RIEJU MRT 50 EURO 4 Owner's Manual page 40

After removal, check to see if the foam filter is blocked, in which
case wash with specific product to clean foam filter, apply some
air filter foam oil, Rieju suggest use LIQUI MOLY FOAM FILTER OIL,
then squeeze the filter material between your hands to remove
surplus oil. Do not wring out the filter element, as this will
damage the foam. When refitting the air filter sure that the
foam fits the box correctly so that no air can enter the engine
without being filtered. It must be cleaned more frequently if the
motorcycle is used in humid or dusty places.
ATTENTION: Don't do the periodic maintenance of this
component and the correct cleaning can be the root cause
of a bad carburetion of the bike and bad function of the
engine and can damage it. Damages for this cause will
exclude of the warranty.
...for everyday adventure
MRT 50 EURO 4
40