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Tuning Your Engine for Best Performance

The engine’s performance depends on the fuel mixture. Turn
the mixture needles clockwise to lean the fuel mixture and
counterclockwise to richen it. Leaning the fuel mixture will increase
engine power up to the engine’s mechanical limits.

Never run the

engine too lean (not enough fuel flow). leaning the engine beyond
the safe allowable limits will result in poor performance and
engine damage.

Indications of an overly lean mixture include:

• Cutting out or sudden loss of power during acceleration.
• Overheating (temperature beyond 300°F at the glow plug).
• Little or no blue smoke coming from the exhaust.
If any of these conditions are present, stop immediately and richen
the high-speed mixture 1/4 turn. The engine will probably be slightly
rich at that setting and you can then re-tune for performance. Always
tune for performance by starting rich and moving toward the ideal
setting. Never try to tune from the lean side. There should always be
a light stream of blue smoke coming from the exhaust.
Before you begin tuning, the engine should be warmed up to its
normal operating temperature and running slightly rich. All final
tuning adjustments must be made to the engine at its normal
operating temperature. You can tell the engine is running rich by
noting any of the following:
• Sluggish acceleration with blue smoke coming from the exhaust.
• There is unburned fuel spraying from the exhaust tip.
• Leaning the high-speed fuel mixture increases performance.

High-Speed Fuel Mixture adjustment

With the engine warm and running at a rich setting, gradually lean the
high-speed fuel mixture in 1/16-turn increments. Make several high-
speed passes with the Nitro Stampede after each adjustment to clear
out the engine and note any change in performance. Continue this
procedure until one of the following conditions exists

There is no longer any performance improvement.

The engine begins to cut out at high-speed.

There is a sudden loss of power during acceleration.

The engine begins to overheat. 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
  • Temperature measurement above 300°F at the glow plug
If any one of the above conditions occurs, the fuel mixture is already
past the maximum safe lean setting. Richen the fuel mixture to the
optimum setting by richening the high-speed needle at least 1/8
turn counterclockwise and retest. This setting will extend engine
component life.

low-Speed Fuel Mixture adjustment

The low-speed mixture is always set after the high-speed needle is
correctly adjusted. The low-speed mixture will be set using the pinch test.

Once the engine is warm, do several high-speed runs to confirm

that the high-speed needle is set correctly.

Bring the vehicle in and pinch closed the fuel line going into the

carburetor. The engine should run for 2-3 seconds, speed up, and
then die.

It is very important to make several high-speed runs with the

Nitro Stampede between adjustments to clear out any excess fuel.
Perform the pinch test immediately after. If the engine is allowed to
idle for a long period of time, it could “load up” with fuel and give
you an inaccurate measurement from your pinch test.

If the engine runs longer than 3 seconds, then lean the low-speed

needle 1/16 turn, make several more high-speed runs, and retest.

If the engine dies immediately without speeding up, then richen

the low-speed needle 1/8 turn, make several more high-speed runs,
and retest.

If the engine dies when you try to accelerate abruptly, then the

low-speed needle is probably set too lean. Richen the low-speed
needle 1/8 turn, make several more high-speed runs, and retest.

When the low-speed needle is set correctly, the engine’s throttle

response should be very quick.

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 screw to control
engine idle.

When the throttle servo is in its neutral position, the throttle slide
should be stopped against the idle speed screw.

Turn the screw clockwise to reduce the idle speed or

counterclockwise 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 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 the
temperature is warmer than your baseline temperature and the air
density is lower. The chart below provides general guidelines on how
weather conditions affect air density when they move higher or lower
than your baseline setting.

Tuning the Engine by Temperature

The following procedures require an optional
infrared temperature probe or on-board
temperature gauge (such as the Traxxas
on-board temperature gauge, part #4091). The
engine 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. 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 atmospheric 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.
The temperature gauge can aid you in tuning by giving you a
relative indication of how your adjustments are affecting the
engine and by preventing you from exceeding maximum engine
temperature. 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