Automatic Scooter Engine, Explained


The cylinder, cylinder-head and piston are some of the most important components in the engine. The upward stroke of the piston draws in the fuel/air mixture from the carburettor and the downward stroke of the piston transfers this mixture up to the cylinder head, the power created by the explosion in the cylinder head (combustion chamber) is transferred to the engine via the crankshaft and connecting rod. The design, material and shape of these components are critical to achieve the desired performance. The upper part of the piston with the help of the piston rings has to provide a gas tight seal within the cylinder. The position of the piston and the size / shape and position  of the cylinder ports, control the flow of gas through the cylinder.


The carburettor is the control for the engine. It feeds the engine with a mixture of air and petrol in a controlled volume that determines the speed, acceleration and deceleration of the engine. 
The carburettor is controlled by a slide connected to the throttle cable from the handlebar twist grip which adjusts the volume of air drawn into the engine. A system of fixed holes (jets) and, adjustable needles control the volume of fuel (petrol) to match the volume of air. The ratio of fuel and air the carburettor delivers can be changed by selecting different jets and adjusting the position of the needles. Sometimes but not very often, the manufacturers restrict the speed of the scooter for certain countries by fitting a reducing washer to the entrance of the carburettor.

Although the carburettor is set by the manufacturer, it is sometimes necessary to make adjustments to match the characteristics of the engine. These are:

  1. Tick-over, this controls the speed of the engine when the twist grip is closed to stop the engine from stalling.
  2. Mixture screw, this controls the volume of fuel that enters the engine when accelerating from a low engine speed. 
  3. The height of the float and float needle controls the amount of fuel available in the carburettor. If the float needle becomes worn it may cause the carburettor to leak and overflow. 
  4. The height of slide needle, controls how quickly the needle is lifted from the main jet, to allow the fuel to pass through the jet. 
  5. Size of the main jet, selecting a main jet with a different size hole will effect the volume of fuel it will pass.

For example when a new exhaust is installed or a custom air filter it is likely that a higher ratio of fuel to air will be necessary and therefore a main jet with a larger hole will be required.

The Variator

The variator is like a continuously variable drive system to give the optimum gear ratio between the engine and the rear wheel. This enables the engine to work at the best speed in comparison to the rear speed.

The variator works upon the ratio of the diameter of the front and rear drive belt pulleys. Both front and rear pulleys are an assembly of moving plates. When viewed from the side, one plate is convex and the other is concave. At low engine speed, the plates of the front pulley are pushed apart by the tension of the belt as it forces itself between the plates. At the same time, the plates in the rear pulley are pushed together by the spring beneath the automatic clutch. Behind the moving plate of the front pulley, there are weight rollers, these are thrown towards the outside edge of the variator by centrifugal force, as the rotation speed of the engine / variator increases. When the weight rollers move towards the outside of the variator, the inside plate is forced together with the outer plate and this moves the drive belt towards the outside of the pulley. This effectively makes the diameter of the front pulley larger. As the belt moves out on the front pulley, the tension in the belt is increased, which overcomes the spring pressure holding the two plates together in the rear pulley. The belt moves towards the center of the rear pulley, which effectively makes the diameter of the rear pulley smaller. In this way, the ratio between the two pulleys is changed and therefore, the speed ratio between the engine and rear wheel is changed. It is common for the scooter manufacturers to restrict the speed of the scooter by fitting a spacer washer under the variator. This restricts the amount the variator is allowed to open. if removed the belt is allowed to go further out into the pulley.


The exhaust has a very important role in the performance of a two stroke engine. The design of the exhaust is critical to achieve the maximum power. Following combustion in the cylinder head the burnt gases are discharged through the exhaust port and they expand into the shaped chamber of the exhaust. The shape, size and length of this chamber are critical to the efficiency of the system.

At the end of the chamber, there is a tapered outlet into the silencer. This tapered outlet creates a reverse pulse, back to the cylinder which is timed to reach the exhaust port at the moment the piston passes it and closes it off. This reverse pulse helps to ensure that none of the incoming fuel/air mixture escapes and the maximum is retained in the cylinder for combustion.  

It is common for the scooter manufacturers to restrict the speed of the scooter for certain countries by tack welding a washer or small restrictor to the entrance of the exhaust or at the exit to the silencer, these can usually quite easily be removed. Sometimes another way for the manufacturers to restrict the scooters speed is for them weld an extra piece of pipe some 15 or 20 cm long to the outside of the exhaust lead in pipe, again a performance increase will be experienced if this is removed, and weld over the hole it leaves.

Each exhaust is designed and tuned to achieve maximum perfor­mance with a specific engine. 
1 - the exhaust evacuates the burnt combustion gases 
2 - the exhaust reduces the sound from the combustion chamber 
3 - the exhaust is tuned to improve the efficiency of the port timing.


The final drive assembly drives the rear wheel; this is a combination of shafts and gears that determine the speed of the wheel in relation to the speed of the engine. In standard form the ratio is set to allow the engine to turn at an optimum speed according to the wheel speed. If the performance of the engine is improved it is possible to change the gears in the final drive assembly for a new ratio.