Yamaha BIG BEAR PROFESSIONAL 400 YFM400FPT Owner's Manual page 302

9. Apply all-purpose grease to the air filter ele-
ment seat.
10. Install the air filter element on the frame.
11. Install the air filter element.
12. Install the air filter case cover by installing the
screws.
13. Install the seat.
NOTE:
_
The air filter element should be cleaned every 20–
40 hours. It should be cleaned and lubricated more
often if the machine is operated in extremely dusty
areas. Each time air filter element maintenance is
performed, check the air inlet to the air filter case
for obstructions. Check the air filter element rubber
joint to the carburetor and manifold fittings for an
airtight seal. Tighten all fittings securely to avoid
the possibility of unfiltered air entering the engine.
_
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