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
Type4-stroke, OHV, single cylinder, inclined by 25°
Displacement389 cm 3 (23.7 cu·in)
Max. horsepower13.0 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
CarburetorHorizontal type, butterfly valve
Cooling systemForced-air
Ignition systemTransistorized magneto ignition
Lubricating systemForced splash
Starting systemRecoil and Starter motor
Stoping systemIgnition primary circuit ground
Fuel usedUnleaded gasoline (octane number 86 or higher)
Fuel consumption6.5 Liters (1.72 US gal, 1.43 Imp gal) / h
PTO shaft rotationCounterclockwise (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 Stroke88 X 64 mm (3.5 x 2.5 in)
Sleeve internal diameter88.000 mm (3.465 in)
Piston skirt outer diameter87.985 mm (3.4640 mm)
Number of piston rings (compression / oil):2 / 1
Ring side clearanceTop/second0.030 – 0.060 (0.0012 – 0.0024)
Ring end gapTop0.20 – 0.40 mm (0.01 – 0.02 in)
Second0.20 – 0.40 mm (0.01 – 0.02 in)
Oil (side rail)0.20 – 0.70 mm (0.01 – 0.03 in)
Ring widthTop2.0 mm (0.08 in)
Second2.0 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 diameter35.985 mm (1.4167 in)
Cylinder Head
Warpage (limit)0.10 mm (0.004 in)
Valve Arrangement:OHV
Stem outer diameter (standard):INTAKE6.590 mm (0.2594 in)
EXHAUST6.550 mm (0.2579 in)
Guaide inner diameter:INTAKE/EXHAUST6.60 mm (0.6260 in)
Cam height:INTAKE32.40-32.80 mm (1.276-1.291 in)
EXHAUST31.89-32.29 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 speed3850 ± 150 rpm
Idle speed1400 ± 150 rpm
Cylinder compression6.0-8.5 kg/cm2 (85-121 psi, 0.59 – 0.83 MPa) at 600 rpm
Valve clearance
Intake valve0.15 ± 0.02 (0.006 ± 0.001)
Exhaust valve0.20 ± 0.02 (0.008 ± 0.001)
Main jet Externally vented carburetor bowl: #105
Internally vented carburetor bowl: #115
Float height13.2 mm (0.52 in)
Pilot screw opening Externally vented carburetor bowl: 2-1/2
Internally vented carburetor bowl: 3
Oil system
Oil typeHonda 4-stroke or an equivalent (SE or later)
Recommended oilSAE 10W-30
Oil capacity1.1 L (1.16 US gal, 0.97 lmp qt)
Ignition system
Ignition timingB.T.D.C. 25° (Fixed)
Spark plugNGK: BP6ES, BPR6ES
Spark plug gap0.7-0.8 mm (0.028-0.031 in)
Air gap0.2-0.6 mm (0.01-0.02 in)
Starter motor
Brush length7.0 mm (0.28 in) limit: 3.5 mm (0.14 in)
Mica depth1.0 mm (0.04 in) limit: 0.2 (0.01)
Author: Sergei Wers
About: Co-founder and CEO of a multi-brand auto repair shop (engine repair, chassis repair, body repair and painting) for over 15 years. I have been developing internet sites with technical specifications for more than seven years.

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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,

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