Yamaha BIG BEAR PROFESSIONAL 400 YFM400FPT Owner's Manual page 304

CAUTION:
_
Never operate the engine with the air filter ele-
ment removed. This will allow unfiltered air to
enter, causing rapid engine wear and possible
engine damage. Additionally, operation with-
out the air filter element will affect carburetor
jetting with subsequent poor performance and
possible engine overheating.
_
8-53