Yamaha YZ85 Manuel D'atelier Du Proprietaire page 482

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 carbu-
retor settings and changing the heat range of
spark plug.
Standard spark plug
5PA70110
In principle, it is advisable to first use spark
G
plugs of standard heat range, and judging
from the discoloration of spark plugs, adjust
carburetor settings.
If the calibration No. of the main jet must be
G
changed by ±30, it is advisable to change the
heat range of spark plugs and newly select
the proper main jet.
NOTE:
When checking the discoloration of spark
G
plugs, be sure to stop the engine immedi-
ately after a run and check.
Avoid racing.
G
When changing the heat range of spark
G
plugs, never attempt to change it more than
±1 rank.
When using a spark plug other than stan-
G
dard, check its heat range against the stan-
dard and check that it is a resistance type.
Note that even if the discoloration seems
G
proper, it may slightly vary with the spark
plug maker and oil in use.
7 - 9
T U N
SETTING
BR10EG/NGK
(resistance type)