20 • NITRO SLASH
See page 26 for more
information on how air
density affects engine tuning.
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, counterclockwise
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 is increased, the low-speed
needle is pulled completely away from the needle seat, leaving it fully
open. At that point, fuel metering is entirely controlled by the high-
speed needle. Again, using our water hose example, when the spray
nozzle at the end of our garden hose is fully open, then the main
water valve can be used to adjust how fast the water flows.
The engine’s performance is directly linked to the fuel mixture.
Richening the fuel mixture increases the amount of fuel in the
air/fuel mixture ratio and leaning the fuel mixture decreases the
amount of fuel in the air/fuel mixture ratio.
• Slightly lean fuel mixtures deliver stronger, more efficient
combustion and more power, but with less lubrication.
• Slightly rich fuel mixtures deliver cooler running and more
lubrication but with slightly less power.
Air
Fuel
Cold Air (More Dense)
(Warm Air) Less Dense
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.
THE TRX 3.3 RACING ENGINE
Water Valve
(Controls Overall Flow)
High Speed Needle
Water Nozzle
(Controls Fine Flow)
Low Speed Needle
Richen
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