In 1972, the AMA announced that the displacement limit for racing motorcycles would be 750cc for all engine types. Suzuki and Kawasaki already had three-cylinder two-stroke production-based bikes ready to be modified into 750 racers. Yamaha did not. Despite this, they campaigned their latest 350cc two-stroke twin, and still managed to win the Daytona 200 in 1972 and 1973 after the poor-handling, 100-horsepower 750 two-stroke Triples had exceeded the capabilities of their chassis and tires.
Yamaha had originally planned to offer a production 750 cc two-stroke four cylinder - the GL750, but the EPA crack-down on emissions had begun, and Yamaha knew such a bike would have a short lifespan in the showrooms. Yamaha had other surprises ready. One was a 500cc Grand Prix bike, the 0W20, based on the GL750 architecture. This was conceptually two of Yamaha’s racing 250cc Twins set side-by-side, each with a thin gear on the inner end of its crank and that pair of gears meshing with a double-width gear on a jackshaft extending to drive a dry clutch on the right.
The other surprise was to hire Giacomo Agostini away from MV Agusta - the team he had ridden for since 1966. And meanwhile they prepared a larger, four-cylinder, 695 cc reed-valve two-stroke engine - coded 0W19
Australian Kel Carruthers, technical chief of the American Yamaha race team, went to Japan to test the new machine. Perhaps knowing Agostini preferred his previous bikes that way, they gave the bike a short 53.5-inch wheel base. Carruthers found the bike to be unstable at high speed. "When I first rode it in Japan I was a little disappointed in it," recalls Kel. "I was Yamaha's number one rider at the time, and I went over there to test it. So I flew over and got ready to ride the bike at a test track. They said, 'Kel-san, the bike is good except for one small problem'. Well, the problem was the thing used to just shake it's head at any speed over 160 miles per hour. Incredible tankslappers! I mean it was scary..."
"Another rider went with me, and after a couple of laps he came in and refused to ride it any more," recalled Carruthers. "The Japanese said, 'Kel-san, now it's your turn. You try'.
"Being the young idiot I was, and being Yamaha's number one rider at the time, I went out and did some more testing."
Carruther’s knowledge of motorcycles helped tame the TZ. He says, "Basically it was too short. We went back to the factory that night, cut through the swingarm, added a couple of inches and lengthened the wheelbase. 95% of the problem was gone right there, then we played with the suspension a bit and got it pretty good."
The bike debuted as the TZ750A in March 1974 at Daytona. Daytona is a challenging place to introduce a new design. The combination of sustained high speed, heavy loading from the banked turns and the 200-mile race distance prove to be exceptional challenges. The new TZ750, piloted by Agostini, survived several challenges not the least of which was heavily damaged Yamaha exhaust pipes. Like the 500 cc GP bike, the new TZ750A’s four pipes were crowded together tightly under the engine. As 10,000 rpm, the pressure pulses hit each flat-sided pipe, they began to physically deform the thin-walled chambers. The resulting flex and pipe contacts covered the track with broken pipe pieces and a very loud Yamaha. Despite this, Agostini won the race. What’s more, TZ750s would win the race every single year from 1974 to 1982.
But those first victories were harrowing. "Everything on it was just too flimsy," confides the 1969 world champ Carruthers. "The first time we tested over here, at Ontario, we had Kenny, Gene Romero and Donnie Castro on the bike. I can't remember what lap times Kenny was turning, but Castro and Romero were like eight seconds slower. Basically, they couldn't ride the bike. It scared them because it wouldn't handle like they wanted it."
"I had just retired to run the race team, but I got on it to do about five laps to see what the problem was. I was about a second and a half off of Kenny's times. I just piddled around the corners and held it wide open around the banking. They were fighting it all the way around the track instead of letting it do its thing. The bike was very flimsy because it had these little-bitty baby fork tubes and chassis."
Carruthers knew that the engine had more, but was limited by the rest of the bike’s design. "It wasn't as fast as it could have been. They were really conservative in the way they built it. After about three or four races, we had another twenty horsepower. Then the chassis really started to flex," laughed Kel.
Initially, they didnt fully address those issues, but they did increase displacement up to 749 cc for the 1975 season - even more power. Ultimately the TZ750 would produce 145 hp, pushing a bike that weighed around 340 pounds.
Despitethis outrageous power to weight ratio, Yamaha started getting customer complaints telling how their new monster bikes were getting beat by 350cc twins.
Suddenly, a series of big changes came. Yamaha’s factory 0W-31 version, introduced in 1976 had what were new features for the day — Better, thicker forks, Longer suspension travel, a more triangulated chassis and swing arm, a rear mono shock, and new wider tire designs with round-profile slick tires.
Not only did the TZ750 run the tables at Daytona for nearly a decade, it instigated a series of chassis and handling changes to the design of motorcycles that we take for granted today.