TRX 2.5R • 14
THE CARBURETOR
The Carburetor
Understanding the carburetor adjustments
The carburetor performs several functions. It
controls the engine’s speed by restricting the
intake of air and fuel into the engine. It atomizes
the fuel (suspends the fuel droplets in the air) and
also controls the air/fuel ratio of the mixture
entering the engine (how much air for a given
amount of fuel). To help provide a better
understanding of engine tuning and why it’s necessary, the
following is a brief explanation of the air/fuel combustion process
that takes place inside the engine.
In order to create the cylinder pressure that results in power, the engine burns the air/fuel
mixture. Both air and fuel, in correct amounts, are needed for proper combustion. It is the
carburetor’s job to mix the air and fuel together (atomize the fuel), in the correct proportion for
the best possible combustion. This is the ideal air/fuel ratio required for the engine. The ideal
air/fuel ratio remains roughly constant. Due to variations in atmospheric conditions
(temperature, humidity, altitude, etc.) fuel flow adjustment valves (called fuel mixture needles)
are required to meter the fuel and maintain the ideal air/fuel ratio in these ever-changing
conditions. For example, colder air is more
dense (more air molecules) for a given
volume of air and therefore requires more
fuel (more fuel molecules) to maintain the
correct air/fuel ratio. Warmer air is less
dense (fewer air molecules) and therefore
needs less fuel to maintain the correct
air/fuel ratio. The tuning needles are there
to adjust how much fuel is made available
for the carburetor to mix with the available
air (atomization).
Air
Fuel
Dense Air
Less Dense Air
Factory Needle Settings
If your factory preset
carburetor adjustments
have been tampered with,
use the following settings:
• Set the high-speed needle
to 4 turns out from closed.
• Set the low-speed needle
so the screw head (
red in
Fig. A
) is flush (even) with
the end of the slide (
yellow
in Fig. A
).
Always use the factory
settings for initial starting.
Only use these settings
when the factory settings
have been lost.
Low Speed
Needle Adjustment
Fig. A
