From Moped Wiki
Yamaha Chappys were introduced as Honda released its small trail bikes. They were produced in the 1970's and early 1980's. The most common models are the LB50 (50cc model) and the LB80 (72cc model). The LB50s was usually known to have pedals installed into the swingarm, while the LB80s normally had the pedal system absent all together.
Because people often modify their Chappys for their own personal riding preferences, pedal existence alone should not be used to identify what model the bike is. On the left side of the engine near the cylinder's base, there is usually a small engraving that states the displacement of the motor, either 50cc or 72cc. The frame will also have a paper label behind the headlight stating the original motor's displacement, the manufacture date, and a VIN number.
edit : the manufacture date is more often the approval date of the model. first three character of the VIN are the version (and there is so much version across the world...)
Both engine models include a 2-stage 2-speed automatic transmissions. Technically, there are 4 speeds all together, but the transmission may only be set to use either the lower two speeds (for extreme uphill climbing, trail riding, etc) or the higher two speeds (for all around on-road cruising). The gear selector (shifter) is located on the left side of the motor, just above the side cover. There is also a neutral gear.
In order for the gear selection to be changed, the bike must come to a complete stop and the engine must have a very low rev. Shifting the bike while moving or reving the motor will usually not work, can be very dangerous (violent jerks often happen), and the transmission gears easily destroyed.
Due to the nature of the transmission, the Chappy is one of the few mopeds/scooters that can actually be bump started even through it is equipped with an automatic clutch. Since the kick starting mechanism is known as the bike's biggest weak point (along with the cable choke), most Chappy owners prefer to bump start their bikes. Bump starting the Chappy has not been proven to cause any type of transmission damage or failure when performed properly.
In extreme instances where the bike does not have an easy cold start, the bike is started easiest by bump starting in low gear ("L"), since the motor will be revving higher as the owner pushes the bike. Once started, it should be ran in low gear for a few minutes until it warms up, and then stopped, shifted into drive ("D"), and ridden normally.
An experienced rider will be able to pedal the bike (if it has pedals) to a slow speed (while in neutral), and then popping the shifter into either Drive or Low. Caution must be taken not to do this at too high a speed, or the transmission may get damaged.
edit : Late automatic models haven't low gear selector, and cannot start without kick starter, as the gear is in free wheel if motor isn't running.
There is a very large difference between the LB50 and LB80 models. While the engine's lower ends of each model are usually identical, the 50cc top end does not pack enough power to push the bike any more than about 21 to 23mph (depending on the rider's weight). The ride feels much like coasting a bicycle down a small incline. In comparison, the LB80 can get to about 35mph stock and packs enough power for the experienced rider to perform a wheelie upon take-off. A kit does exist from Polini to convert an LB50 to a 72cc displacement top end.
edit : The 50cc version with 4 speed manual gearbox (like swiss ones) ride about 40-45mph, stock, certainly the most powerful 50cc version of the Chappy, with an engine like a patchwork of other yamaha engine (pw50, yz80, rd50 and fs1 parts...) Euro 50cc 2 speed automatic gearbox reach 27-30 mph, and 30-34 for older models with moutain gear switch before the max 28mph law in most euro countries. Polini kit fit only on 49.9cc, and is a 62cc. It can reach more higher rpm, up to 7hp with bigger carb/intake/exhaust. Stock exhaust are the most performance limitation for 50 and 72cc.
Fuel/Air Intake System
Stock Chappys are usually equipped with a 19mm intake, a Mikuni Carburetor featuring a cable choke, a hose running to an air filter/air box, and a reed valve. There are two fittings on the side of the intake. One is for oil injection and the other is to vacuum fuel from the petcock. Since the original air filter for the Chappy is no longer available, it is recommended to replace the air box & hose with an aftermarket clamp-on filter. Care must be take into choosing a smaller one so that it does not interfere with the plastic cover above the motor.
edit : euro intake are 16mm. Polini kit can have a double reed valve from other yamaha bike without mod.. Original air filter is still available from Yamaha !
If replacing the carburetor, one must be selected that has a cable choke. If a lever choke is used, the plastic cover above the carburetor will either have to be removed or have a hole cut in it large enough to be able to reach the lever. The intake can also be converted to a 21mm and still function correctly.
edit : most common conversion is with a polini 19-21 intake, with 17,19 or 21 pgbh delorto carb - phgb version need to have left side air screw, cable choke and 70° gas cable options. These need lot of jet setup, and less restrictive exhaust to run correctly.
Since the Chappy is not equipped with a fuel gauge, the gas tank features two "pockets" near the bottom, each having their own outlet valve. Once the fuel has dissipated in the pocket with the active outlet valve, the petcock can be switched to use the other outlet valve (by selecting "Reserve") and the bike can usually be ridden long enough to find a gas station.
The Chappy gas tank is capable of holding just over 1 gallon of gasoline.
The Chappy's generator outputs three separate circuits, which are Ignition, AC Accessory, and one to charge the battery.
The first is used exclusively for ignition. It is usually identified by a black wire with a white stripe, and runs to the external ignition module, the ignition switch (key switch), and the kill switch lever on the right-side handlebar controls. When the engine is shut off via the key or kill switch, the wire receives a direct line to ground and the entire circuit is therefore "shorted". This makes spark impossible and the engine will be forced to shut off.