Honda TRX300EX Owner's Manual page 199

Emission Control Systems
Crankcase Emission Control System
The engine is equipped with a closed crankcase system to prevent
discharging crankcase emissions into the atmosphere. Blow-by gas is
returned to the combustion chamber through the air cleaner and the
carburetor.
Problems That May Affect ATV Exhaust Emissions
If you are aware of any of the following symptoms, have the vehicle
inspected and repaired by your Honda ATV dealer.
Symptoms:
1. Hard starting or stalling after starting
2. Rough idle
3. Misfiring or backfiring during acceleration
4. After-burning (backfiring)
5. Poor performance (driveability) and poor fuel economy
Federal regulations prohibit removing or disabling a device or element
of design that may affect your engine's emission performance unless
your ATV will be used exclusively in competition. If you modify your
engine for use in sanctioned competition events, you must deface or
destroy the emission control information label.
Technical Information
190