Yamaha TZ250M1 Owner's Service Manual page 267

A
B
C
SETTING
Next run a few laps of the circuit with this
setting and check for any difference in
engine revolutions. If no difference is
noticed, use a main jet with a much smaller
calibration number.
w
Example: #560 ≥ #540
Factory-set main jet
2. Checking of spark plug and piston for dis-
coloration
Repeat the adjustment in the above "1"
several times. If the engine begins to run at
more than 12,000 rpm at full throttle, pro-
ceed to the "spark plug chopping" step
(refer to P7-9) to check for the discoloration
of the spark plug(s) and piston(s).
Refer to the photo for judgment on the dis-
coloration.
As a novice will find it difficult to determine
how much smaller number main jet can be
used just by looking at discoloration, he
should consult an experienced person for
his own experience, too.
Whether the setting is proper or not can be
judged by engine revolutions.
Approximate criteria for such judgment are
given below, on condition that the sec-
ondary reduction ratio is fit for conditions of
the circuit.
•13,000 rpm in 1st and 2nd
•12,500 rpm in 5th and 6th
A Normal
B Over burned (too lean)
C Oil fouled (too rich)
3. Adjustment of main nozzle
The main nozzle 1 adjustment follows the
completion of the adjustment of the main
jet. Check that engine revolutions smoothly
respond to throttle opening from where
throttle is about to be opened to 1/2 throttle
opening. Use a main nozzle of a smaller
size if engine revolutions appear to falter at
the beginning of throttle opening and then
suddenly respond to further throttle open-
ing.
Example:S-4 ≥ S-3
7-3
TUN
#580