The Honda GX31 is a 31 cc (1.9 cu·in) natural aspirated single-cylinder four-stroke air-cooled internal combustion small gasoline engine with a horizontal shaft, manufactured by Honda Motor Company for general-purpose applications.
Cylinder bore and piston stroke are 39.0 mm (1.54 in) and 26.0 mm (1.02 in), respectively. The compression ratio rating is 8.0:1.
The Honda GX31 engine produced 1.5 PS (1.12 kW; 1.5 HP) at 7,000 rpm of maximum output power and 1.64 N·m (0.2 kg·m, 1.2 ft·lb) at 6,500 rpm of peak torque.
The Honda GX31 features:
- Overhead valve (OHV) design
- Transistorized magneto ignition system
- Diaphragm type carburetor
- Centrifugal governor
- Pumping spray lubrication system
- Forced air cooling system
- Recoil starter
General information
| Engine Specifications | |
| Engine model | GX31 |
| Engine type | 4-stroke, OHV, single-cylinder |
| Displacement | 31 cm3, (1.9 cu·in) |
| Max. horsepower | 1.5 PS (1.12 kW; 1.5 HP) at 7,000 rpm |
| Max. torque | 1.64 N·m (0.2 kg·m, 1.2 ft·lb) at 6,500 rpm |
| Cooling system | Forced air |
| Air cleaner | Dry type |
| Lubricating system | Pumping spray |
| Ignition system | Transistorized magneto ignition |
| Fuel system | Diaphragm type carburetor |
| Fuel consumption | 250 g/HPh |
| Fuel tank capacity | 0.65 L (0.17 US. gal. 0.14 Imp. gal.) |
| Fuel used | Unleaded gasoline (octane number 86 or higher) |
| Starting system | Recoil starter |
| Stoping system | Ignition primary circuit ground |
| PTO shaft rotation | Counterclockwise (from PTO shaft side) |
| Dimensions (L x W X H) | 210 x 251 x 250 mm (8.3 x 9.9 x 9.8 in) |
| Dry weight | 3.4 kg (7.5 lbs) |
Cylinder Block and Head
| Cylinder block | |
| Compression ratio: | 8.0:1 |
| Cylinder bore: | 39.0 mm (1.54 in) |
| Piston stroke: | 26.0 mm (1.02 in) |
| Cylinder internal diameter (standard): | 39.000 mm (1.5354 in) |
| Piston skirt outer diameter (standard): | 38.990 mm (1.535 in) |
| Number of piston rings (compression / oil): | 2/1 |
| Ring end gap (Top): | 0.100-0.250 mm (0.0039-0.0098 in) |
| Ring end gap (Second): | 0.100-0.250 mm (0.0039-0.0098 in) |
| Ring width (Top): | 0.980 mm (0.0386 in) |
| Ring width (Second): | 0.015-0.050 mm (0.0006-0.002 in) |
| Crankshaft center distance: | 13.0 mm (0.51 in) |
| Cylinder head | |
| Valve arrangement: | OHV |
| Valves: | 2 |
| Valve steam diameter (INTAKE): | 3.985 mm (0.1569 in) |
| Valve steam diameter (EXHAUST): | 3.950 mm (0.1555 in) |
| Camshaft lobe height (INTAKE): | 22.790 mm (0.8972 in) |
| Camshaft lobe height (EXHAUST): | 22.790 mm (0.8972 in) |
| Camshaft journal diameter : | 5.020 mm (0.1976 in) |
Service Data
| Idle speed: | 3,100 ± 200 rpm |
| Cylinder compression: | 2.9-5.9 kg/cm2 (0.3-0.6 MPa, 41-84 psi) at 1,200 rpm |
| Valve clearance | |
| Intake valve clearance: | 0.12 ± 0.02 mm (0.005 ± 0.001 in) |
| Exhaust valve clearance: | 0.15 ± 0.02 mm (0.006 ± 0.001 in) |
| Oil system | |
| Lubrication system: | Pumping spray |
| Oil type: | Honda 4-stroke or an equivalent (SG, SF) |
| Recommended oil: | SAE 10W-30 |
| Oil capacity: | 0.1 L (0.11 US. qt, 0.09 Imp. qt.) |
| Fuel system | |
| Type: | Diaphragm type carburetor |
| Main jet: | #40 |
| Ignition system | |
| Type: | Transistorized magneto ignition |
| Ignition timing: | 27° B.T.D.C. (Fixed) |
| Spark plug: | NGK C5HSB, CR5HSB DENSO U16FS-UB, U16FSR-UB |
| Spark plug gap: | 0.6-0.7 mm (0.0236-0.0276 in) |
| Ignition coil air gap | 0.35-0.5 mm (0.0138-0.0197 in) |
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