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 (68.3) |
| Operating weight, kg (lb) |
37.0 (81.6) |
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
| Engine | |
| 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) |
| Carburetor | |
| 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 DENSO: W20EP-U, W20EPR-U |
| 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) |
Author:
What is the air gape on the coil to flywheel. I dont have any spark? Any sujestions?
what is the air gap on igncoil weak spark kicks back valves and everything seems fine , anthing else to look for! thank you
Air gap at flywheel: 0.2-0.6 mm (0.01-0.02 in)
What is a normal exhaust gas tempature for the GX390?
What is the direct replacement for the GX390K1 model? I cant find the older version.
Thanks,
What is the bolt pattern dimension to mount a pump adaptor
what is the crankshaft end float measurement?
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
Very informative site !!!
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Roger
what is head bolt torque on a 390 Honda motor
Great website! Thank you!!!
The oil fill has an odd conversion, 1.1 liter shouldn’t equal a gallon. Probably 1.16 qt.
After 10 minutes the engine shuts off.I suspect because it gets hot.What can I do to fix the problem?
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,
What do you check when you have no spark at the plug?