Yamaha's Zuma 50 scooter was fitted with a 14 mm Teikei-built carburetor to feed the 50 cc two-stroke single-cylinder engine. To many scooter owners, the carburetor is a mysterious device that appears to work through some sort of sorcery, creating an utter reluctance to approach any type of maintenance or repair that the carburetor may need. Understanding how the carburetor works can help alleviate any fears you may have about it.
Basic Carburetor Operation
A carburetor's primary duty is to mix air and fuel into a highly-combustible mixture that will be ignited within the engine. Air is pulled in through the scooter's air box and the carburetor's inlet through the vacuum pressure generated by the engine's pistons. The speed of the incoming air flow is increased as it passes through the inner carburetor bore, which tapers from the wider inlet to the narrower outlet leading to the engine. Along the way, fuel is introduced into the air flow through a series of nozzles called jets; each jet meters a specific amount of fuel specific to demands of the engine at varying throttle positions.
The carburetor stores fuel in a bowl-shaped chamber attached to the bottom of the carburetor body. Fuel passes from the fuel tank and into the chamber through a special valve that is controlled by a float. As the fuel level increases, the float rises and seals the valve to stop the flow of fuel into the chamber. Fuel is pulled into the carburetor bore through the fuel jets during operation, affecting fuel level and opening the valve to admit more fuel into the chamber. This process repeats over and over until the engine is stopped.
The Zuma's engine speed is controlled by the throttle grip on the right handlebar, which in turn operates a pulley that raises or lowers a cylindrical valve within the carburetor bore. When the valve is raised, the increased flow of air pulls a greater amount of fuel into the mixture and raises the engine's rpm. Likewise, lowering the valve reduces the rpm. At idling speeds, the valve is lifted just far enough to admit a small amount of air into engine. The engine idle speed is adjusted using a small screw near the throttle pulley; turning the screw clockwise pushes the valve open in small increments, while turning the screw counterclockwise closes the valve.
There are two jet systems at play within the Zuma's carburetor, both of which rely on several components to operate properly. The first of these is the pilot system, which provides fuel from idle to a one-quarter open throttle position. Incoming air flow is directed through a small channel built into the carburetor inlet, where it passes the pilot -- or mixture -- screw. Turning the mixture screw clockwise restricts the air flow and reduces the amount of fuel drawn out through the pilot jet and into the air and fuel mixture; turning the screw counterclockwise allows more air to pass through and pull more fuel into the mixture. In either case, the resulting pilot mixture is dispersed through a passage located just behind the throttle valve. The pilot jet is housed in a small tower within the float chamber, adjacent to the central main jet tower.
The main jet system, which provides fuel from one-quarter to full throttle, relies on a tapered needle attached to the bottom of the throttle valve. The "jet" needle's taper allows for a progressively increasing amount of fuel to be drawn out of the main jet as engine speeds increase. The Zuma's jet needle is adjustable via a set of slots cut into the upper end of the needle and can be repositioned by moving the needle clip into a different slot. Raising the clip lowers the needle and reduces fuel flow at wider throttle openings. Lowering the clip raises the needle and admits a greater amount of fuel earlier in the throttle's movement.
A typical motorcycle carburetor uses a third fuel jet system called an enrichener. This is often mistaken as a choke, since it is only needed when the Zuma is first started and until it has warmed to its operating temperature. Unlike a choke, which restricts air flow to create a fuel-rich mixture, the enrichener adds more fuel to the normal air flow intake to help the engine start from a complete stand-still. The enrichener is often controlled by a lever attached to the handlebar or directly to the carburetor. In the Zuma's case, the enrichener is electronically controlled for the rider's benefit. The enrichener is operated by an electro-magnetic switch that lowers or raises a plunger over the enrichener jet. The plunger is held in an open position until the engine warms to its normal operating temperature. Once warm, the switch closes and pushes the plunger firmly over the enrichener jet.
The Care and Feeding of a Carburetor
Carburetors of any type are extremely sensitive to debris trapped in the fuel and gasoline's natural tendency to break down into a green sludge. This sludge is the primary cause of many carburetor problems, ranging from a complete inability to start to inconsistent fuel delivery, The sludge acts in the same manner as dirt or rust caught in the fuel flow and clogs the carburetor jets to restrict fuel flow. In worst-case scenarios, fuel flow can be restricted completely by sludge or debris. Preventing fuel sludge is as simple as riding the scooter often and keeping the tank filled with fresh gasoline at all times. A fuel stabilizer should be added to the fuel supply if the scooter is placed in storage for longer than two weeks. Debris, however, can often be screened out of the incoming fuel flow with the use of an inline fuel filter. As the name implies, the filter is placed between the fuel tank and the carburetor and will trap dirt and other contaminants before it enters the carburetor.
- Yamaha YW50S Service Manual; Yamaha Motor Corporation
- The Professional Motorcycle Repair Program: Fuel Systems, Volume 9; Professional Career Development Institute