Yamaha MM700A, Manual, Page: 86 / 282

80 CHAPTER THREE
mixture. Turning the pilot mixture screw counterclock-
wise
the fuel mixture.
1. Open the shroud.
NOTE
Figure 45
shows
the carburetors removed
for clarity.
2. Locate the pilot mixture screw on the underside of
each carburetor (Figure 45).
3. Turn all pilot mixture screws in until lightly seated.
Back the screws as follows:
a. On 593 models, back the pilot mixture screw out
two turns.
b. On 698 cc models, back the pilot mixture screw out
one and a half turns.
4. Connect a tachometer according to the manufacturer's
instructions.
5. Start the engine and allow it to warm to normal operat-
ing temperature.
6. Adjust the idle speed screw (Figure 46) so the engine
idles at 1500- 1700
7. Adjust each pilot mixture screw (Figure 45) individu-
ally. (Turning the pilot mixture screw clockwise leans the
fuel mixture.) Slowly rotate the pilot mixture screw to
find the setting
the engine idles at maximum rpm.
This may require screw rotation in both directions to find
the best setting.
8. Readjust the idle speed screw, if necessary, so the en-
gine idles at 700 rpm.
Adjust the remaining pilot mixture screws.
10. Operate the snowmobile and check performance. If
the engine hesitates during acceleration from a stop, rotate
each pilot mixture screw slightly counterclockwise. Re-
check engine performance and readjust the pilot mixture
screw setting as needed.
NOTE
Operation in other than normal conditions,
extremely warm or cold weather or high
altitude operation, may require changing
the pilot jet to obtain satisfactory low speed
performance.
Carburetor tuning
snowmobiles arc tuned for sea level condi-
tions. However, when the snowmobile is operated at a
higher altitude, engine performance will drop because of a
change in air density. At sea level, the air is denser than
the air at 10,000 ft. The power output will decrease by
3
percent for every 1000 ft. increase in elevation. This de-
crease in power is caused by a drop in cylinder pressure
and a change in the ratio. For example, an engine
that produces 40 hp at sea level will produce approxi-
mately
38.8 hp at 1000 ft. At 10,000 the engine pro-
duces 29.5 hp. With the carburetor adjusted for proper
operation at sea level, the engine would run extremely
rich at 10,000
Also consider air temperature when jetting the
tor. For example, the carburetors are set by the manufac-
turer to run at temperatures of
F to
F (0 to
C)
at sea level. If operating the snowmobile under conditions