Historical Context:
The Yamaha QT50 was a popular lightweight, air-cooled, two-stroke moped targeted at urban commuting and recreational use in the 1980s.
Competitors:
Competed with similar models from Honda (e.g., Hobbit, Spree), Suzuki (e.g., FA50), and Tomos.
Features:
Compact size, easy operation, fuel efficiency, simple maintenance, distinctive styling for its era.
Manual Value:
Provides essential technical data for restoring and maintaining the classic Yamaha QT50, ensuring longevity and correct operation.
Primary Coverage:
Comprehensive repair and service procedures for the 1985 Yamaha QT50.
Secondary Coverage:
Maintenance schedules, troubleshooting guides, electrical diagrams, torque specifications.
Estimated Page Count:
100-200
Type:
Repair Service Manual
Target Audience:
Professional Technicians and Experienced DIY Mechanics
Engine:
Disassembly, assembly, inspection, troubleshooting, carburetor tuning, cooling system.
Fuel System:
Fuel tank, fuel lines, fuel filter, carburetor (VM12SC) servicing and adjustment.
Ignition System:
CDI ignition, spark plug, ignition timing, troubleshooting.
Transmission:
Automatic V-belt drive system (CVT), torque converter, clutch, variator inspection and service.
Brakes:
Front and rear drum brake system, brake shoe replacement, brake cable adjustment.
Suspension:
Front telescopic fork, rear monoshock absorber, fork oil replacement, shock absorber inspection.
Electrical System:
Wiring diagram, lighting system, horn, starter system (kick start), battery maintenance, fuse replacement.
Frame And Body:
Chassis components, seat, handlebars, fenders, body panel removal and installation.
Maintenance:
Routine checks, lubrication points, filter changes, spark plug replacement.
Troubleshooting:
Diagnosis of common problems, fault finding charts, diagnostic flowcharts.
Repair:
Detailed step-by-step instructions for component disassembly, repair, and reassembly.
Adjustment:
Carburetor idle speed, throttle cable, brake cables, parking brake.
Specifications:
Torque values for all fasteners, engine clearances, fluid capacities.
Depth:
Comprehensive, covering full rebuilds and advanced diagnostics.
Engine Type:
2-stroke, air-cooled, reed valve induction
Displacement:
49cc (49 cm³)
Bore X Stroke:
38.0 mm x 43.0 mm
Carburetor Model:
Mikuni VM12SC
Carburetor Settings:
Idle speed: 1400-1600 rpm; Pilot jet: #17.5; Main jet: #75; Needle: 4J12-3 (2nd clip from top); Air screw: 1.5 turns out from seated.
Horsepower Approximate:
Approx. 3.5 - 4.0 HP @ 6500 RPM
Torque Approximate:
Approx. 3.0-3.5 Nm @ 5500 RPM
Fuel Type:
Unleaded gasoline, 87 octane (minimum)
Oil Mixing Ratio:
100:1 (Yamalube 2-stroke oil or equivalent) for premix, or via automatic oil injection system.
Fuel Tank Capacity:
2.5 Liters (0.66 US gallons)
Ignition System Type:
CDI (Capacitor Discharge Ignition)
Spark Plug Type:
NGK B7HS or equivalent
Spark Plug Gap:
0.6 - 0.7 mm (0.024 - 0.028 in)
Ignition Timing:
13° BTDC @ 2000 RPM (static)
Transmission Type:
Automatic V-belt drive system (CVT - Continuously Variable Transmission)
Drive System:
Shaft drive (from transmission output to rear wheel)
Clutch Type:
Centrifugal clutch
Variator Type:
Torque converter type with movable drive and driven sheaves.
Gear Ratios:
Variable (2.53 - 0.86)
Transmission Oil Type:
Yamaha Gear Oil GL-4 SAE 80W
Transmission Oil Capacity:
0.1 Liters (0.03 US quarts)
Length:
1,640 mm (64.6 in)
Wheelbase:
1,165 mm (45.9 in)
Dry Weight:
57 kg (126 lbs)
Wet Weight:
61 kg (135 lbs) (approximate with full fluids)
Front Tire Size:
2.50-16 4PR
Rear Tire Size:
2.50-16 4PR
Front Tire Pressure:
200 kPa (2.0 kgf/cm², 29 psi)
Rear Tire Pressure:
225 kPa (2.3 kgf/cm², 33 psi)
Wheel Type:
Spoked wheels with aluminum rims
Safety Rating:
Not applicable for this class of vehicle in its era; relies on basic design for rider protection.
Braking System:
Front and rear drum brakes.
Lighting:
Headlight, taillight, brake light, turn signals, position lights.
Frame Construction:
Steel tube frame for structural integrity.
Rider Controls:
Handlebar mounted controls for throttle, brakes, and lights.
Oil Change Transmission:
Every 3,000 km (1,800 miles) or annually.
Spark Plug Cleaning And Gap Check:
Every 1,000 km (600 miles).
Air Filter Cleaning:
Every 1,000 km (600 miles); replace if damaged.
Carburetor Cleaning:
As needed, or every 3,000 km (1,800 miles) for optimal performance.
Brake Adjustment:
Every 2,000 km (1,200 miles) or as needed.
Transmission Oil:
Yamaha Gear Oil GL-4 SAE 80W or equivalent.
Fork Oil:
Approximately 10W fork oil (specific volume to be confirmed in manual, typically ~100-150ml per leg).
Fuel Mixture Oil:
Yamalube 2-stroke oil or API TC classified oil (100:1 ratio recommended).
Common Problems:
Carburetor gumming from old fuel, worn drive belt, ignition system (CDI) failures, worn brake shoes, fork seal leaks.
Troubleshooting Tip Carburetor:
Ensure correct pilot jet and needle setting for clean idle and smooth acceleration. Check for vacuum leaks on intake manifold.
Troubleshooting Tip Drive Belt:
Inspect for cracking, fraying, or excessive wear. Replace if width is reduced or if slippage occurs under load.
Brand Focus:
Yamaha Motor Company, renowned for its motorcycles, musical instruments, and marine products, has a long history of innovation in engine technology.
Predecessors:
The QT50 built upon Yamaha's experience with small-displacement, two-stroke engines and automatic transmissions in earlier moped designs.
Successor Models:
Yamaha continued to produce various moped and scooter models, evolving engine technology and chassis designs over the years.
Production Period:
Mid-1980s (specifically 1985 model year covered by manual).
Manufacturing Location:
Likely Japan, though specific assembly locations could vary.
