Honda CB250 Nighthawk Owner's Manual page 189

Emission Control Systems
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.
180
Technical Information
Problems That May Affect Motorcycle
Exhaust Emissions
If you are aware of any of the following
symptoms, have the vehicle inspected and
repaired by your authorized Honda
motorcycle dealer.
Symptoms:
1. Hard starting or stalling after starting
2. Rough idle
3. Misfiring or backfiring during
acceleration
4. After-burning (backfiring)
5. Poor performance (driveability) and
poor fuel economy