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Honda G400 Engine

The Honda G400 is a four-stroke L-head single cylinder horizontal shaft air-cooled small gasoline engine from G-series. A 24.8 cu. in (406 cc) engine has a cylinder bore of 3.38" (86 mm), piston stroke of 2.76" (70 mm), and rated power of 10 Hp (7.5 kW) at 3600 rpm. The compression ratio rating is 6.5:1 or 6.8:1. The Honda G400 engine is equipped with a splash type lubrication system, horizontal carburetor with butterfly valve, transistorized magneto or CDI, and recoil starter. The operating weight is 72.8 lbs (33 kg).

General Technical Data

Engine Specifications

Engine Model ................. Honda G400
Engine Type ................. 4-cycle L-head 1-cylinder air-cooled small gasoline
Number of Cylinders ................. Single
Displacement, cu.in (cc) ................. 24.8 (406)
Cylinder Bore, in (mm) ................. 3.38 (86.0)
Piston Stroke, in (mm) ................. 2.76 (70.0)
Rated Engine Power, hp (kW) ................. 10.0 (7.5)
Rated Engine Speed, rpm ................. 3600
Maximum Torque, Nm (ft-lbs) ................. 22.5 (16.6)
Torque Speed, rpm ................. 3300
Compression Ratio ................. 6.5:1 or 6.8:1 (G400K1)
Lubrication System ................. Splash type
Cooling System ................. Forced air
Ignition System ................. Transistorized or flywheel magneto
Starting System ................. Recoil or electric starter
Electrical System Rating, Volts .................. 12

Fuel System

Fuel System Type ................. Carburetor
Carburetor Type ................. Horizontal with butterfly valve
Fuel Consumption, g/HPh ................. 270
Fuel Type ................. Gasoline (86 or higher)
Fuel Tank Capacity, qts (L) ................. 6.3 (6.0)

Dimensions and Weight

Overall Length, in (mm) ................. 16.9 (430)
Overall Width, in (mm) ................. 15.4 (390)
Overall Height, in (mm) ................. 19.7 (500)
Net Weight, lbs (kg) ................. 72.8 (33.0)

Maintenance Data

Engine Oil

Engine Oil Type (Summer) ................. SAE 10W-40
Engine Oil Type (Winter) ................. SAE 10W
API Service Classifications ................. SE or higher
Oil Capacity, qts (L) ................. 1.3 (1.2)

Carburetor Adjustment

Carb Main Jet ................. #92
Float Height, in (mm) ................. 0.34 (8.6)
Pilot Screw Opening ................. 2 turns
Low Idle Speed, rpm ................. 1400±150
High Idle Speed, rpm ................. 3850±150

Ignition System

Spark Plug Type (Magneto) ................. NGK B4H, BR4HS
Spark Plug Type (CDI) ................. NGK BP4HS-10, BPR4HS-10
Spark Plug Gap (Magneto), in (mm) ................. 0.0236-0.0276 (0.60-0.70)
Spark Plug Gap (CDI), in (mm) ................. 0.035-0.039 (0.9-0.10)
Ignition Timing, deg ................. 20 BTDC
Spark Plug Torque, Nm (ft-lbs) ................. 20 (14.7)

Service Specifications

Cylinder Block

Cylinder Compression, psi (kPa) ................. 85 (588)
Cylinder Bore Inside Diameter (Std), in (mm) ................. 3.39 (86.00)
Cylinder Bore Inside Diameter (Wear Limit), in (mm) ................. 3.396 (86.10)
Connecting Rod Small End I.D (Std), in (mm) .................. 0.63 (16.00)
Connecting Rod Small End I.D (Wear Limit), in (mm) .................. 0.633 (16.08)
Connecting Rod Big End Radial Clearance, in (mm) .................. 0.0019-0.0026 (0.048-0.065)
Connecting Rod Big End Axial Clearance, in (mm) .................. 0.004-0.032 (0.1-0.8)
Crankpin Outside Diameter (Std), in (mm) .................. 1.46 (37.00)
Crankpin Outside Diameter (Wear Limit), in (mm) .................. 1.451 (36.85)
Piston Skirt Outside Diameter (Std), in (mm) ................. 3.39 (86.00)
Piston Skirt Outside Diameter (Wear Limit), in (mm) ................. 3.383 (85.85)
Piston Pin Outside Diameter (Std), in (mm) .................. 0.75 (19.00)
Piston Pin Outside Diameter (Wear Limit), in (mm) .................. 0.748 (18.97)
Top Ring Width, in (mm) .................. 0.08 (2.00)
Middle Ring Width, in (mm) .................. 0.10 (2.50)
Oil Ring Width, in (mm) .................. 0.16 (4.00)
Top Ring Side Clearance, in (mm) .................. 0.0008-0.0024 (0.02-0.06)
Middle Ring Side Clearance, in (mm) .................. 0.0004-0.002 (0.01-0.05)
Oil Ring Side Clearance, in (mm) .................. 0.0004-0.002 (0.01-0.05)
Top Ring End Gap, in (mm) .................. 0.008-0.016 (0.20-0.40)
Middle Ring End Gap, in (mm) .................. 0.008-0.016 (0.20-0.40)
Oil Ring End Gap, in (mm) .................. 0.008-0.012 (0.20-0.30)

Cylinder Head and Valves

Cylinder Head (Max Out of Flatness), in (mm) ................. 0.004 (0.10)
Intake Valve Clearance, in (mm) ................. 0.003-0.006 (0.08-0.16)
Exhaust Valve Clearance, in (mm) ................. 0.004-0.007 (0.11-0.19)
Intake Valve Stem Outside Diameter (Std), in (mm) ................. 0.28 (7.00)
Intake Valve Stem Outside Diameter (Wear Limit), in (mm) ................. 0.272 (6.91)
Exhaust Valve Stem Outside Diameter (Std), in (mm) ................. 0.28 (7.00)
Exhaust Valve Stem Outside Diameter (Wear Limit), in (mm) ................. 0.271 (6.89)
Intake Valve Guide Inside Diameter, in (mm) ................. 0.28 (7.00)
Exhaust Valve Guide Inside Diameter, in (mm) ................. 0.28 (7.00)
Intake Valve Spring Free Length, in (mm) ................. 1.68 (42.7)
Exhaust Valve Spring Free Length, in (mm) ................. 1.68 (42.7)
Intake Valve Seat Width, in (mm) ................. 0.042 (1.06)
Exhaust Valve Seat Width, in (mm) ................. 0.060 (1.40)
Camshaft Journal Outside Diameter (Std), in (mm) ................. 0.70 (17.8)
Camshaft Journal Outside Diameter (Wear Limit), in (mm) ................. 0.697 (17.716)
Intake Cam Height, in (mm) ................. 1.52 (38.6)
Exhaust Cam Height, in (mm) ................. 1.53 (38.6)

Tightening Torque Specs

Cylinder Head Bolt, Nm (ft-lb) ................. 24-26 (17-19)
Connecting Rod Bolt, Nm (ft-lb) ................. 12.6-15.4 (9.1-11)
Flywheel Nut, Nm (ft-lb) ................. 110-120 (80-87)
Crankcase Cover, Nm (ft-lb) ................. 20-24 (15-17)
Fuel Tank Bolt, Nm (ft-lb) ................. 20-28 (15-20)
Fuel Valve, Nm (ft-lb) ................. 12-15 (8.7-10.8)
Oil Drain Plug, Nm (ft-lb) ................. 15-25 (11-18)
Tappet Cover (M6x12), Nm (ft-lb) ................. 8-12 (5.8-8.7)

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