NITRO RUSTLER • 29
ThE TRx 2.5 RacING ENGINE
Fine-Tuning the Carburetor
After fine-tuning your TRX 2.5 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 the
previous page 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 (Traxxas
on-board digital temperature gauge,
part #4091). 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
