Yamaha TX 750

The TX750 of 1972 was one of Yamaha's few commercial failures. It was designed as a larger, smoother, more powerful stable-mate to the XS650 twin. Larger it was, at 763cc, and certainly smoother, with its twin-cylinder vibration tamed to a considerable extent by a complex balancer system, named omni-phase by Yamaha, consisting of two counter-rotating weights housed to the rear of the crankshaft and acting to cancel out the amplitude of the shaft. But more powerful?

Hardly. The makers were at pains to point out that the 750 was a tourer, with electric starting, quieter mechanicals and a soggier ride to help distinguish it from the gutsy 650. But a true top speed of around 10Omph and leisurely acceleration earned it no more than a lukewarm reception among motor cylists accustomed to Honda four-style performance from a 750.

Within months of its introduction in the USA disturbing stories about unreliability depressed already mediocre sales. Yamaha accepted the situation, dropped the big twin, and turned to another configuration, the 120° triple, for their next, more successful, 750.

History

Until the early 1970s Yamaha focused on two-stroke bikes, but concerns about pollution had begun to make two-strokes increasing unpopular. Around this same time competitors started to release new four-stroke multi-cylinder bikes with larger engines and greater levels of comfort. Yamaha added four-stroke engines to their line-up when they released the XS650 XS1 in 1970. This bike was powered by a Hosk-derived four-stroke air-cooled twin of 654 cc.

Work began on a successor to the XS650 two years before its 1972 debut. Although superficially similar to the XS650, the TX750 was an almost entirely new design. The TX750 was featured in an article in Yamaha News magazine in October 1972, and appeared on Yamaha's stand at the 19th Tokyo Motor Show one month later.

The standard road bike was model number 341, while the police version was model number 435.  The police version came with a distinctive valve-cover.

The bike was released in Japan in July 1972, and towards the end of the same year in the United States, where it retailed for a price of US$1554.00.

Shortly after being launched in Europe reports of engine failures began to come in.  Several causes for the failures were suggested. One suggested cause for the failures was the oddly shaped and located oil filter.  Others blamed the bike's complicated oil system for starving parts of lubrication.  Others blamed the quality of the oil available at the time.

One of the two most commonly suspected causes was excessive heat coming from the balance tube connecting the exhaust ports.  The other most common claim was that the Omni-Phase balancer's weighted shafts were whipping oil in the sump into a froth at high rpms, aerating the oil and starving the crank of lubrication which then resulted in bearing failure.  Yamaha described the cause as "a problem with heat build-up that prevented stable engine performance and insufficient machine durability".

One ready solution was to add an external oil cooler. Coolers from a variety of sources were retrofitted to the bikes, often mounted high and out of sight.  Eventually Yamaha came out with a cooler of their own, mounted low in the air-stream.  Yamaha recalled the bikes, a first for the industry, and sent techs out to dealerships to install the oil coolers.

Problems specific to the balance system also occurred. The chain driving the counterbalance shafts would stretch, resulting in the counterweights being out of phase with the crankshaft and the engine vibrating more severely than a regular twin would. The tight timing tolerances between the two balance shafts caused some reviewers to worry that a slight maladjustment would cause the eccentric weights on the shafts to run into each other.  Yamaha's fix in this case was to revert to an external chain adjuster, which required a modified sump.

In all 18 engineering change orders, including a deeper sump, were issued for the engine.  The extensively modified 1974 TX750A model does not suffer from reliability issues.

Despite Yamaha's efforts to solve the problems, sales of the TX750 never recovered.

Production figures

Model year	Months	VINs	Units
1972	August - December	341-000001 - 015511	15511
1973	January - March	341-015512 - 023281	7770
1973	April - December	341-100101 - 103460	3360
1974	January - December	341-103461 - 104360	900
1975	January - December	341-104361 - 104760	400
All years (Europe)		341-50XXXX	~5500
All years (Police)		435-XXXXXX
Total			~33441

Reviews and road tests

In general reviewers praised the effectiveness of the Omni-Phase balancer system. A June 1973 review by Big Bike magazine said that at low rpms the TX750 vibrated as much as a typical British twin, but that at higher engine speeds the bike became smoother. Popular Science reviewer Ray Hill called the TX750 "one of the smoothest-riding bikes I've ever been aboard." Cycle World magazine wrote in their October 1972 issue: "The result is smoothness beyond belief," and "Shut your eyes and you are on a four. It couldn’t be a twin." Cycle magazine's issue of March 1973 called the TX750's engine the "Most advanced two-cylinder engine in motorcycling"

The controls and instruments were described as top quality and easy to use. The three warning light system drew mixed reviews, with some expressing appreciation for the extra redundancy built into the lighting system and others voicing annoyance at the way the warning system operated, lighting a red lamp under normal conditions and flashing in the event of a problem.

The bike was characterized as a low-rpm tourer rather than a high-revving sports-bike.  The bike's throttle-response and low-end torque were ranked high.  In a March 1973 article Cycle magazine called the TX750 "one of the ten fastest production motorcycles in the world" based on its quarter-mile speed. Testers reported noticeable driveline lash, and clunking and lurching when shifting out of neutral.  Other complaints focused on a lack of preload in the front suspension, and excessive softness in the suspension and seat pad in general.  Handling was described as adequate, with some reviews describing the bike as stable on the straights and in corners, and others reporting a tendency to squirm or wobble when pushed into faster corners.

Some reviews recorded high oil temperatures even in testing and recommended prospective owners consider adding an external oil cooler.

Motorsports

In September 1972 one of the first TX750s exported to Australia was prepared to challenge the One Hour, Twelve Hour and Twenty-four Hour Australian endurance records. A combination of mechanical problems and confusion over the existing records caused the attempt to be stopped after six hours. The team claimed the previously unrecorded Six Hour record after having covered 370.23 mi (595.8 km) at an average speed of 61.68 mph (99.3 km/h).

The same TX750 was part of a team that appeared at the 1972 Castrol Six Hour race at Amaroo Park. Yamaha put together a six-bike, twelve-rider team for the event. After a series of both major and minor mechanical failures and some rider errors only three of the six were running at the end of the race. The highest placed TX750 finished in fifth place.

One of the first TX750s to arrive in Europe was ridden to a sixth-place finish by "Texas" Henk Klassen at the 6 Hour event held at the Zandvoort circuit in the summer of 1973.

Yamaha Motor Amsterdam contracted Porsche A.G. Research to prepare the TX750 to be entered in endurance racing events in 1974. Two bikes were run in the Bol d'Or 24 at Le Mans. The Porsche-prepared engines were replaced with Yamaha engines but both bikes failed during the night with snapped cam chains.  The bikes also ran at the Thruxton 500 Miles in England. Both TX750 Endurance racers are reported to have been destroyed after their racing careers were over.

Source wikipedia