The S5 engine was introduced in 1968 as a successor to the previous S2 and S3 engines. The S5 engine had a displacement of 49.8 cc and a bore and stroke of 39 x 41.8 mm. It had a single cylinder with a piston port intake and a rotary valve exhaust. The compression ratio was 9.5:1 and the power output was about 3.5 hp at 6000 rpm. The engine had a three-speed gearbox with a wet multi-plate clutch and a chain drive. The ignition system was either points or electronic.
The S5 engine was available in different versions, such as the S5E (enduro), the S5K (cross), the S5N (street), and the S5T (trial). The main differences were in the cylinder head design, the carburetor size, the exhaust pipe shape, and the gear ratios. Some versions also had different cooling systems, such as air-cooled or liquid-cooled.
In the early 1990s, the S5 engine was upgraded to the S5GS version, which had a larger bore and stroke of 40 x 39.3 mm and a displacement of 50 cc. It also had a two or four reed valve intake and a larger carburetor of 14 or 16 mm. The power output was increased to about 6 hp at 9000 rpm. The S5GS engine was used by Husqvarna, KTM, LEM, Rivara, and DBM.
Features of the Franco Morini S5 Engine
The Franco Morini S5 engine was known for its reliability, durability, and simplicity. It was easy to maintain and tune, as well as to modify for higher performance. Some of the features that made the S5 engine stand out were:
The kick-start lever operating forward: This feature was unique to the S5 engine and allowed for easier starting and less stress on the kick-start mechanism.
The rotary valve exhaust: This feature improved the scavenging efficiency and reduced the back pressure in the exhaust system.
The wet multi-plate clutch: This feature provided smooth and precise clutch operation and reduced wear and tear on the clutch plates.
The electronic ignition: This feature improved the spark timing and reduced maintenance costs.
Performance of the Franco Morini S5 Engine
The Franco Morini S5 engine was designed to provide good performance for mini bikes in various terrains and conditions. It had a strong low-end torque and a decent top speed. Depending on the version and the tuning, the S5 engine could reach speeds of up to 50 mph or more. The S5 engine also had good fuel efficiency and low emissions.
The Franco Morini S5 engine was also adaptable to different modifications and upgrades. Some of the common modifications that enthusiasts made to their S5 engines were:
Increasing the compression ratio by shaving the cylinder head or using a thinner head gasket.
Installing a larger carburetor or a reed valve intake for better airflow and fuel delivery.
Replacing the exhaust pipe with a performance pipe or a custom pipe for better exhaust flow and sound.
Changing the ignition system to a CDI or a racing ignition for better spark and timing.
Changing the gearbox to a four-speed or a five-speed for better acceleration and top speed.
Conclusion
The Franco Morini S5 engine was one of the most popular and successful two-stroke engines for mini bikes in the 20th century. It was used by many motorcycle manufacturers and riders around the world. It was praised for its reliability, durability, simplicity, and performance. It was also easy to maintain and modify for different purposes and preferences. The Franco Morini S5 engine is a classic example of Italian engineering and craftsmanship that has left a lasting legacy in the mini bike world.
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