The TRX 2.5 Racing Engine
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. The ideal air/fuel ratio required for
the engine 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).
The Fuel Mixture Needles
The amount of fuel metered and atomized by the carburetor is
controlled by the two mixture needles, the high-speed needle
and the low-speed needle. The low-speed needle is used to
meter the fuel used by the engine at idle and low RPM (part-
throttle) engine speed. The high-speed needle is used to meter
the fuel when the throttle is open from part throttle to wide-
open throttle (WOT). Two needles on the TRX 2.5 provide precise
control of the air/fuel ratio across the engine’s entire RPM range.
The maximum possible fuel flow is always controlled by the
high-speed needle. It works like the main water valve on a
garden hose. Turn it clockwise to close the valve, counter-
clockwise to open it. When the throttle is at idle or partially
open, the low-speed needle again meters the fuel flow at the
outlet (needle seat) where the fuel enters the carburetor venturi.
This second valve acts like the spray nozzle at the end of the
garden hose in our example. When you accelerate from idle, the
throttle opens and the low-speed needle is pulled away from
the needle seat. This allows more fuel to flow with the increased
air flow. As the throttle is increased, the low-speed needle is
28
See page 35 for
more information on
how air density
affects engine tuning.
Air
Fuel
Dense Air
Less Dense Air
