NITRO 4-TEC 3.3 • 21
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.
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 readjust 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 top
dead center (TDC), see page 34 for instructions on freeing the engine.
The Idle Speed Adjustment
The idle speed 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 adjustment screw. Always use
the idle speed adjusting screw to control engine idle. Do not use the
throttle trim on the transmitter to adjust idle speed. The idle speed
should be set as low as possible and still maintain reliable running.
Water Valve
(Controls Overall Flow)
High Speed Needle
Water Nozzle
(Controls Fine Flow)
Low Speed Needle
Richen
Lean
Rich
See
Tuning Your TRX 3.3
Racing Engine
on page 26
for complete information on
adjusting the air/fuel mixture
and idle speed.
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.
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.
Low Speed
Needle Adjustment
Fig. A
THE TRX 3.3 RACING ENGINE
