Yamaha MZ125

Yamaha MZ125The Yamaha MZ125 is a 123 cc (7.5 cu-in) single-cylinder air-cooled 4-stroke internal combustion general-purpose gasoline engine with horizontal shaft manufactured by Yamaha Motor Co., Ltd.

The Yamaha MZ125 engine has an OHV (overhead valve) design; float type carburetor; centrifugal flyweight governor; mechanical splashing lubrication system; Transistor Controlled Ignition (T.C.I.) and recoil or electrical (option) starter. The Yamaha MZ125 used aluminum cylinder block with the spiny cast-iron sleeve. For exhaust valve used high heat resistant SUH3 steel.

The cylinder bore is 56.0 mm (2.20 in) and the piston stroke is 50.0 mm (1.97 in), compression ratio rating is 8.3:1. The engine produced 3.4 PS (2.5 kW) at 3,600 rpm of maximum horsepower and 2.9 PS (2.1 kW) at 3,600 rpm of continuous rated power. Maximum torque is 7.3 N·m (0.7 kg·m, 5.4 ft·lb) at 2,400 rpm.

General information

Engine Specifications
Model MZ125
Type 4-stroke, OHV, single cylinder
Displacement 123 cm 3 (7.5 cu-in)
Max. horsepower 3.4 PS (2.5 kW) at 3,600 rpm
Max. torque (crank PTO) 7.3 N·m (0.7 kg·m, 5.4 ft·lb) at 2,400 rpm
Carburetor Float type
Cooling system Forced-air
Ignition system Transistor Controlled Ignition (T.C.I.)
Lubricating system Mechanical splashing
Starting system Recoil or Electrical (option)
Fuel used Unleaded gasoline (octane number 86 or higher)
Fuel tank capacity 4.5 L (1.18 US Gal)
PTO shaft rotation Counterclockwise (from PTO shaft side)

Dimensions and Weight

Yamaha MZ125
Length, mm (in) 325 (12.8)
Width, mm (in) 352 (13.8)
Height, mm (in) 370 (14.6)
Dry weight, kg (lb) 15.5 (34.2)

Cylinder Block and Head

Cylinder block
Compression ratio: 8.3:1
Bore x Stroke 56 X 50 mm (2.20 x 1.97 in)
Sleeve internal diameter 56.005-56.015 mm (2.0249-2.2053 in)
Piston skirt outer diameter 56.000 mm (2.205 in)
Number of piston rings (compression / oil): 2 / 1
Ring side clearance Top 0.040-0.080 (0.0016-0.0030 in)
Second 0.020-0.060 (0.0008-0.0024 in)
Ring end gap Top 0.20-0.40 mm (0.008-0.016 in)
Second 0.20-0.40 mm (0.008-0.016 in)
Oil 0.20-0.40 mm (0.008-0.016 in)
Piston pin outer diameter 15.995-16.000 mm (0.6297-0.6299 in)
Connecting rod small end inner diameter 16.006-16.020 mm (0.6301-0.6307 in)
Connecting rod big end inner diameter 28.000-28.015 mm (1.1023-1.1029 in)
Crankpin outer diameter 28.00 mm (1.10 in)
Cylinder Head
Warpage (limit) 0.10 mm (0.004 in)
Valve Arrangement: OHV
Valves: 2
Valve head diameter INTAKE 21.0 mm (0.83 in)
EXHAUST 19.0 mm (0.75 in)
Stem outer diameter (standard): INTAKE 5.5 mm (0.22 in)
EXHAUST 5.5 mm (0.22 in)
Cam height: INTAKE 26.9 mm (1.06 in)
EXHAUST 26.68 mm (1.05 in)
Camshaft journal diameter: 14.193-14.989 mm (0.5895-0.5901 in)

Tightening torque specs

Tightening torque specs
Cylinder head 20 Nm; 2.0 kg·m; 14 ft·lb
Cylinder head cover 10 Nm; 1.0 kg·m; 7 ft·lb
Crankcase cover 22 Nm; 2.2 kg·m; 16 ft·lb
Connecting rod 12 Nm; 1.2 kg·m; 9 ft·lb
Flywheel magneto 65 Nm; 6.5 kg·m; 47 ft·lb
Governor arm 8 Nm; 0.8 kg·m; 6 ft·lb
TCI unit 10 Nm; 1.0 kg·m; 7 ft·lb
Oil drain plug 17 Nm; 1.7 kg·m; 12 ft·lb

Service Information

Valve clearance
Intake valve clearance 0.10 mm (0.004 in)
Exhaust valve clearance 0.10 mm (0.004 in)
Oil system
Oil type Yamalube 4-stroke or an equivalent (SE or SF)
Recommended oil SAE 10W-30, 10W-40
Oil capacity 0.6 L (0.63 US. qt, 0.53 Imp. qt.)
Model Mikuni BV-18-11
Bore size 18/11
Main jet #78.8
Pilot jet #78.8
Float height 16.0 mm (0.63 in)
Pilot screw opening 2 turns out
Ignition system
Spark plug NGK: BPR4ES
Spark plug tightening torque 18 Nm; 1.8 kg·m; 13 ft·lb
Spark plug gap 0.7-0.8 mm (0.028-0.031 in)
Ignition coil air gap 0.5 mm (0.02 in)
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