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Nitro 4-Tec 3.3
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23
The TRX 3.3 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 3.3
Racing Engine 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 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
Air
Fuel
Cold Air (More Dense)
(Warm Air) Less Dense
See page 30 for more
information on how
air density affects
engine tuning.
Start
1/8 Turn
45°
Full Turn
360°
1/2 Turn
180°
1/4 Turn
90°
3/4 Turn
270°
A “turn” refers to
tightening (“turning in”) or
loosening (“turning out”)
mixture needles. A “full
turn” refers to turning the
needle 360°, so a “1/2 turn”
would be 180°, a “1/4 turn”
would be 90°, and so on.