Honda FourTrax Recon TRX250TM 2019 Owner's Manual page 197

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 appropriate
carburetor settings including oxidation catalytic converter and
secondary air injection system.
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.
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.
Technical Information 187