Honda TRX500FM Owner's Manual page 220

Emission Control Systems
Source of Exhaust Emissions
The combustion process produces carbon monoxide (CO), oxides of
nitrogen (NOx) and hydrocarbons (HC). Control of hydrocarbons and
oxides of nitrogen is very important because, under certain conditions,
they react to form photochemical smog when subjected to sunlight.
Carbon monoxide does not react in the same way, but it is toxic.
Honda Motor Co., Ltd. utilizes various systems to reduce carbon
monoxide, oxides of nitrogen and hydrocarbons.
Exhaust Emission Control System
The exhaust emission control system is composed of an oxidation
catalyst and appropriate carburetor settings. No adjustments should be
made except for an idle speed adjustment with the throttle stop screw or
carburetor adjustment for high altitude operation.
The exhaust emission control system is separate from the crankcase
emission control system.
Technical Information 211