Honda GX390 Engine Instruction Manual
Honda GX390 Engine
Honda GX390 Engine Instructions
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Honda GX390 Engine Specifications
| Feature | Specification |
|---|---|
| Engine Model | Honda GX390 (PTO shaft type S, with fuel tank) |
| Engine Type | 4-stroke, overhead valve, single cylinder |
| Displacement | 389 cm³ (23.7 cu-in) |
| Bore × Stroke | 88.0 × 64.0 mm (3.5 × 2.5 in) |
| Net Power (SAE J1349) | 8.2 kW (11.1 PS, 11.0 bhp) at 3,600 rpm |
| Max. Net Torque (SAE J1349) | 25.1 N·m (2.56 kgf·m, 18.5 lbf·ft) at 2,500 rpm |
| Dimensions (L × W × H) | 380 × 450 × 447 mm (15.0 × 17.7 × 17.6 in) |
| Dry Mass | 31.5 kg (69.4 lbs) |
| Engine Oil Capacity | 1.1 L (1.2 US qt, 1.0 Imp qt) |
| Fuel Tank Capacity | 6.1 L (1.61 US gal, 1.34 Imp gal) |
| Cooling System | Forced air |
| Ignition System | Transistor magneto |
| PTO Shaft Rotation | Counterclockwise |
| Recommended Fuel | Unleaded gasoline, pump octane 86 or higher (research octane 91 or higher) |
| Maximum Alcohol Content in Fuel | 10% ethanol (E10) or 5% methanol by volume |
| Recommended Engine Oil | SAE 10W-30, API SJ or later, for general use |
| Reduction Case Oil Capacity (1/2 reduction, applicable types) | 0.30 L (0.32 US qt, 0.26 Imp qt) |
| Oil-Bath Air Cleaner Oil Capacity | 80 cm³ (2.7 US oz, 2.8 Imp oz) |
| Recommended Spark Plugs | BPR6ES (NGK), W20EPR-U (DENSO) |
| Spark Plug Gap | 0.7–0.8 mm (0.028–0.031 in) |
| Spark Plug Wrench Size | 13/16 inch |
| Standard Idle Speed | 1,400 ± 150 rpm |
| Valve Clearance (cold) | IN: 0.15 ± 0.02 mm; EX: 0.20 ± 0.02 mm |
| Recommended Battery (electric starter types) | 12 V – 18 Ah to 12 V – 30 Ah |
| Paper Filter Max. Cleaning Air Pressure | 207 kPa (2.1 kgf/cm², 30 psi) |
| High Altitude Carburetor Modification Threshold | Above 1,500 m (5,000 ft) |
| Power Loss with Altitude | Approx. 3.5% per 300 m (1,000 ft) of altitude, even with carburetor modification |
| Fuse Cover Screw | 6 × 12 mm special screw |
Honda GX390 Engine Details
The Honda GX390 is a general-purpose engine widely used to power generators and other equipment. The owner's manual says it should be treated as a permanent part of the engine. It also reminds operators that the instructions supplied by the equipment manufacturer cover additional startup, shutdown and adjustment procedures. The engine produces carbon monoxide, so it must never run indoors or in partly enclosed areas. During operation, keep it at least 1 meter (3 feet) from buildings and other equipment.
A key protective feature is the Oil Alert system (applicable types). It automatically stops the engine before the crankcase oil level falls below a safe limit, while the engine switch stays in the ON position. If the engine stops and won't restart, check the oil level first. Electric starter versions also have a circuit protector for the battery charging circuit. A short circuit or reversed battery polarity trips it, and a green indicator pops out. Push the button to reset it.
Starting follows a clear sequence. Turn the fuel valve ON and close the choke for a cold start. Move the throttle about 1/3 of the way toward MAX, switch the engine ON and operate the starter. Pull the recoil starter lightly until you feel resistance, then pull briskly. Never run the electric starter for more than 5 seconds at a time, and wait at least 10 seconds before trying again. To stop the engine, move the throttle to MIN, turn the switch OFF and close the fuel valve.
The maintenance schedule calls for an engine oil change after the first month or 20 hours, then every 6 months or 100 hours. Clean the air cleaner every 3 months or 50 hours and replace the paper element yearly or every 300 hours. Cyclone types follow 150-hour and 600-hour intervals. Clean the sediment cup and service the spark plug every 6 months or 100 hours. A dealer should clean the combustion chamber every 500 hours. Check the fuel tube every 2 years.
Several air cleaner versions are covered: standard dual-element, cyclone dual-element, oil bath, single-element and low profile. Tap paper elements clean or blow them out from the inside. Wash foam elements, soak them in engine oil and squeeze out the excess, or the engine will smoke. The optional spark arrester must be cleaned every 100 hours with a brush. To reach it, remove the 8 mm muffler nuts, the 4 mm deflector screws and the 5 mm protector screws.
When installing spark plugs, tighten a new plug an extra 1/2 turn after it seats and a reused plug 1/8–1/4 turn. Electric starter models protect the starter relay and charging circuits with a fuse. If it blows, the engine can still be started manually, but the battery won't charge. Never fit a fuse with a higher rating. The throttle and choke levers have holes for remote cable linkage. When you use a remote throttle, loosen the friction nut.
For storage, use fuel stabilizer or drain the tank and carburetor, because gasoline can deteriorate in as little as 30 days. Pour 5–10 cc of engine oil into the cylinder. Then pull the starter until the pulley notch lines up with the cover hole, which closes the valves. Recharge the battery monthly and don't use a plastic cover. When transporting the engine, let it cool for at least 15 minutes and keep it level with the fuel valve OFF.
Frequently Asked Questions (FAQs)
Can't find the answer you need? Here are some frequently asked questions about the Honda GX390 Engine, and we answer them below.
