Honda CB250 Nighthawk Owner's Manual page 187

Emission Control Systems
Noise Emission Requirements
The EPA also requires that motorcycles
built after January 1, 1983 comply with
applicable noise emission standards for
one year or 3,730 miles (6,000 km) after
the time of sale to the ultimate purchaser,
when operated and maintained according
to the instructions provided.
Warranty Compliance
Compliance with the terms of the
Distributor's Warranties for Honda
Motorcycle Emission Control Systems is
necessary in order to keep the emissions
system warranty in effect.
178
Technical Information
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 lean
carburetor settings and other systems to
reduce carbon monoxide and
hydrocarbons.