The 1974-1995 Yamaha RS100, RXS100, RS125, RXS125 repair and service manual by Haynes, covers the following models:
Book Excerpt: 1974-1995 Yamaha RS/RSX100, RS,RXS125 Repair Manual
- Yamaha RS100 (97cc), September 1974 to October 1983
- Yamaha RXS100 (98cc), April 1983-on
- Yamaha RS125 (123cc), September 1974 to January 1976
- Yamaha RXS125DX (123cc), January 1976 to July 1984
ENGINE, CLUTCH AND GEARBOX
DISMANTLING THE ENGINE: REMOVING THE LEFT-HAND ENGINE CASING FITTINGS
FUEL SYSTEM AND LUBRICATION
- Remove the three screws securing the gear selector drum end cap and then the cap. The cap holds in position a horseshoe-shaped retainer, fitted in a groove in the end of the selector drum. This will drop away when the cap is removed.
- Withdraw the clutch pushrod, checking to see that the ball between the two pushrods is not lost. It may remain inside the casing and if so should be shaken out.
- Unscrew and withdraw the neutral indicator light switch from the top of the casing, also the plunger housing, spring and ball assembly situated directly behind it.
- Remove the circlip from the gearchange pedal shaft and check finally that nothing has been missed before turning the unit over to work on the right-hand casing.
DISMANTLING THE ENGINE: REMOVING THE RIGHT-HAND CASING AND FITTINGS
- With the unit supported on suitable blocks, slacken and remove the outer cover screws, and lift the cover away, complete with the oil pump and drive gear.
- Lift off the tachometer plastic drive gear and washers from its support pin (where fitted).
REED VALVE INDUCTION SYSTEM: MODE OF OPERATION
- Of the various methods suitable for controlling the induction cycle of a two-stroke engine, Yamaha has adopted the reed valve, a non-mechanical device located between the carburetor and the cylinder.
Each valve comprises two flexible stainless steel strips attached to a die-cast aluminum alloy casting.
The strips seat on a gasket formed from heat and oil-resisting synthetic rubber, which is welded to the casing by the heat from the engine.
A specially-shaped valve stopper, made of cold rolled stainless steel plate, acts as a form of stop, to control the extent to which the valves are free to move.
- The valves open of their own accord as the pistons commences to rise in the cylinder creating a vacuum within the crankcase. Atmospheric pressure forces the valves open and causes a fresh fuel/air charge to be rammed into the crankcase.
The existing mixture, already in the combustion chamber is fully compressed and then ignited by the sparking plug. The explosion drives the pistons downwards again, expelling the exhaust gases through the exhaust port as it is uncovered by the falling piston.
Although the reed valves are closed by the time the piston has reached the top of its stroke, they open again while the compressed charge in the crankcase passes into the combustion chamber via the transfer ports, through the inertia caused by the stream of fuel/air mixture entering the cylinder.
This additional new charge is admitted by the seventh port and is not permitted to pass into the crankcase. Instead, it is used to expel the remainder of the exhaust gases out of the cylinder, so they do not conflict with the incoming charge that is about to be compressed and fired. In other words this second action is of a scavenge nature only.
From this point onwards the reed valves close until the piston is again on the ascent and new charge in the combustion chamber is compressed and ignited.
The reed-valves open again to admit a new charge of fuel/air mixture to the crankcase and the whole cycle of operations is repeated.
Subject: 1974, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995 Yamaha RS100, RXS100, RS125, RXS125 service, maintenance, repair. ISBN-10: 1859600557 | ISBN-13: 9781859600559 | Haynes 331