Honda CRF50F 2007 Owner's Manual page 148

Source of 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.
Emission Control Systems
Exhaust Emission Control System
The exhaust emission control system is
composed of 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.
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
139