Yamaha YZ250(W)/W1 Owner's Service Manual page 566

SETTING
EC71M010
Change of the heat range of spark plugs
Judging from the discoloration of spark plugs, if
they are found improper, it can be corrected by
the following two methods; changing carburetor
settings and changing the heat range of spark
plug.
Standard spark plug
9 In principle, it is advisable to first use spark
plugs of standard heat range, and judging from
the discoloration of spark plugs, adjust carbure-
tor settings.
9 If the calibration No. of the main jet must be
changed by ±15, it is advisable to change the
heat range of spark plugs and newly select the
proper main jet.
NOTE:
9 When checking the discoloration of spark
plugs, be sure to stop the engine immediately
after a run and check.
9 Avoid racing.
9 When changing the heat range of spark plugs,
never attempt to change it more than ±1 rank.
9 When using a spark plug other than standard,
check its heat range against the standard and
check that it is a resistance type.
9 Note that even if the discoloration seems prop-
er, it may slightly vary with the spark plug
maker and oil in use.
7-11
TUN
BR8EG/NGK
(resistance type)