Yamaha YTZ250S 1986 Service Manual page 46

Four of the components, the jet needle, needle
jet, main jet, and pilot jet, regulate the flow of
fuel; the slide and pilot air screw requlate the flow
of air. The following chart indicates the working
range of each component. Note how the work-
ing ranges overlap each other as the throttle valve
moves from closed to fully open.
SLIDE VALVE CARBURETOR
WORKING RANGE OF EACH CARBURETOR
COMPONENT
1/8 1/4
1/2
CLOSED
1. Main jet
2. Needle jet, jet needle
I
@
3/4
FULL OPEN
3. Throttle valve cutaway
4. Pilot air screw & jet
r--""
I
I
I FULL
I
If you note a particular symptom of rich or lean
running in a specific range, use the chart to
determine which component needs changing.
Use the following information to decide what
changes to make.
Main jet
The main jet has its greatest effect in the
3/4-to-full-throttle range. The number of the
main jet, stamped on the bottom or side of the
jet, indicates the relative size of the hole in the
jet which meters fuel. The larger the number
on the main jet is, the bigger the hole and the
3-5
more fuel it will pass; hence, larger numbers
mean richer jetting. Smaller numbers, of
course, mean leaner jetting. Make main-jet
changes one step at a time.
1. Jet number
Jet needle and needle jet
The jet needle and needle jet together have their
greatest effect in the 11 4-to-31 4-throttle range.
The needle moves in and out of the needle jet;
since the needle is tapered, its position in the jet
determines the amount of fuel allowed through.
There are five grooves in the top of the needle
in which a circlip fits. This clip locates the needle
in the slide and, therefore, determines its posi-
tion relative to the needle jet. Moving the clip
down has the effect of pulling the needle further
out of the jet; the mixture is thereby richened.
Moving the clip up leans the mixture. Change the
clip position one step at a time.
1. Clip position
2. Leaner
3. Richer