the oil split up with 80% pressure fed through the crankshaft and big end rod bearings with the small end and cylinder walls splash fed. The other 20% runs up the oil line behind the cylinder to the valve train and cam chain. After performing its lubricating chores here the oil drops into the sump and through a screen filter. Now the return pump picks up the oil and splits it up again with 80% going back to the tank and 20% forced through the shift mechanism, then dropping onto the gears for lubrication. Complex? You bet.
With a claimed output of 63 horsepower at 6500 rpm we found the performance pleasant, though not shattering by any means. Even if the power claim is correct it's just not sufficient to overcome the very fat weight figure of 518 pounds wet. Comparing weight figures, we note that this is seven pounds lighter than the Honda 750 Four, but five more than the XLH Sportster, 51 greater than the XS2 650 and a whopping 60 above the BMW 750. The total effect of this fatness is moderate acceleration times and speeds and excessive stopping distances. We spent a day at Irwindale Raceway with Yamaha engineer Sunny Nagashima observing acceleration, braking and speedometer correction tests on 350, 650 and 750 Yamahas. Acceleration wise the TX 750 is a moderate performer. The best Tech Editor Tony Murphy could record was 13.87 e.t. and 95.84 mph, both on the same run. This would compare as slower than the Honda 750 and Moto Guzzi but slightly quicker than a BMW or full dress Harley 74.
The performance isn't all bad, just disappointing. Fortunately the engine has good low-speed pulling power from 2500 up to 6500. It doesn't surge or bog, just pulls at a moderate, constant rate. Everyone who rode the machine found it easy to ride, predictable and smooth. We spent 400 of our 1200 miles riding two-up and found other interesting information. Riding through winding mountain roads taxes the engine efficiency and there is a noticable increase in operating temperature and resulting decrease in performance. Sunny informed us that he had recorded oil temperature readings of 240 degrees during testing in 115-degree desert heat. This was with a solo rider. We found the engine was happy to hold 80 mph for miles and miles across the desert floor with two up, but again with a sharp increase in operating temperature over riding solo. We are sure that an engine oil cooler would be of benefit for the touring rider. The enclosed oil tank, small fins and polished castings just are not complementary to good heat dissipation.
We did find that the engine runs clean as a whistle and didn't lose a drop of oil during the test. However the engine and oil tank breathers spewed a constant mist over the tail section of the machine and left small puddles in our otherwise clean garage. Enough oil accumulated on the centerstand to cause this writer to drop the machine once when a foot slipped off it while trying to raise the heavyweight to center. It left a nasty gash in a friend's truck side. More breather restriction and rerouting of tubes is in order.
Fuel consumption was also disappointing for a touring mount. Our overall average came out to 34.4 mpg, identical to a XLH Sportster. Our worst was 29.1 and best was 39.6 mpg. With the small 14-liter (3.7-gallon) gas tank we once ran it bone dry at an embarrassing 108 miles. With this as a minimum it would make it impossible to travel cross country in many areas without a backpack of extra fuel. Oddly enough the Japanese domestic model comes standard with a 19-liter (5.0-gallon) tank. Don't get your hopes up yet about this as an option. The Japanese tank isn't available here and would be an extra if it were. Considering it would involve having to install a shorter seat with the bigger tank, the cost would be more than prohibitive.
The TX 750 comes with a superb hydraulic disc front brake and a good drum rear binder. Both are identical to the XS2 650. The front disc uses a dual puck actuating system rather than one forcing the disc against a fixed pad. This permits side flotation of the pucks (in the case of an irregular or warped surface) and therefore equal pressure and friction on the disc. Our braking tests gave us a best stop from 60 mph of 118' 6" and from 30 mph, 29' 9". Average for a 3000-ound sedan is 130 feet at 60 mph and 30 at 30. It must be noted that the TX 750 is quite difficult to haul down at 60 mph due to the tremendous weight transfer on the front. The XS2 650 is easier and more controllable to stop and at 60 mph will come to a halt in 100 feet flat.
Probably the real pride and joy of the Yamaha is the instrumentation and electrics. The very handsome 140 mph speedometer comes with a tripmeter and the super-easy to read 10,000 rpm tachometer has steady needle movement. Both would fit nicely in a Ferrari. The clever warning lights are in preparation for U.S.A. and Japanese safety standard requirements. There's a cluster of three red warning lights at the bottom of the instrument panel. The left light warns of insufficient oil pressure. Center is a light that comes on when the rear brake lining wear exceeds 2mm (.080") and is actuated by a sensor resting on the shoe cam. But for some odd reason there is not a similar warning light for the front brake which takes 70-80% of the stopping work load. Right is a light that comes on and off with the brake lamp. going a step further, this light will flash on and off when the brake(s) is applied and the bulb is burned out, to indicate need for replacement. Below the two (right and left) turn indicator lights is an interesting white lamp. This one comes on in the event that the high or low headlamp beam fails, but at the same time it transfers the unused current to the remaining good light. For daytime riding it ensures one headlight beam will burn at all times.
The soft fork springs and poor shock damping doubtlessly minimize what feels to be a fairly good handling chassis. The front forks use a marginal double damping system that allows the units to constantly clack when topping. The straight-wound springs are rated at 35.7 inch/pounds and give a soft, cushy ride. Unfortunately they sacked out prematurely from lack of sufficient pre-load and allowed the machine to rest 2 inches below its 5 1/4 inches of travel. When completely collapsed the wheelbase shortened almost two inches and also lessened the trail to cause quick handling under load. The rear shock springs are progressive-wound with a minimum tension of 118 inch/pounds and a maximum of 138 for the three inches of travel. The springs felt right for the machine, but damping was absent, allowing that pogo stick action to occur. These springs would probably work well over Koni dampers.
High-speed cruising with the TX 750 leaves something to be desired in the comfort department. The wide handlebars are swept back too much and place the rider's wrist at an awkward angle. riding two-up accentuates this and causes premature rider fatigue plus leading to high-speed squirming as the pilot holds on to the grips too hard. The saddle uses foam that is far too soft. You can push to the steel base of the seat with one finger. This lets the rider's and passenger's tail bones rest on the hard bottom and suffer that electric-type vibration. Yamaha might think of studying the saddles of the BMW and Moto Guzzi for both depth and length.
For the first time Yamaha has used alloy rims on one of its street mounts. The D.I.D. aluminum rims are replicas of the famous Akront hoops from Italy. No tire bead locks are used. The rear 4.00x18-inch tire works very well for all road conditions. The front 3.50x19-inch rib-style rubber is tremendously overworked with two up or during hard braking. A K87 Dunop would be an improvement.
Starting chores are handled by a chain-driven electric motor mounted in the forward part of the engine. We sometimes had to crank it considerably to get the fires burning, but it always did the trick. Carburetion is the same as the XS2 650, a matching pair of 38mm Mikuni Solex vacuum slide (SU type) units. Performance is smooth under most conditions, though they do spit erratically, like the slide diaphrams are vapor locking when the engine gets hot.
After completing our lengthy test on the TX 750 we tried to evaluate whether all the engine complexity, counterbalance system and machine fatness were necessary. Engine complexity such as Yamaha has used might be justified if it performed in the top ten percent of its class, but this isn't the case. Servicing is indeed going to be an intricate task for any mechanic. The counterbalance system is novel, but also costly, heavy and involved for its level of efficiency. The old Matchless 650 twin was a very smooth engine, every bit as much as the TX 750, yet a compact 40-inch vertical twin free of extra weight, counterbalance flywheels and complexity. The heaviness of the machine is certainly not appealing or advantageous. Weighing in at more than the electric start Sportster seems hard to justify.
The official price of $1554 has just been established. This will drop the Yamaha under the hornet's nest comprised of Honda's 750 Four, Norton's Commando twin, Moto Guzzi's twin, Triumph's triple, Ducati's twin and Kawasaki's 900 four. The void that Yamaha's TX 750 was intended to fill was narrow at the outset; a super-smooth twin that would make a multi or rubber mounting of engine unnecessary. Did they pull it off? Mechanically, it hardly seems likely. For the TX is not the fastest, quickest, lightest or smoothest tourer. Neither is it the most comfortable, most economical or best handling roadster. Test evaluations place it down the road, in varying degrees, in all categories except electrical safety equipment. Price-wise, The TX is meeting head on with Triumph's venerable 650 and even their new 750 vertical twins. Tough Competition. Let's just say that Yamaha is still king of the two-strokes.