Until 1968, Kawasaki was mainly involved in developing two cycle motorcycles, although the company did have a long history of developing four cycle engines. In 1937, Meguro (merged with Kawasaki in 1963) manufactured 500cc single engines, and the engineers who developed this technology moved to Kawasaki. These engineers played a major role in developing the 650cc W series motorcycles.
This experience provided Kawasaki with the basic skills to develop four cycle engines.In 1967 Kawasaki made a decision to develop a high-performance motorcycle which would far exceed the 650W1, the largest motorcycles in Japan that time.
As the United States was targeted as the main market for these high performance motorcycles, the development team was sent to the U.S. where they secretly worked out a plan for the new model.
Finally, the displacement of the new model was set at 750cc and a mock-up was completed in October 1968.
However, Honda announced a new 750cc single-over-head-cam (SOHC) motorcycle at the
Tokyo Motor Show held the same year. The Kawasaki management staff realized it was meaningless to come out with a similar model after Honda had already introduced theirs, so all development efforts on Kawasaki's 750cc model were stopped.

In 1970, the Z1 (development code T103) developing project team was reunited with the best staff in all the fields joining the project. This group repeated research and experiments to develop a better .
Kawasaki resumed U.S. market research in March of 1970 and collected customers' opinions from various sources such as random samplings of dealers and editors of major motorcycle magazines.
Finally, the management staff concluded there was a strong market for a high-speed, eye-appealing motorcycle with enough power to use as a reliable touring model.
Kawasaki's answer to this market was a 1,000cc class, four cycle, four cylinder model. The main requirements for the Z1 engine were high speed, high stability, and ease of dealing with pollution problems. A four cycle unit meeting these requirements would be met by strong market demand.

The first prototype was completed in the spring of 1971. This prototype was ridden by
American test riders with minor adjustments made step by step. In the fall of that year, the final prototype was completed and after testing, the unit was approved for mass production. The first production model was completed in February 1972, and this unit was subjected to repeated severe road testing after which all parts, including even the nuts and bolts, were examined. After reworking all weak points, the first mass-production model was built in May 1972.

The 903cc displacement of the Z1 made it the largest motorcycle in Japan. Worldwide, it was larger than Italian Moto Guzzi 850 and comparable to Harley-Davidson 1000 and 1200.
The specifications called for an air-cooled four-cycle four-cylinder engine with a double-over-head-cam (DOHC) mechanism.
The DOHC was necessary to realize overall high performance from low speed to high speed range. In motorcycle markets around the world, there were only one or two other samples of this type of engine, and it was the first engine for Kawasaki to adopt this advanced valve train.

900 Z1 ENGINE

The Z1's maximum horsepower was 82ps at 8,500rpm, 0 to 400m acceleration was 12 seconds, and the maximum speed was above 210km/h. The Z1 power was 8ps higher than the H2, and had great potential considering the average horsepower of the 1,200cc automobile was 77ps at that time.
However, horsepower per displacement was comparatively lower than the H1 and H2 because
Kawasaki changed their engine design policy so that the powerband was not set near the engine's limit, thereby pursuing elegance and smooth engine performance. It is also noteworthy that the Z1 engine was based on a policy to prevent pollution and was equipped with anti-air-pollution devices such as a positive crankcase ventilation system.
The main features of the Z1 were the reliable double-cradle steel tube frame, a safe and reliable disc brake system, and ease of maintenance. Since the Z1 utilized the complicated DOHC mechanism, ease of maintenance was carefully considered at the design stage. As a result, the Z1 could be maintained without removing the engine from the body except for maintenance of crankshaft related parts.
The Z1 style was fresh, but cool, without the look of a 900cc heavy weight machine. The style was achieved with tail-up mufflers, a light tear-drop formed fuel tank, and a slim, flowing seat.
All Z1 parts were individually examined and tested time after time resulting in a five year development period. Five years is not a short development period for one model, although as noted earlier, development was at one time stopped altogether. In this sense, the Z1 was the Kawasaki's flagship model.

In September 1972, the Z1 was introduced to the U.S. public, and sales started in November of that year. Since the development stage, Z1 was nicknamed "The New York Steak," and the Z1 was enthusiastically welcomed by markets as the "mouth watering motorcycle" when sales started. The suggested retail price was $1,900 and the initial sales plan called for 1,500 vehicles per month including the European markets.

900 Z1 IMAGE SKETCH

900 Z1 MOCK UP

The Z1 was introduced to the Japanese public at the Tokyo Motor Show in October of 1972 and drew the strongest attention among numerous new models developed by our competitors.

In December 1972, Kawasaki held a press conference at Tokyo Takanawa Prince Hotel and invited guests from 17 companies in the motorcycle industry, and reporters from magazines and newspapers. At the conference, a new model, the Z2, was introduced as a brother model for the Japanese market.Production of Kawasaki's 750RS Z2 started in January 1973. It was a 746cc machine with newly designed pistons and crankshaft parts to express the same feeling as the Z1. The maximum Z2 horse power was 69ps at 9,000rpm with a maximum speed of 190km/h.

Sale of the Z2 started in March 1973 and were 10% higher than our competitors' 750cc class motorcycles. The 900cc class body size and the DOHC engine attracted Japanese riders all at once because motorcycle equipped with a DOHC engine had not existed in Japan prior to the Z2.
During the first two years of production, Kawasaki built 80,000 Z1 and Z2 motorcycles, and the sales of these models established Kawasaki's reputation as a heavy weight motorcycle manufacturer.

(Source: "50 Years' History of Akashi Works" This book is not for sale.)