Honda 2009 TRX90X Owner's Manual page 197

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Emission Control Systems
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 various systems to reduce carbon
monoxide, oxides of nitrogen and hydrocarbons.
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.
188 Technical Information