How To Tune the TRX 2.5, 2.5R and 3.3 for Best Performance

Article #49807


Tuning Your TRX 2.5, 2.5R and 3.3 Engine for Best Performance

The Fuel Mixture

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). Never lean the engine until it begins to cut out or stall. Leaning the engine beyond the safe allowable limits will result in poor performance and almost certain engine damage. Indications of an overly lean mixture include:

  • Cutting out or sudden loss of power during acceleration.
  • Overheating (temperature beyond 270° 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 retune for performance. Always tune for performance by starting rich and moving leaner 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
  • The vehicle may not shift into second gear (if installed)
  • There is unburned fuel spraying from the exhaust tip
  • Leaning the high-speed fuel mixture increases performance


High-Speed Fuel Mixture Adjustment

Base setting for high-speed needle = four turns out from closed

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 vehicle after each adjustment to clear out the engine and note any change in performance. TRX racing engines are extremely powerful. Remember to apply the throttle gradually to prevent wheelies or loss of control. Continue this procedure until one of the following conditions exist:

  1. There is no longer any performance improvement
  2. The engine begins to cut out at high speed (Danger!)
  3. There is a sudden loss of power during acceleration (Danger!)
  4. 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 270° 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

Base setting for low speed needle = flush with the grey ring around the
low-speed adjustment screw (see engine/screwdriver pic above)

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.

  1. Once the engine is warm, do several high-speed runs to confirm that the high-speed needle is set correctly.
  2. Bring the vehicle in and pinch closed the fuel line going into the carburetor. The engine should run for two-to-three seconds, speed up, and then die.
  3. If the engine runs longer than three seconds, lean the low-speed needle 1/16 turn, make several more high speed runs, and retest.
  4. If the engine dies immediately without speeding up, 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, possibly even to the point of making it difficult to keep the vehicle from doing a wheelie when you accelerate!


Idle Speed Adjustment

Base setting for idle stop screw = 0.5mm-1.0mm gap between the slide
valve and the inside of the carburetor inlet tract (venturi).

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.

  1. Turn the throttle trim on the transmitter so that the brakes are applied. This ensures that the throttle slide is resting against the idle adjustment screw.
  2. If necessary, remove the air filter to gain access to the idle adjustment screw.
  3. 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.
  4. Reset the throttle trim on the transmitter.

Note: If the idle speed is set too high it could prevent the T-Maxx from shifting between forward and reverse. If you experience this, simply reduce the idle speed.


Fine-Tuning the Carburetor

After fine-tuning your TRX 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 below provides general guidelines on how weather conditions affect air density when they move higher or lower than your baseline setting.

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