How to Service a GX120 Honda Engine

Honda GX120If you own professional grade lawn care equipment, chances are you have at least one GX120. That popularity is owed to Honda’s legendary reliability, but even the best engine needs occasional maintenance.

Before You Begin

Before performing any maintenance, shut off the engine. If the engine needs to be run for any reason, make sure it’s located in a well ventilated area to prevent carbon monoxide poisoning.

Maintenance Schedule

Each use: Check the engine oil level, reduction gear oil (if equipped) and air cleaner

First month and every 20 hours of use thereafter: Change the engine oil and reduction gear oil (if equipped)

Every three months or 50 hours: Clean the air cleaner. This may need to be done more frequently if the engine is used in dusty areas.

Every 6 months or 100 hours: Clean the sediment cup, check and clean the spark plug and clean the spark arrester (if equipped)

Every year or 300 hours: Bring your engine to a service center to have the fuel tank and strainer cleaned and have the valve clearance adjusted.

Every two years: Have a service center check the fuel line.

Checking and Changing the Oil

The GX120 holds 0.63 quarts (0.6 liters) of SG SF grade 10W-40 engine oil. Some models have a gear reduction case that holds oil. Look on the side of the motor next to the output shaft for a large metal case. If the case has a dipstick/filler cap, it has 1/2 gear reduction. If the case is round and has a filler bolt on top, it has 1/6 gear reduction. 1/2 gear reduction cases hold 17 oz. (500 cc) while 1/6 reduction cases hold 5.1 oz. (150 cc). Oil doesn’t need to be changed in 1/6 cases, just kept up to the minimum level.

To check the oil level in the engine and in 1/6 reduction cases, wipe the dipstick clean and insert it into the filler hole without screwing it in. Make sure the oil comes up to the marks on the dipstick.

To change the engine oil:
1. Position a drain pan beneath the oil drain plug, located at the base of the engine.
2. Remove the oil filler cap followed by the oil drain plug.
3. Once all the oil has drained out of the motor, replace the drain plug. Add oil through the oil filler cap opening.
4. Put the oil filler cap back in the engine.

To change the oil in 1/2 reduction gear cases:
1. Position a drain pan beneath the drain bolt on the bottom of the case. Remove the dipstick/filler cap followed by the drain bolt.
2. Once the oil has drained, replace the drain plug. Add oil.
3. Screw the oil filler cap into the case.

To add oil in 1/6 reduction gear cases:
1. Remove the fill bolt and oil level bolt.
2. Add oil until it starts to drip out of the oil level bolt hole.
3. Screw the fill bolt and oil level bolt back into the case.

Air Cleaner

Your engine will have a dual element cleaner, a cyclone cleaner or an oil bath cleaner.

Cyclone and dual element air cleaners should be assembled in the following order, starting at the carburetor base: rubber gasket, paper element surrounded by foam element, wide wing nut, cover and small wing nut. The inside of the cyclone housing can be accessed by removing three screws on the top of the cover.

Oil bath cleaners should be assembled in this order: screen, element, cover, washer and wing nut. Clean the base and refill with 2 oz. (60 cc) of fresh oil.

To clean a paper element, tap it against a hard surface to dislodge any dirt.

To clean a foam element, wash it with soap and water or rinse it with a solvent. Once dry, soak the foam in fresh oil and squeeze out any excess.

To clean the cyclone, disassemble the cover, then wash and dry the interior of the housing.

Sediment Cup

1. Turn the fuel valve off
2. Remove the cup and o-ring, located directly below the fuel valve.
3. Wash and dry both pieces. Replace the o-ring if it has been damaged.
4. Screw the cup and o-ring into the carburetor.

Spark Plug

The spark plug gap should be between 0.028-0.031 in (0.7-0.8 mm). Replace the plug if it shows any sign of damage. Tighten 1/8-1/4 turn after it’s hand-tight to ensure it’s seated. If the plug is new, tighten 1/2 turn.

Spark Arrester (If Equipped)

Not all GX120 engines come with a spark arrester, but one can be added to the muffler to comply with local fire safety laws.

1. Remove two 8mm nuts and separate the muffler assembly from the engine.
2. Remove three 4mm screws and take off the exhaust deflector.
3. Remove eight 5mm screws and take off the muffler protector.
4. Remove the 4mm screw and take off the spark arrester.
5. Use a brush to remove deposits from the arrester. Replace the arrester if it is cracked.
6. Assemble in reverse order.

Where to Buy Parts for Your GX120 is a certified Honda small equipment dealer, carrying everything you need to keep your GX120 running. Starting at our Honda engine section, you can match up part numbers with factory diagrams so you know you’re ordering exactly what you need. Live in Canada? We can ship to you, too.

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