Honda GX390 K1

Honda GX390K1 (13.0 HP)The Honda GX390 K1 is a 0.39 l (389 cc, 23.7 cu·in) single-cylinder air-coolled 4-stroke internal combustion small gasoline engine with horizontal shaft, manufactured by Honda Motor for general-purpose applications, such as: pressure washers, tillers and cultivators, generators, construction or industrial equipment, water pumps.

From spring 2010 onwards, Honda producing the updated version of GX390 engines. These engines meet the most stringent exhaust gas emission regulation, EPA Phase III.

The Honda GX390 K1 has overhead valve design, single-cylinder inclined by 25°, transistorized magneto ignition system and horizontal carburetor with a butterfly valve. It was equipped with recoil or electric starter.

The Honda GX390K1 produces 13.0 HP (10.0 kW) at 3,600 rpm and 27 Nm (2.7 kg·m, 20.0 ft·lb) of torque at 2,500 rpm. It has a bore and stroke of 88.0 mm x 64.0 mm. Compression ratio rating is 8.0:1.

General information

Engine Specifications
Model GX390K1
Type 4-stroke, OHV, single cylinder, inclined by 25°
Displacement 389 cm 3 (23.7 cu·in)
Max. horsepower 13.0 HP (10.0 kW) at 3,600 rpm
Max. torque (crank PTO) 27 Nm (2.7 kg·m, 20.0 ft·lb) at 2,500 rpm
Carburetor Horizontal type, butterfly valve
Cooling system Forced-air
Ignition system Transistorized magneto ignition
Lubricating system Forced splash
Starting system Recoil and Starter motor
Stoping system Ignition primary circuit ground
Fuel used Unleaded gasoline (octane number 86 or higher)
Fuel consumption 6.5 Liters (1.72 US gal, 1.43 Imp gal) / h
PTO shaft rotation Counterclockwise (from PTO shaft side)

Dimensions and Weight

Honda GX390K1 (Type S)
Length, mm (in) 380 (15.0)
Width, mm (in) 450 (17.7)
Height, mm (in) 443 (17.4)
Dry weight, kg (lb) 31.0
weight, kg (lb)

Cylinder Block and Head

The Honda GX390 has cast iron cylinder sleeve, ball bearing supported crankshaft, improved piston design, heavy duty balancer shaft. The piston has two compression rings and one oil ring. Precision camshaft design offers precise valve timing and optimal valve overlap for better fuel efficiency.

Cylinder block
Compression ratio: 8.0:1
Bore x Stroke 88 X 64 mm (3.5 x 2.5 in)
Sleeve internal diameter 88.000 mm (3.465 in)
Piston skirt outer diameter 87.985 mm (3.4640 mm)
Number of piston rings (compression / oil): 2 / 1
Ring side clearance Top/second 0.030 – 0.060 (0.0012 – 0.0024)
Ring end gap Top 0.20 – 0.40 mm (0.01 – 0.02 in)
Second 0.20 – 0.40 mm (0.01 – 0.02 in)
Oil (side rail) 0.20 – 0.70 mm (0.01 – 0.03 in)
Ring width Top 2.0 mm (0.08 in)
Second 2.0 mm (0.08 in)
Connecting rod small end I.D. 20.005 mm (0.7876 in)
Connecting rod big end I.D. 36.025 mm (1.4183 in)
Crankshaft outer diameter 35.985 mm (1.4167 in)
Cylinder Head
Warpage (limit) 0.10 mm (0.004 in)
Valve Arrangement: OHV
Valves: 2
Stem outer diameter (standard): INTAKE 6.590 mm (0.2594 in)
EXHAUST 6.550 mm (0.2579 in)
Guaide inner diameter: INTAKE/EXHAUST 6.60 mm (0.6260 in)
Cam height: INTAKE 32.40-32.80 mm (1.276-1.291 in)
EXHAUST 31.89-32.29 mm (1.256-1.271 in)
Camshaft outer diameter: 15.984 mm (0.6293 in)

Tightening torque specs

Tightening torque specs
Connecting rod bolt (M8x1.25 Special) 14 Nm; 1.4 kg·m; 10 ft·lb
Cylinder head bolt (M10x1.25) 35 Nm; 3.5 kg·m; 25 ft·lb
Flywheel nut (M16x1.5 Special) 115 Nm; 11.5 kg·m; 83 ft·lb
Rocker arm pivot lock nut (M6x0.5) 10 Nm; 1.0 kg·m; 7 ft·lb
Rocker arm pivot bolt (M8x1.25 Special) 24 Nm; 2.4 kg·m; 17 ft·lb
Crankcase cover bolt (M8x1.25) 24 Nm; 2.4 kg·m; 17 ft·lb
Oil level switch joint nut (M10x1.25) 10 Nm; 1.0 kg·m; 7 ft·lb
Fuel filter joint nut (M10x1.25) 2 Nm; 0.2 kg·m; 1.4 ft·lb
Muffler mounting nut (M8x1.25) 24 Nm; 2.4 kg·m; 17 ft·lb
Air cleaner wing nut (M6x1.0) 9 Nm; 0.9 kg·m; 6.5 ft·lb
Oil drain bolt (M12x1.5) 23 Nm; 2.3 kg·m; 17 ft·lb
Fuel tank bolt (M8x1.25) 24 Nm; 2.4 kg·m; 17 ft·lb
Air cleaner mounting nut (M6x1.0) 10 Nm; 1.0 kg·m; 7 ft·lb
Fuel strainer cup (M24x1.0) 4 Nm; 0.4 kg·m; 2.9 ft·lb
Standard torque values
Bolt, nut (5 mm) 5.5 Nm; 0.55 kg·m; 4.0 ft·lb
Bolt, nut (6 mm) 10 Nm; 1.0 kg·m; 7 ft·lb
Bolt, nut (8 mm) 24 Nm; 2.4 kg·m; 17 ft·lb
Bolt, nut (10 mm) 37.5 Nm; 3.75 kg·m; 27 ft·lb
Bolt, nut (12 mm) 55 Nm; 5.5 kg·m; 40 ft·lb

Maintenance data

Maximum speed 3850 ± 150 rpm
Idle speed 1400 ± 150 rpm
Cylinder compression 6.0-8.5 kg/cm2 (85-121 psi, 0.59 – 0.83 MPa) at 600 rpm
Valve clearance
Intake valve 0.15 ± 0.02 (0.006 ± 0.001)
Exhaust valve 0.20 ± 0.02 (0.008 ± 0.001)
Main jet Externally vented carburetor bowl: #105
Internally vented carburetor bowl: #115
Float height 13.2 mm (0.52 in)
Pilot screw opening Externally vented carburetor bowl: 2-1/2
Internally vented carburetor bowl: 3
Oil system
Oil type Honda 4-stroke or an equivalent (SE or later)
Recommended oil SAE 10W-30
Oil capacity 1.1 L (1.16 US gal, 0.97 lmp qt)
Ignition system
Ignition timing B.T.D.C. 25° (Fixed)
Spark plug NGK: BP6ES, BPR6ES
Spark plug gap 0.7-0.8 mm (0.028-0.031 in)
Air gap 0.2-0.6 mm (0.01-0.02 in)
Starter motor
Brush length 7.0 mm (0.28 in) limit: 3.5 mm (0.14 in)
Mica depth 1.0 mm (0.04 in) limit: 0.2 (0.01)
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  1. what is the air gap on igncoil weak spark kicks back valves and everything seems fine , anthing else to look for! thank you

  2. What is the bolt size for outer pattern pump mount 1″ shaft engine? 10mm feels tight is it 3/8″ 1″1/2 HT UNC

  3. The oil fill has an odd conversion, 1.1 liter shouldn’t equal a gallon. Probably 1.16 qt.

  4. After 10 minutes the engine shuts off.I suspect because it gets hot.What can I do to fix the problem?

  5. if your honda is kicking back, the valves are not right, .006 intake .008 exhaust
    their is a triangle stamped where the pull rope engauges make sure it is straight up and down before you adjust the valves, you have to pull the pullrope assembly off to see it, also pull the spark plug,

  6. we haven’t used our motor for a year and we cleaned everything up but found that the crank case was full of gas…what and why. Can you shed any light on this. We are putting all new oil in but I am not convinced that the “gas leak” is fixed.

    • Same thing happened to mine because I forgot to shut the fuel switch off before putting into storage. Every drop of gas made it’s way into the crankcase. Drained the crankcase, refilled with oil, put gas in the tank and runs great.

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