Yamaha MM700A Manual page 88

82
Mid-range tuning
The jet needle and needle jet control the mixture at me-
dium speeds, from approximately 114 to 3!4 throttle (Fig-
ure 47). The tapered bottom half of the jet needle extends
into the needle jet (Figure 49). Fuel cannot flow through
the space between the needle jet and jet needle until the
throttle valve is raised approximately 114 open. When the
throttle valve rises, the jet needle's tapered portion moves
out of the needle jet (Figure 49).
The top of the jet needle has five evenly spaced grooves
(Figure 50) that permit adjustment of the jet needle posi-
tion in the needle jet. Changing the position of the jet nee-
dle in the jet needle alters the fuel mixture ratio. If the
retaining clip in the top of the jet needle is moved to a
higher groove on the needle jet (thus dropping the needle
deeper into the jet), the mixture will be leaner. Moving the
clip to a lower groove (raising the needle) will result in a
richer mixture.
The jet needle clip is positioned in the jet needle groove
using two shims (Figure 51). The shims allow more pre-
cise tuning. For instance, the jet needle position may be
2.0 or 2.5 depending on the arrangement of the shims and
clip.
1. Refer to Chapter Five and remove the throttle valve as-
sembly (Figure 52).
2. Unscrew the jet needle retaining plate screws
53) and remove the jet needle.
3. Note the position of the clip (Figure 49) and shims be-
fore removal. Remove the clip and shims and reposition
them on the jet needle. Make sure the clip seats in the nee-
dle groove completely.
4. Reverse to install.
High-speed tuning
The main jet controls the mixture from 3/4 to fall throt-
tle and has some effect at lesser throttle openings (Figure
47). Each main jet is stamped with a number (Figure 54).
(Figure
Larger numbers provide a richer mixture, smaller num-
bers a leaner mixture. Refer to the tables at the end of
Chapter Five for main jet sizes at sea level conditions.
When operating the snowmobile in relatively warm
weather or at a higher altitude, install a smaller main jet.
When operating the snowmobile in abnormally cold
weather conditions, install a larger main jet.
CHAPTER THREE
Throttle
valve
Clip
Jet needle
Needle jet
Main jet
Adjusted
Lean
Rich
Jet needle