Honda TRX420TM Owner's Manual page 209

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.
Emission Control Systems
Technical Information
201