Traxxas 4902, Owner'S Manual

T-MAXX 2.5R

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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 TDC, see
page 43 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 2.5R

Racing Engine 

on page 31

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 2.5R Racing Engine