Honda CB250 Nighthawk Owner's Manual page 188

Exhaust Emission Control System
The exhaust emission control system
consists of lean carburetor settings, and no
adjustment should be made except idle
speed adjustment with the throttle stop
screw.
Emission Control Systems
Evaporative Emission Control System
(California only)
This motorcycle complies with the
requirements of the California Air
Resources Board (CARB) evaporative
emission regulations. Fuel vapor from the
fuel tank and carburetor is directed into
the charcoal canister and air cleaner where
it is absorbed and stored while the engine
is stopped. When the engine is running
and the purge control diaphragm valve is
open, fuel vapor in the charcoal canister
and air cleaner is drawn into the engine
through the carburetor.
Technical Information
179