‘THE YAMAHA GL750 COULD WELL HAVE CHANGED THE FACE OF MOTORCYCLING’

350cc engine (each with a 10mm gear) together which drove a single 20mm geared countershaft. It also used a pair of 65mm bore x 56mm stroke water-cooled, seven-port ‘Torque Induction’ 350 twin-cylinder barrels, with a capacity totalling 743cc. (The 694cc TZ750A had a 64 x 54mm bore and stroke, and later TZ750 models were bored out to 66.4mm which gave 747cc).

While the GL750 was graced with beautifully polished engine covers, the only common components between it and the TZ750 racer were the horizontally-split crankcases, and crankshafts. However, the engines shared the same reverse-rotation 180° firing order, where two cylinders fired at the same time – Yamaha’s first ‘big bang’ engine, but only because the engine ran a countershaft to get the sprocket rotating in the right direction.

Yamaha cleverly placed the alternator of the motor behind the crankshaft to make a narrow engine that offered less wind resistance and excellent ground clearance. It also developed reed valves to restrict carburettor blow-back and increase low-speed pulling power. A multiplate wet clutch drove a five-speed constant-mesh gearbox.

Power output was a conservative 70hp (probably with Mikuni carbs) at 7000rpm, with 54lb.ft of torque at 6500rpm. The low 7500rpm redline was displayed on an allnew dash that incorporated a temperature gauge for the thermosyphon (as opposed to pumped) 4.2-litre water-cooling system, another first for a non-competition Yamaha.

However, the real show-stopper was the fuel injection system which Yamaha initially developed for its experimental US racing snowmobile project. The ‘manual’ GL injection consisted of a rubber hose connected to a small open nipple fitted to each of the four crank chambers. The hose led to an approximately 30mm-diameter diaphragm pump, activated by the positive and negative crankcase pressures generated by the pumping piston action. A pump was located under each of the four intake manifolds.

The gravity-fed fuel came from twin outlet fuel taps found on each side of the tank going down to the pump, where Yamaha may have relied on some kind of mechanical needle valve to determine injected fuel volumes, in harmony with the butterfly valves and revs. With sharp opening and closing throttle variations, the pump on its own wouldn’t have been capable of metering correct fuel quantity and duration; the pressure inside the crank chamber also plateaued at a given rpm.