Traxxas 49077 - Manual (Page 16)

16 • T-MAXX 3.3 OWNER’S MANUAL
The maximum possible fuel flow is always controlled by the high-speed
needle. It works like the main water valve on a garden hose. Turn it
clockwise to close the valve, counter-clockwise to open it. When the
throttle is at idle or partially open, the low-speed needle meters the
fuel flow at the outlet (needle seat) where the fuel enters the carburetor
venturi. This second valve acts like the spray nozzle at the end of the
garden hose in our example. When you accelerate from idle, the throttle
opens and the low-speed needle is pulled away from the needle seat.
This allows more fuel to flow with the increased air flow. As the throttle
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.
• 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 28 for instructions
on freeing the engine.
The Idle Speed Screw
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.
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.
THE TRX 3.3 RACING ENGINE
Water Valve
(Controls Overall Flow)
High Speed Needle
Water Nozzle
(Controls Fine Flow)
Low Speed Needle
Richen
Lean
Rich
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.
See Tuning Your TRX 3.3 Racing Engine on page 20 for complete
information on adjusting the air/fuel mixture and idle speed.
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
A “turn” refers to tightening
(“turning in”) or loosening
(“turning out”) mixture needles.
A “full turn” refers to turning
the needle 360°, so a “1/2 turn”
would be 180°, a “1/4 turn”
would be 90°, and so on.
Start
1/8 Turn
45°
Full Turn
360°
1/2 Turn
180°
1/4 Turn
90°
3/4 Turn
270°
KC2077-49077-T-Maxx-TQi-Owners.indd 16 6/6/14 11:50 AM