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RS125R Parts combination according to model years
If used machines are bought, previous owner’s specifications should be fully checked whether they are bought from private owners or retailers.
Since the RS racers have had improvements year by year, they are different in specifications depending on model years even if they have similar exterior appearance. Accordingly, before installing parts, manufacture years and parts combination according to years should be confirmed. Wrong combination would lead to degraded performance and failures.
Shown below are parts combinations according to manufacture year for the RS125R.

Parts combination for RS125R
Parts name Criteria '97 '98 '99 '00 '01
Cylinder Cylinder stamp Head No.6 Head No.7 Head No.8 Head No.9 Head No.0
Cylinder head Plug seating surface Plateau Plateau Plateau Plateau Recess
ECU Type No. CI662C CI708 CI708 CI708 CI708B
Carburetor Carburetor stamp   09A 09A 09D 09J
Criteria for each part
  1) Cylinder
Cylinders should be identified by the first digit of the 8-digit stamped numbers placed to the left when the exhaust side is faced forward. Basically, manufacture year minus one (first digit).
  2) Cylinder head
Photo below to the left shows a cylinder head with elevated plug-seating surfaces for 1997 through 2000 year model. Photo below to the right shows a cylinder head with recessed plug-seating surface for the 2001 year model.
Viewing from the combustion chamber side, plug-seating surfaces are opposite each other (2001 year model has elevated seating surface).
Cylinder head with elevated plug-seating surfaces for 1997 through 2000 year model
Cylinder head with recessed plug-seating surface for the 2001 year model.
'97 -'00 cylinder head with plateau shape spark plug-seating surface.   '01 cylinder head with concaved spark plug seating-surface.
  3) ECU can be identified by the model number [CI###] in white-letter on the upper right the body.
  4) Carburetor
Can be identified by stamp indicated by white lines on the photo.
Features of settings by year
'97 Unleaded gasoline was introduced for the first time. Adjust ignition timing to avoid rich feeling in summer and detonation in winter (Retarding ignition timing is effective against detonation)
'98-'99 Pwj fitted carburetor was introduced for the first time.
The Pwj became effective in preventing detonation (Pwj designed to prevent detonation). (Pwj of smaller #40 or #42 were used)
'00 Like the 250, Pwj was used for carburetor settings. Larger Pwj#48 is cut after peak output power for smoother high revving (Pwj designed for setting)
'01 Increased low-to-middle torque oriented engine characteristics, effective for rising at the corner. Pwj #48 with MJH#3.9, rich setting designed not to lose powerful middle speed range.
  For users who bought used machines and want to improve potential performance, it is recommended that parts be replaced with parts of latest year model for more potential.
  Parts for '01 RS125R  
'97 RS125R * - * * * * * *
'98 RS125R - - - * * * * -
'99 RS125R - - - * * * * -
'00 RS125R - - - * * * - -
  *=Can be used if all the related parts are replaced
-=Same specifications (can be used even if different in part number)