NITRO SLASH • 27
THE TRX 3.3 RACING ENGINE
Idle Speed Adjustment
Once the high- and low-speed mixtures have been set, reduce the idle
speed to the minimum reliable idle speed. Remember, this adjustment
should be made while the engine is running at normal operating
temperature. Always use the idle speed adjustment screw to control
engine idle.
1. When the throttle servo is in its neutral position, the throttle slide
should be stopped against the idle speed adjustment screw.
2. Turn the screw counterclockwise to reduce the idle speed or clockwise
to increase it. The idle speed should be set as low as possible while still
maintaining reliable running characteristics.
Fine-Tuning the Carburetor
After fine-tuning your TRX 3.3 Racing Engine at the end of the break-
in procedure, no major adjustments to the fuel mixture are usually
necessary. Make note of the temperature, humidity, and barometric
pressure at the time you finished fine tuning your carburetor. Current
weather conditions can be found online from national websites, local TV
news websites, and television. This information will be considered your
baseline setting.
You may need to adjust your carburetor needles to compensate for
changes in temperature and barometric pressure (air density) from day to
day. Generally, you’ll need to richen the fuel mixture when the weather is
colder than your baseline temperature and the air density is higher. Lean
the fuel mixture when weather is warmer than your baseline temperature
and the air density is lower. The chart on page 26 provides general
guidelines on how weather conditions affect air density when they move
higher or lower than your baseline setting (see page 20 for detailed info
on how air density affects mixture settings).
Tuning the Engine by Temperature
The following procedures require an optional infrared temperature probe
or on-board temperature gauge. Engine temperature can be used as
an effective tuning aid when you understand the relationship between
engine temperature and ambient temperature. The engine operating
temperature, when tuned for maximum performance, will vary according
to atmospheric conditions, engine load, gauge accuracy, and many other
factors. The atmospheric condition that has the most influence on engine
temperature is air temperature. Expect the engine temperature to vary
almost in direct proportion to air temperature. Assuming you tuned
the engine for the same maximum performance each day, the engine
will run about twenty degrees hotter when it’s ninety degrees outside
than it would in seventy-degree weather. For this reason, we cannot
give you a definitive temperature range that indicates the best possible
engine tuning.
There is NO optimal temperature that can be used as a target to deliver
the best engine tuning. Do not rely on a temp gauge alone to tune
your engine. Tune the engine by paying very close attention to how
it responds to changes in fuel mixture (more smoke/less smoke, fast/
sluggish, reliable/stalling, smooth sound/ muffled sound, etc)
.
Once the engine is tuned, then observe the temperature.
The temperature gauge can aid you in tuning by giving you a relative
indication of how your adjustments are affecting the engine and to help
prevent you from reaching excessive engine temperatures. For example,
as you lean the fuel mixture, the engine performance will increase along
with the temperature. If you continue to lean the fuel mixture and the
temperature increases but the engine performance does not change,
then you have exceeded the maximum safe lean setting. Make note of the
engine temperature. Generally, try to keep your engine from exceeding
270°F when measured at the glow plug. If necessary, increase airflow
to the engine by cutting out the rear of the body, windshield, and front
valance. In some situations, the engine may perform very well with no
stalling, lagging, or hesitation at temperatures above 270°F, particularly in
very hot climates.
If richening the fuel mixture to bring the temperature down to 270°F
results in poor, sluggish performance (engine never cleans out), then
return the engine back to a satisfactory state of tune based on how it
sounds and performs (always with a visible stream of blue smoke coming
from the exhaust) . If engine temperature is exceeding 270°F with proper
cooling and no signs of abnormal running, then avoid running the engine
at its maximum lean setting. Watch closely for
any
signs of overheating.
Richen the fuel mixture slightly to provide a safety margin of additional
cooling lubrication. Symptoms of overheating include:
• Steam or smoke coming from the engine (not exhaust).
• Hesitation or stalling during acceleration.
• Popping or clattering sound when decelerating (detonation).
• Fluctuating idle speed.
