![spain].gif](spain].gif){kind=small}
(As used in some Aprilia, Italjet, Malaguti, Beta, Benelli, MBK, Motron, Yamaha scooters etc.)
The Yamaha engines are in three types, two air-cooled and one water-cooled, all made under license by Minarelli in Italy and supplied to a number of manufacturers.
The oldest was originally used in the Yamaha BWS. It has an upright cylinder and the carb feeding through a reed valve into the rear of the cylinder barrel.
The later engine unit was originally used in the Yamaha Jog, which has a similar layout to the BWS motor, but has a horizontal not vertical cylinder with the carb feeding into the crankcase not the cylinder. This motor was originally air-cooled and for the last few years has been available in water-cooled form.
Up until recently the motors were all drum brake at the rear with either 10" or 13" wheels. Now the latest models are available with a disc brake on the rear. These water and air cooled motors had an update in 1995/96 when the transmission clutch and rear pulley system was revised.
All the restricted motors are restricted by a spacer between the front pulleys, restricters in the exhausts (not always) and a rev limiter built into the CDI unit. The rev limiter can be disconnected on early motors by disconnecting one wire. On later motors (’96 onwards) there was an increase in the number of manufacturers using the latest CDI units, which must be replaced to remove the rev limiter. These replacement CDI units can have standard ignition timing or variable ignition timing with no rev limiter (which is useful when higher states of tune are used).
One of the problems with all these engine units is that the RPM varies greatly as the pulleys change diameter, and that the standard transmission spring will not cope with 70cc power outputs without obtaining belt slip at low speeds. The easiest way to help both these problems is to use an aftermarket variator and rear pulley spring to enhance transmission performance. The rear half of the rear pulley can be changed so that RPM change is reduced to a minimum as the pulleys change diameter. This rear pulley half is referred to as a torque driver pulley.
The higher state of tune aluminium MHR or race cylinders with race expansions can beneficially use the race/fast-road transmission set-up, which uses a matched rear pulley assembly and variator and a special drive belt. The only extra part required is a clutch assembly to obtain a complete transmission system.
If an exhaust is used with a cylinder that between them generates a wider powerband then the only transmission part required is an uprated rear spring so that belt slip is eliminated.
The Yamaha engines are in three types, two air-
The oldest was originally used in the Yamaha BWS. It has an upright cylinder and the carb feeding through a reed valve into the rear of the cylinder barrel.
The later engine unit was originally used in the Yamaha Jog, which has a similar layout to the BWS motor, but has a horizontal not vertical cylinder with the carb feeding into the crankcase not the cylinder. This motor was originally air-
Up until recently the motors were all drum brake at the rear with either 10" or 13" wheels. Now the latest models are available with a disc brake on the rear. These water and air cooled motors had an update in 1995/96 when the transmission clutch and rear pulley system was revised.
All the restricted motors are restricted by a spacer between the front pulleys, restricters in the exhausts (not always) and a rev limiter built into the CDI unit. The rev limiter can be disconnected on early motors by disconnecting one wire. On later motors (’96 onwards) there was an increase in the number of manufacturers using the latest CDI units, which must be replaced to remove the rev limiter. These replacement CDI units can have standard ignition timing or variable ignition timing with no rev limiter (which is useful when higher states of tune are used).
One of the problems with all these engine units is that the RPM varies greatly as the pulleys change diameter, and that the standard transmission spring will not cope with 70cc power outputs without obtaining belt slip at low speeds. The easiest way to help both these problems is to use an aftermarket variator and rear pulley spring to enhance transmission performance. The rear half of the rear pulley can be changed so that RPM change is reduced to a minimum as the pulleys change diameter. This rear pulley half is referred to as a torque driver pulley.
The higher state of tune aluminium MHR or race cylinders with race expansions can beneficially use the race/fast-
If an exhaust is used with a cylinder that between them generates a wider powerband then the only transmission part required is an uprated rear spring so that belt slip is eliminated.
