Yamaha YTZ250S 1986 Service Manual page 47

1. Jet needle number
If changing the clip position doesn't provide the
proper setting, the needle jet may be chang-
ed. This will make a large difference in the
amount of fuel provided. On the side of the
needle jet are stamped two figures, a letter
followed by an even number; these figures in-
dicate the relative size of the needle jet. The
letter ranges from "N" (leaner) to
"Q"
(richer).
The number will stand for one of five sizes
within each letter's range. The smaller the
number is, the leaner the needle jet. Change
the needle jet one step at a time, and make fine
adjustments with the jet needle clip.
When changing the Jet Needle for the stan-
dard Needle Jet, the jet needle length must be
proper so that it will not slip off the needle jet
at full throttle operation. (If the jet needle slips
off, the machine may become uncontrollable.)
Throttle valve
The throttle valve may be changed to affect
the mixture in the 1 18-to-1 /2-thtottle range.
The bottom portion of the throttle valve which
faces the rear of the carb is cut at an angle;
this is called the cutaway. The height of the
cutaway determines the characteristic of the
airflow. The height is indicated by the number
stamped on the bottom of the throttle valve.
A smaller number means a smallar cutaway,
and a sma liar cutaway provides a richer mix-
ture. Conversely, a lager cutaway makes the
mixture leaner. Make throttle valve changes in
increments of 0.5.
3-6
CD _ _
,..:::",.._~
1. Cutaway
2. Slide number
Pilot jet and pilot air screw
The pilot jet and pilot screw control the mix-
ture in the closed-to-1 18-throttle range. To ad-
just the mixture in this range, the pilot air screw
can be turned to change the airflow through
the circuit, or the pilot jet can be changed to
provide more or less fuel. Start by turning the
pilot air screw. Screwing it in richens the mix-
ture, and turning it out leans the mixture. Pilot
air screw specs indicate the turns out from a
lightly seated position. Make changes in
1 12-turn increments. If turning the screw bet-
ween one and two-and-a-half turns doesn't
provide the desired results, change the pilot jet.
This jet has a number stamped on it which in-
dicates its size; the larger the number is, the
richer the jet. Make one-step changes in the
pilot jet, and fine-tune with the pilot screw.
1. Pilot air screw
2. Pilot jet number