NITRO RUSTLER • 21
ThE TRx 2.5 RacING ENGINE
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.
Tuning the engine means finding the perfect balance between the two;
excellent power to meet your needs while maintaining good lubrication
for long engine life.
The optimal fuel mixture setting is rich
to provide
a safety margin against having a lean condition if some variable
changes (such as the temperature from one day to the next).
General fuel mixture settings are measured by the number of turns
the needles are turned out from fully closed. The fuel mixture settings
have been pre-set from the factory to typical break-in settings. Do
not re-adjust your carburetor from the factory settings until after the
engine is started and running, and you have been able to observe the
engine running to assess what minor adjustments may be required to
compensate for fuel, temperature, and altitude. Adjustments are usually
made in 1/8 or 1/16-turn increments. If the engine sticks at TDC, see
page 35 for instructions on freeing the engine.
The Idle Speed Adjustment
The idle speed adjustment screw controls the closed position of the
throttle slide. When the throttle servo is in its neutral position, the
throttle slide should be stopped against the idle speed adjustment
screw. Always use the idle speed adjustment screw to control engine
idle. See page 28 for instructions on adjusting the idle speed.
Water Valve
(Controls Overall Flow)
High-Speed Needle
Water Nozzle
(Controls Fine Flow)
Low-Speed Needle
Richen
Lean
Rich
Higher nitro requires a richer
fuel mixture. When running
33% fuel, richen your high
speed needle 3/4 turn if
previously running 20% nitro
and then re-tune the engine
for maximum performance.
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.
See
Tuning Your TRX 2.5 Racing
Engine
on page 28 for complete
information on adjusting the air/
fuel mixture and idle speed.
Low-Speed
Needle Adjustment
Fig. A
