9
en
adjustIng the carBuretor
understanding Rich and Lean fuel Mixture
Adjusting the carburetor is one of the most critical facets of running a
nitro powered RC vehicle. The fuel mixture is referred to as being “rich”
when there is too much fuel and “lean” when there is not enough fuel
for the amount of air entering the engine. The amount of fuel entering
the engine is adjusted with high- and low-speed threaded needle
valves. The low-speed needle is located in the front of the moving
slide. The high-speed needle sticks straight up at the back of the
carburetor. Both feature a slotted head that is used as a reference and
receptacle for a flat blade screwdriver for adjustments. The mixture is
made richer by turning the needle counterclockwise and leaner by turn-
ing clockwise. An overly “rich” mixture will yield sluggish acceleration
and performance with thick smoke from the exhaust. A “lean” mixture
can cause the engine to hesitate before accelerating or, in some cases,
to lose power momentarily after the initial acceleration. A lean mixture
also makes the engine run hotter than desired and does not provide
enough lubrication for the internal engine components, causing prema-
ture wear and damage. It is always advisable to
run the engine slightly
rich
and
never lean
to avoid overheating and possible damage.
Base start-up settings from the factory
High-Speed Needle
4 1/4 turns out from bottom
Low-Speed Needle
2 3/4 turns out from bottom
engine tuning
After the engine is broken in, you can tune it for optimum performance.
When tuning, it is critical that you be cautious of overheating as severe
damage and premature wear can occur. You want to make all carbure-
tor adjustments in “one hour” increments.
low-speed adjustment
The low speed adjustment affects the idle and slightly-off-idle
performance. The optimum setting allows the motor to idle for at least
8−10 seconds. The model should then accelerate with a slight amount
of sluggishness and a noticeable amount of smoke. The simplest way
to check this is to make sure the engine has been warmed up and let
the engine idle for 8−10 seconds. If the low-speed mixture is so far off
that the engine won’t stay running that long, turn the idle stop screw
clockwise, increasing the idle speed. With the engine at idle, pinch and
hold the fuel line near the carburetor, cutting off the flow of fuel, and
listen closely to the engine rpm (speed). If the low-speed needle is set
correctly, the engine speed will increase
only slightly and then die. If the engine
increases several hundred rpm before
stopping, the low-speed needle is too
and trying again. If the engine speed
does not increase but simply dies, the
needle is too lean and needs to be
richened up
by turning the
needle counter-
clockwise one
hour before trying
again. After you
have optimized
the low-speed
setting, the
engine will probably be idling faster. You will have
to adjust the idle stop screw counterclockwise to
slow the engine idle speed down. The engine
should accelerate at a constant pace without
hesitating.
stoPPIng the VehIcle
Press the end of a screwdriver or rag against the exhaust
opening to stop the engine. Avoid direct contact with
the hot exhaust.
1 H
our
2 H
our
carb adjustments
Make all carburetor adjustments
in one-hour increments.
Imagine the slot in the needle is
the hour hand on a clock. Adjust
it as though you were moving the
hour hand from one hour to the
next or previous one.
Slower
Faster
Richer
Leaner
Low-Speed Needle
Leaner
Richer
Idle Speed Screw
As you turn this clockwise,
it increases the carburetor
opening at idle and increases
idle speed. Turning it counter-
clockwise decreases the
opening and idle speed.
As you turn this clockwise
(leaner), less fuel enters
the engine. Turning it
counterclockwise (richer)
allows more fuel to enter
the engine.
High-Speed Needle
As you turn this clockwise
(leaner), less fuel enters
the engine. Turning it
counterclockwise (richer)
allows more fuel to enter
the engine.
Carb
Opening
