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Honda GX390 Specifications

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• Honda GX390 Engine Specifications
• Honda GX390 Engine Troubleshooting

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)
Cylinder Bore, in (mm) .............. 3.47 (88.0)
Piston 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 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)

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

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)

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 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 Ring Width, in (mm) .............. 0.08 (2.00)
Second Ring Width, in (mm) .............. 0.08 (2.00)
Top Ring Groove Side Clearance, in (mm) .............. 0.0012-0.0024 (0.030-0.060)
Second Ring Groove Side Clearance, in (mm) .............. 0.0012-0.0024 (0.030-0.060)
Top Ring End Gap, in (mm) .............. 0.010-0.020 (0.20-0.40)
Second 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)

Cylinder Bore Inside Diameter (Max), 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 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
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

Main Jet (Externally Vented Bowl) .............. #105
Main Jet (Internally Vented Bowl) .............. #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

Check Engine Oil Level .............. Every 10 hours
Clean Air Filter Element .............. Every 50 hours
Change Engine Oil .............. Every 100 hours
Check and Adjust Spark Plug .............. Every 100 hours
Clean Fuel Strainer .............. Every 300 hours
Check and Adjust Idle Speed .............. Every 300 hours
Check Carburetor Adjustments .............. Every 300 hours
Replace Spark Plug .............. Every 300 hours
Replace Air Filter Element .............. Every 300 hours
Check and Adjust Valve Clearance .............. Every 300 hours
Clean Combustion Chamber .............. Every 300 hours
Clean Fuel Line .............. Every 300 hours

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)

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