Tuning - Yamaha YTZ250S 1986 Service Manual

Tuning
Tuning a carburetor is not the mysterious
science many racers believe it to be. One needs
only to establish a basic knowledge of the iden-
tification and function of carb components as
well as how they work together to do the job
well.
Some basics
The carburetor mixes air and fuel to burn in the
engine. To allow the engine to produce op-
timum power under a variety of conditions, the
carburetor must provide the correct ratio of air
and fuel at all times. To accomplish this, the
carb has several overlapping systems, and each
system has its greatest effect in a specific rpm
range. Thus, the tuner can consider each
general engine-speed range one at a time; he
can then adjust the appropriate systems to
achieve smooth, strong power delivery
throughout the entire rpm range of the engine.
If a carburetor is in need of tuning, the engine
usually is said to be "running too rich" or "run-
ning too lean." These terms refer to the amount
of fuel relative to the volume of air being taken
into the engine. Hence, an engine that is too
rich needs less fuel for a given volume of air;
conversely, an engine running too lean needs
more fuel for the air being taken in.
AIR-FUEL MIXTURE
10:1
CD
12:1
r-+------..1---------/"
14:1
16:1
18:1
20 40
60
80
100
THROTTLE VALVE OPENING
(%)
1. Maximum power mixture ratio
2. Theoretical mixture ratio
3-3
12-13 9
CD
~---:--~----------~
TO DEVELOP MAXIMUM POWER, ONE GRAM
OF FUEL IS REQUIRED PER
12
TO
13
GRAMS OF
AIR.
1. Fuel 2. Air
Tuning changes usually are required to accom-
modate changes in altilude. At higher altitudes,
the air is "thinner" or less dense. This means
there are fewer molecules of oxygen in a given
volume of air to mix and burn with the fuel.
The carb therfore needs to be made "leaner"
to provide less gas and maintain the proper
ratio of fuel to air at high altitudes.
;R
100
0
r
90
3:
0
-I
~
u..
a:
<i:
o
500 1000 1500 2000
ALTITUDE
(m) _
• Change due to elevation (at constant tem-
perature).
If you race in an area with a wide range of
temperatures, you might have to make carb ad-
justments for them. Higher air temperatures re-
quire leaner carb tuning. Higher humidity, on
the other hand, demal"lds carb settings which
are richer. Such adjustments, however, should
rarely be necessary for the majority of racers;
motocross engines in general are sensitive only
to the most extreme changes in such weather
conditions.