Honda CRF50F 2004 Owner's Manual page 149

Emission Control Systems
Source of Emissions
The combustion process produces carbon
monoxide (CO) and hydrocarbons (HC).
Control of hydrocarbons 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 lean carburetor
settings and other systems to reduce carbon
monoxide and hydrocarbons.
Exhaust Emission Control System
The exhaust emission control system is
composed of lean carburetor settings, and no
adjustments should be made except idle speed
adjustment with the throttle stop screw.
The exhaust emission control system is separate
from the crankcase emission control system.
140
Technical Information
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.