Honda GX120 Owner's Manual page 33

Paper air filter element: Tap the filter element several times on a hard
surface to remove dirt, or blow compressed air [not exceeding 30 psi
(207 kPa, 2.1 kgf cm )] through the filter element from the inside.
Never try to brush off dirt; brushing will force dirt into the fibers.
Foam air filter element: Clean in warm soapy water, rinse, and allow to
dry thoroughly. Or clean in nonflammable solvent and allow to dry.
Dip the filter element in clean engine oil, then squeeze out all excess
oil. The engine will smoke when started if too much oil is left in the
foam.
6. Wipe dirt from the inside of the air cleaner base and cover, using a
moist rag. Be careful to prevent dirt from entering the air duct that
leads to the carburetor.
7. Place the foam air filter element over the paper element, and
reinstall the assembled air filter. Be sure the gasket is in place
beneath the air filter. Tighten the air filter wing nut securely.
8. Install the air cleaner cover, and tighten the cover wing nut securely.
SERVICING YOUR ENGINE
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