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Honda GX-390 Engine

General Technical Data

Honda GX390 Engine Specifications

Engine Model ................. Honda GX390
Engine Type ................. 4-cycle overhead valve single-cylinder air-cooled small gasoline engine
Number of Cylinders ................. 1
Displacement, cu.in (cc) ................. 23.7 (389)
Engine Bore, in (mm) ................. 3.47 (88.0)
Engine Stroke, in (mm) ................. 2.53 (64.0)
Rated Engine Power, hp (kW) ................. 13.0 (10.0)
Rated Engine Speed, rpm ................. 3600
Maximum Torque, Nm (ft-lbs) ................. 27 (20)
Torque Speed, rpm ................. 2500
Compression Ratio ................. 8.0:1
Valve Arrangement ................. OHV
PTO Shaft Rotation ................. Counterclockwise (from PTO side)
Lubrication System ................. Forced splash type
Air Cleaner ................. Dual type, semi dry type
Cooling System ................. Forced air
Ignition System ................. Transistor type magneto
Stopping System ................. Ignition primary circuit ground
Starting System ................. Recoil or electric starter

Fuel System

Fuel System Type ................. Carburetor
Carburetor Type ................. Horizontal, butterfly valve
Governor Type ................. Mechanical centrifugal
Fuel Tank Capacity, qts (L) ................. 6.9 (6.5)
Fuel Type ................. Gasoline (86 or higher)

Dimensions and Weight

Length, in (mm) ................. 15.9 (405)
Width, in (mm) ................. 17.7 (450)
Height, in (mm) ................. 17.4 (443)
Net Weight, lbs (kg) ................. 68.3 (31.0)

Honda GX390 Maintenance Data

Cylinder Block and Valves

Cylinder Bore Inside Diameter (Maximum), in (mm) ................. 3.476 (88.170)
Cylinder Compression, psi (kPa) ................. 85-121 (600-850)
Cylinder Head (Max. Out of Flatness), in (mm) ................. 0.004 (0.10)
Intake Valve Clearance, in (mm) ................. 0.006 (0.15)
Exhaust Valve Clearance, in (mm) ................. 0.008 (0.20)

Engine Oil and Filter

Engine Oil Type (Summer) ................. SAE 10W-30
Engine Oil Type (Winter) ................. SAE 5W-30
API Service Classifications ................. SE or higher
Oil Capacity, qts (L) ................. 1.16 (1.10)
Air Filter Element Part Number ................. 17210-ZE3-010

Carburetor Adjustment

Main Jet (Externally/Internally Vented Bowl) ................. #105 / #115
Idle Fuel ................. 2-1/4 turns out
Float Height, in (mm) ................. 0.52 (13.2)
Low Idle Speed, rpm ................. 1400±150
High Idle Speed, rpm ................. 3850±150

Ignition System

Spark Plug Type ................. NGK BPR6ES or Denso W20EPR-U
Spark Plug Gap, in (mm) ................. 0.028-0.032 (0.70-0.80)
Module Air Gap, in (mm) ................. 0.008-0.024 (0.20-0.60)
Ignition Timing, deg ................. 25 BTDC
Spark Plug Torque, Nm (ft-lbs) ................. 20 (14)

Service Intervals

Service Air Cleaner Element ................. Every 50 hours
Change Engine Oil ................. Every 100 hours
Check and Adjust Spark Plug ................. Every 100 hours
Clean Fuel Strainer Cup ................. Every 100 hours
Check Carburetor Adjustment ................. Every 200 hours
Check and Adjust Valve Clearance ................. Every 300 hours
Replace Spark Plug ................. Every 300 hours

Honda GX390 Service Specifications

Cylinder Block

Cylinder Bore Inside Diameter (Std), in (mm) ................. 3.4711-3.4716 (88.000-88.015)
Cylinder Bore Inside Diameter (Wear Limit), in (mm) ................. 3.476 (88.170)
Connecting Rod Small End Inside Diameter (Std), in (mm) ................. 0.7876 (20.005)
Connecting Rod Small End Inside Diameter (Wear Limit), in (mm) ................. 0.790 (20.07)
Connecting Rod Big End Inside Diameter (Std), in (mm) ................. 1.4183 (36.025)
Connecting Rod Big End Inside Diameter (Wear Limit), in (mm) ................. 1.420 (36.07)
Connecting Rod Big End Oil Clearance, in (mm) ................. 0.0016-0.0026 (0.040-0.066)
Crankshaft Outside Diameter (Std), in (mm) ................. 1.4167 (35.985)
Crankshaft Outside Diameter (Wear Limit), in (mm) ................. 1.415 (35.93)

Cylinder Head and Valves

Number of Valves ................. 2
Cylinder Head (Max. Out of Flatness), in (mm) ................. 0.004 (0.10)
Intake Valve Clearance, in (mm) ................. 0.005-0.007 (0.13-0.17)
Exhaust Valve Clearance, in (mm) ................. 0.007-0.009 (0.18-0.22)
Intake Valve Stem Outside Diameter (Std), in (mm) ................. 0.2589-0.2594 (6.575-6.590)
Intake Valve Stem Outside Diameter (Wear Limit), in (mm) ................. 0.254 (6.44)
Exhaust Valve Stem Outside Diameter (Std), in (mm) ................. 0.2573-0.2579 (6.535-6.550)
Exhaust Valve Stem Outside Diameter (Wear Limit), in (mm) ................. 0.252 (6.40)
Intake Valve Guide Inside Diameter, in (mm) ................. 0.2598-0.2604 (6.600-6.615)
Exhaust Valve Guide Inside Diameter, in (mm) ................. 0.2598-0.2604 (6.600-6.615)
Intake Valve Guide To Stem Clearance, in (mm) ................. 0.0004-0.0016 (0.010-0.040)
Exhaust Valve Guide To Stem Clearance, in (mm) ................. 0.0020-0.0031 (0.050-0.080)
Valve Seat Width (Intake/Exhaust), in (mm) ................. 0.04-0.05 (1.0-1.2)
Valve Spring Free Length, in (mm) ................. 1.54 (39.00)
Cam Height (Intake), in (mm) ................. 1.276-1.291 (32.40-32.80)
Cam Height (Exhaust), in (mm) ................. 1.256-1.271 (31.89-32.29)
Camshaft Outside Diameter (Std), in (mm) ................. 0.6286-0.6293 (15.966-15.984)
Camshaft Outside Diameter (Wear Limit), in (mm) ................. 0.627 (15.92)

Piston and Piston Rings

Piston Pin Bore Inside Diameter (Std), in (mm) ................. 0.7875 (20.002)
Piston Pin Bore Inside Diameter (Wear Limit), in (mm) ................. 0.7891 (20.042)
Piston Skirt Outside Diameter, in (mm) ................. 3.4640 (87.985)
Piston to Cylinder Bore Clearance, in (mm) ................. 0.0006-0.0020 (0.015-0.052)
Piston Pin Outside Diameter (Std), in (mm) ................. 0.787 (20.000)
Piston Pin Outside Diameter (Wear Limit), in (mm) ................. 0.785 (19.95)
Top Compression Ring Width, in (mm) ................. 0.08 (2.00)
Second Compression Ring Width, in (mm) ................. 0.08 (2.00)
Top Compression Ring to Groove Side Clearance, in (mm) ................. 0.0012-0.0024 (0.030-0.060)
Second Compression Ring to Groove Side Clearance, in (mm) ................. 0.0012-0.0024 (0.030-0.060)
Top Compression Ring End Gap, in (mm) ................. 0.010-0.020 (0.20-0.40)
Second Compression Ring End Gap, in (mm) ................. 0.010-0.020 (0.20-0.40)
Oil Ring End Gap, in (mm) ................. 0.010-0.030 (0.20-0.70)
Ring End Gap (Wear Limit), in (mm) ................. 0.04 (1.00)

Honda GX390 Tightening Torque Specs

Crankcase Cover (M8x1.25), Nm (ft-lb) ................. 24 (18)
Cylinder Head Cover Bolt (M10x1.25), Nm (ft-lb) ................. 35 (25)
Rocker Arm Pivot Bolt (M8x1.25), Nm (ft-lb) ................. 24 (18)
Oil Drain Plug Bolt (M12x1.5), Nm (ft-lb) ................. 23 (17)
Connecting Rod Bolt (M8x1.25), Nm (ft-lb) ................. 14 (10)
Flywheel Nut (M16x1.5), Nm (ft-lb) ................. 115 (83)
Air Cleaner Wing Nut (M6x1.0), Nm (ft-lb) ................. 10 (7)
Muffler Bolt (M8x1.25), Nm (ft-lb) ................. 24 (17)
Oil Level Switch Bolt (M10x1.25), Nm (ft-lb) ................. 10 (7)
Fuel Strainer Cup (M24x1.0), Nm (ft-lb) ................. 3.9 (2.9)
Spark Plug (M14x1.25), Nm (ft-lb) ................. 20 (14)