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Honda Technical Information

Notes on maintenance (engine)
RS125R / RS250R
The RS provides the high potential for circuit use. Lack of the basic maintenance, however, will result largely degraded performance. Servicing should be regularly performed and parts be periodically replaced, if needed.

Periodic replacement parts
Item Intervals Criteria
Plug cap Every 1000km (cleaning for each maintenance)  
Cylinder Every 2000km Wear or damage
Piston Every 500km Damaged skirt or wear
Piston ring Every 500km Partial loss of identification point, wear
Piston pin clip Every 500km Burning, damage, stepped wear
Piston pin clip Every 300km (Every time clip is removed)  
Connecting rod small end bearing Every 500km Burning, wear, damage
Crank shaft Every 2000km Deformation, damage
Clutch outer Every 2000km Wear, damage
Spark plug   Wear
Ignition coil   Wear, damage
Reed valve   Loose, damage

Item Interval
Front/rear suspension Every 2000 km or every 4 races
Cables (throttle or clutch) Every 2 to 3 races (every time races are done in a rainy day)
Brake master cylinder Every season
Steering stem Every season (check needed after falling down)
Suspension link section At new vehicle, every time parts replaced
Wheel bearing Abnormal noise, play, defective operation

Checking other fasteners
  Check the brake system, front section, or rear axle section for loose fasteners. Also check wire lock.

Reasons for specifying intervals in terms of kilometers for periodic replacement parts
Example: Pistons for which replacement is specified at every 500km
Durability: Reduced streaks on piston surface (oil bank)
occurrence of cracks
Degraded performance: Deformed pistons
Blow up (ring grooves struck by rings, wear)

As described above, intervals in km are the ranges within which an appropriate racer can provide its inherent high performance or which will not cause durability related failure. (Likewise, these intervals have been specified for other parts. See owners manual)

Engine maintenance
  With increased power, the 2001 model year RS125R and RS250R have cylinder bore shape and timing similar to the manufacturer's kit engine.
Accordingly, cylinder maintenance has become more important than before.
Basically, it is recommended that the items described in the manual be followed. In particular, mentioned below is the description on the necessity for chamfering.
  The chamfer on the upper surface of the exhaust port may be glittering when obliquely viewed from the skirt side of the cylinder taken from engines which have traveled to some degree.  
  The figure above shows a magnified view of the cross section of the port chamfer. The piston ring slightly protrudes into the exhaust port when the piston goes up and down.
This chamfer again rests in the bore surface. If there is, however, an angular (even obtuse-angle) section at the cylinder chamfer interface section, the piston ring movement gradually becomes bad, resulting in strengthened contact by the ring on chamfers.
For this reason, chamfer should be made. When chamfering is made for upper and lower ports, wet abrasive papers will be applied to the interface of the bore surface and the port chamfer at small angles (10 to 15 degrees) to make the interface round.
Chamfer maintenance would avoid stuck rings or peeled plate at the cylinder port.

  Piston contact

Piston contact should not usually be removed. Severe contact with the cylinder is attributed to some other reasons. Pistons have special profile (out of round), which is designed to produce heat emission, providing proper contact when pistons and cylinders are subject to deformation.
Accordingly, careless piston modifications in profile will degrade the ideal profile, resulting in more contact with other portions.

Ring sticking
  With the increase of service engine speeds year after year, even if keystone rings are used, sticking may result.
If rings and pistons are new, ring groove modifications are not required against sticking. However, if stuck rings are found at the time of maintenance, the modifications must be done. Stuck rings may cause engine troubles as well as a significant power drop.
Stuck rings can be identified by signs of a failure exhibited at the engine start, such as unwillingness to start engines or abnormal engine noise emitted from the silencer.

Piston rings
  The abutment of piston rings should be chamfered at the gap using a round file. This is intended to prevent stopper pins from being cut by rotational force generated when pistons are moving up and down.  

Piston sizes (applicable until the year 2002 RS250R)
  Cross reference of piston and cylinder combination  
  *Cylinders for the 2001 year and later models bear no A/B identification markings. Instead, they have NXA stamped at the side.  

Handling cylinders and cylinder heads
  Extreme care should be exercised during handling cylinders or cylinder heads.
In particular, flawed mating faces may deteriorate adherence, causing blow by.
If there are projections caused by flaw or dent, they should be removed using abrasive paper to make the surface flat.
However, if flaws are present across O-rings or flaws are present in the O-ring grooves, appropriate cylinders or cylinder heads should not be used since O-rings cause blow by.

RC valve (applicable until 2002 year RS250R)
  For the RS250R, unadjusted RC valves will result in significant power drop (1 to 5 PS). Even if piston maintenance is not performed, RC valves, once heated, should be readjusted since the valves displace.  

Clutch outer section
  The clutch outer section should be periodically serviced and replaced if necessary. If excessive plays or cracks are found during servicing, defective parts should be replaced as soon as possible. (clutch outer fasteners)
(Protruded fasteners may damage pulsar plates for the RS125R or casing cover for the RS250R.)

Water seal
  When water pump seals are replaced to fix coolant leak from water pump, water pump shaft should be visually checked. If the shaft exhibits color change or rough surface, just seal replacement will not solve the problem. In this case, both seals and shafts should be replaced. If coolant leak is again found after replacement of both parts, water pump bearing and balancershaft bearing should be replaced as well.  

  When cleaning carburetors, do not apply high pressure air to carburetors with floats installed. High pressure air would cause oil level in carburetors to get out of order, resulting in significantly wrong settings.  

Carburetor float
  Carburetor float section is a very delicate part. Extreme care should be exercised during disassembly.
If oil level should go wrong, see owners' manual for proper adjustment. Wrong oil level would cause rich or lean mixture, or hesitation. Also, check that floats are smoothly going up and down.
If floats do not move smoothly, rich feeling or hesitation may result when beginning to open the throttle. Also, unstable fuel mixture would cause frequent detonation.
  Float oil level confirmation position  
  Float level should be checked at the point at which float valve comes into slight contact with float lip.
Oil level height should be measured at the highest point of the float, instead of at the point of main jet.

Notes on RS125R
2001 year model cylinder head
Secondary volume for the 2001 and later models is less than that for 2000 and earlier modeled by 0.2cc in cylinder head volume. The 2001 year model has a cylinder head with the center protruded, making impossible the measurement of the volume when the head assembled (because of creation of air puddle). Single head only allows measurement.
Secondary volume criteria
Item Volume when assembled Single head volume
'00 11.6cc 11.1cc
'01 11.4cc 10.9cc
Non-standard combination will result in unwillingness to rev highly and vibration, leading to deteriorated potential performance.
The 2000 year model exhaust joint is different in joint diameter from the 2001 year model, making impossible its common use.

Notes on RS250R
Exhaust joint
  The 2001 year model has exhaust joint length and color which are opposed to the 2000 year model.

Machine #1 front side #2 rear side
'00model 34.2mm black 29.2mm silver white
'01model 27.5mm silver white 29.5mm black
  Non-standard combination will result in unwillingness to rev highly and vibration, leading to deteriorated potential performance.
The 2000 year model exhaust joint is different in joint diameter from the 2001 year model, making impossible its common use.

Exhaust joint gasket
  The 2001 year model has a thin stainless gasket in comparison with its former thick copper gasket (for improved durability). Accordingly, like installation of exhaust joints to chambers, liquid gasket should be applied before exhaust joints are installed.

Reed valve
  The 2001 year and later model has undergone changes to a reed valve body, having altered angle as compared with 2000 year model (2001 model has steep angles).
Making reed valves thinner allows easy opening, and installation of supports secures functioning of valves when wide-open.
As described in the manual, valves, supporter and stoppers should be so installed to ensure their position.