Honda GX engine oil, plugs and separate oil reservoirs

The engine on pressure washers, generators and compactors

The GX240, GX270, GX340 and GX390 are single-cylinder, overhead-valve, forced-air engines that share one owner’s manual. The GX240 and GX270 displace 270 cubic centimeters and the GX340 and GX390 displace 389 cubic centimeters. All four take the same 1.1 liter (1.2 US quart) oil capacity.

Honda’s oil-bath air cleaner, on engines that have one, takes a separate 60 cubic centimeters (2.0 US ounces) on the GX240 and GX270 and 80 cubic centimeters (2.7 US ounces) on the GX340 and GX390, of the same oil as the engine. That is the source of the “80 cc” figure some listings quote, and it is not the engine’s crankcase capacity.

The Honda GX240, GX270, GX340 and GX390 engines schedule

Parts to buy for the Honda GX240, GX270, GX340 and GX390 engines

These engines have no oil filter. Cyclone and oil-bath air cleaners use different elements, so match the part to the air cleaner on your engine.

Choosing between the GX240, GX270, GX340 and GX390

T&L Equipment's GX comparison cautions that between series, for example GX240 to GX270 to GX340, you should check every position in the mounting-bolt pattern, and notes that equipment such as reversible plate compactors is built around one of the three depending on the machine's weight.

Honda's engine page explains that its horsepower is a net figure measured to the SAE J1349 standard with the production muffler and air cleaner fitted, which is why ratings printed on equipment can differ from a bare-engine number.

Altitude, Oil Alert and the pre-start checks on a GX240 to GX390

The Honda owner’s manual says the standard carburetor mixture is too rich at high altitude, which lowers power, raises fuel use and can foul the spark plug and cause hard starting. If you always run above 1,500 meters (5,000 feet), it tells you to have a servicing dealer modify the carburetor.

Even then, power falls about 3.5% for each 300 meters (1,000 feet) of altitude. It also warns that a modified carburetor is too lean for low altitude and can overheat the engine below 1,500 meters.

On types fitted with it, the Oil Alert system stops the engine before the crankcase oil falls below a safe level, and the engine switch stays in the ON position. The manual's advice follows from that: if the engine stops and will not restart, check the oil level before troubleshooting anything else. It still says to check the oil before every start, along with the fuel level, the air filter element and the equipment the engine drives, and on applicable types the reduction case oil.

Stabilizer, storage and transport for Honda GX engines

To use a stabilizer, the manual says to fill the tank with fresh gasoline, because air in a part-filled tank promotes deterioration, add the stabilizer as directed, then run the engine outdoors for 10 minutes so the treated fuel replaces the untreated fuel in the carburetor.

For storage, keep the engine level, because tilting can leak fuel or oil, in a ventilated dry place away from any flame appliance or spark-producing motor. Cover it only when cool and never with sheet plastic, because a nonporous cover traps moisture and promotes rust. Electric-start types need the battery recharged once a month.

If the cylinder was coated with oil for storage, the manual says brief smoke at the next start is normal. Before loading equipment onto a truck, let the engine cool for at least 15 minutes.

The horizontal-shaft engine under half the pressure washers and pumps in the shed

The GX120, GX160 and GX200 are Honda’s general-purpose horizontal-shaft engines. They turn up on pressure washers, water pumps, generators, plate compactors and a long list of other equipment, which is why their figures are among the most looked-up on this site. They are four-stroke, overhead-valve, single-cylinder engines.

What separates them is size: 122, 163 and 196 cubic centimeters, and with it a little more oil each step up.

All three share the same oil type, plug and gap. The oil is SAE 10W-30 of API service category SJ or later. The spark plug is an NGK BPR6ES, or the Denso equivalent W20EPR-U, set to a gap of 0.7 to 0.8 mm (0.028 to 0.031 inch).

Idle speed is 1,400 RPM, plus 200 or minus 150. The valve clearance tolerance is plus or minus 0.02 mm, and Honda points to the dealer or the shop manual for setting it.

Honda’s schedule, in the order you will do it

Honda gives cyclone-type air cleaners their own intervals: cleaned every 6 months or 150 hours and replaced every 2 years or 600 hours. Two details are specific to this family. Engines fitted with a reduction case, the gearbox on the output, carry a separate oil capacity of 17 US ounces (0.50 L), and that oil is checked and changed on the same schedule as the engine oil.

And the sediment cup under the fuel valve is a maintenance item, cleaned every six months, which is one reason a GX that sat all winter starts better once the cup is emptied.

Parts to buy for the Honda GX120, GX160 and GX200

There is no oil filter on these engines. The Locate model plates and engine IDs shows where the engine model and serial number sit if you need to confirm a part.

How the GX160 and GX200 differ

Green Packs notes the GX160 is the smaller, shorter engine, with a shorter stroke than the GX200, and E-Catalog's comparison lists the GX200 at 200 cm³ and 4.8 kW against 163 cm³ and 3.6 kW for the GX160, both four-stroke and air-cooled, with a larger fuel tank on the GX200 (3.6 L against 3.1 L).

Engine Tech Specs gives the GX200's rated power as 6.5 hp (4.8 kW) at 3,600 RPM. Because the family shares its service items, the oil, plug and filter information here applies across them, and the displacement is what to check when buying a replacement part.