| Component | Specification |
|---|---|
| Production Years | 1977 – 1983 |
| Engine Oil Type | Yamaha 2-Cycle Oil (JASO FC/FD) |
| Oil Capacity | 1.2 Liters |
| Tire Size (Front/Rear) | 4.00-8 4PR |
| Tire Pressure (Front) | 1.25 bar (18 psi) |
| Tire Pressure (Rear) | 1.50 bar (22 psi) / 2.25 bar (32 psi) |
| Battery Type | 6N4-2A-4 (6 Volt) |
| Spark Plug | NGK BP6HS |
The Yamaha LB 50 2M Chappy, specifically the 551 variant, remains a marvel of mid-century urban engineering. A unique historical highlight of the Chappy series is its centrifugal clutch system paired with a high/low range sub-transmission—a rare engineering choice for a 50cc machine that allowed it to tackle gradients that would stall contemporary mopeds. This “dual-range” capability cemented its reputation as a versatile utility bike rather than a mere toy.
Engineering Overview
At the heart of the 551 model is a robust 49cc two-stroke, air-cooled engine. Unlike the more common single-speed mopeds of the era, the 2M (2-speed Manual/Automatic variant) provides a distinct mechanical advantage in urban stop-and-go traffic. The frame design, a step-through steel backbone, offers high torsional rigidity while maintaining the compact “monkey bike” footprint.
Maintenance Guide
1. Lubrication System
The Chappy utilizes an Autolube system, meaning manual pre-mixing is not required under standard operation. For peak performance and to prevent carbon buildup, MotoSpecsLab recommends Yamaha 2-Cycle Oil (JASO FC/FD). The system has a total capacity of 1.2 liters. Monitoring the oil level is critical, as these small-displacement air-cooled engines rely heavily on the oil film for heat dissipation.
2. Electrical and Ignition
The 551 model operates on a 6-volt electrical system, powered by a 6N4-2A-4 battery. Ignition reliability is highly dependent on the spark plug condition. The factory-specified NGK BP6HS should be checked every 2,000 km for electrode wear and gap. A chocolate-brown color on the insulator indicates a healthy air-fuel mixture.
3. Wheels and Chassis
The small 8-inch wheels are fitted with 4.00-8 4PR tires. Due to the small diameter, tire pressure is critical for handling stability. We recommend 1.25 bar (18 psi) for the front. The rear requires 1.50 bar (22 psi) for solo riders, but must be increased to 2.25 bar (32 psi) if carrying a load to prevent sidewall deformation.
MotoSpecsLab Editorial Verdict
The Yamaha LB 50 2M Chappy (551) is a masterclass in functional miniaturization. Its mechanical durability is legendary, provided the 2-stroke oil injection system is maintained. For collectors and enthusiasts, the 551 represents a specific era of Yamaha’s dominance in the leisure bike market, offering a more “motorcycle-like” feel than the smaller-wheeled competitors of the late 70s.