63 Tiger Cub Motorcycle

FRS 106, Michael Littman – Spring 2011

Top End

Team – Farhan, Luke, Katie

The engine in our 1963 Tiger Cub is a single cylinder, four stroke engine.  In order to organize our part of the ’63 Tiger Cub, we divided the top end into several sections.  The first division we made was into the cylinder, the head, and the carburetor.  The cylinder and head are discussed in subsections, while the carburetor is detailed below.



The carburetor provides fuel and oxygen to the engine in a way that allows for maximum power from the combustion phase without overheating. It thus has to provide the right amount of fuel and the right ratio of fuel to air, quantities which vary depending on throttle opening. The carburetor has to allow the engine to run efficiently at start, at full throttle, and every throttle opening in between.





A carburetor mixes fuel and air in the right proportions and delivers it to the engine. It also atomizes the fuel, turning the liquid gas in the carburetor into a gaseous mixture with the fuel dissolved in air.

The main parts of the carburetor are the bowl, the jets, and the venturi (the open tube that goes through the center of the carb). The bowl is what holds the fuel. The jets are tubes that allow fuel to flow up from the bowl into the venturi. The venturi is a hollow tube running the length of the carburetor and attached to the head of the engine.

The basic process in a carburetor is as follows:

  • air flows through the venturi and into the head
  • the motion of the air creates a negative pressure in the venturi
  • the negative pressure exerts a force on the fuel in the jets
  • the fuel is sucked up towards the venturi
  • en route, it is mixed with air in the jets and atomized
  • the fuel further mixes with air upon entering the venturi
  • the entire mixture is sucked into the head to be burned

Different controls are used to achieve the optimal fuel:air ratio for given throttle openings and to allow the engine to run at idle.

Air to Fuel Ratio

In theory, for complete combustion under normal conditions, the air : fuel ratio is 15 : 1. The actual ratio in an engine varies from this level, however. A lower ratio is needed to start the engine because when the engine is cold, some of the gas in the cylinder condenses on the walls of the chamber and can’t be burned. At full throttle, the ratio is again lower because this lower ratio achieves maximum power at high speed. In the mid-range, the ratio in the engine is slightly lower than the theoretical value to achieve better fuel efficiency.

An air : fuel ratio that is lower than the desired ratio is termed “rich”, while one that is higher than the desired ratio is termed “lean.”

At different throttle openings, the engine also requires different absolute amounts of fuel. This is accomplished using different fuel systems.


Fuel Controls

Throttle Valve

The throttle valve is a piston that sits in the middle of the venturi. It is attached to the throttle cable such that when the throttle is opened, the piston is pulled up, and when it is closed the piston comes down, blocking off more of the venturi.

The throttle valve has a triangular section removed facing the intake side called the cutaway. The cutaway helps to create a negative pressure in the venturi because air hitting the valve at the cutaway is forced down and under the valve. This forces the air to move faster (think of a river that narrows – the water has to move faster) and creates a negative pressure in the venturi. A greater cutaway creates a lower pressure in the venturi, and causes more fuel to be sucked up through the jets. Similarly, a smaller cutaway creates a relatively higher pressure in the venturi and less fuel is sucked up.

Tuning the throttle valve has the greatest effect on the engine between approximately 1/8 throttle and 1/2 throttle.

Idle Adjusting Screw

The idle adjusting screw sits in the venturi and holds the throttle valve up at a certain minimum height. This controls the amount of air that flows through the venturi at idle and thus the pressure in the venturi at idle, and how much fuel is drawn from the jets when the throttle is closed. If the screw is further unscrewed into the venturi, the motorcycle will tend to have a faster idle (the engine will be running faster at idle), and vice versa.

The idle adjusting screw only affects the fuel flow to the engine at idle.

Pilot Jet

At low throttle openings, the pressure in the venturi is not low enough to draw fuel from the main fuel system, so a pilot fuel system is employed. The pilot jet is a smaller jet than the others allowing fuel to flow up through it into the venturi at higher pressure. The fuel from the pilot jet is metered with air from the air screw and they are mixed in the jet itself before being drawn into the venturi, mixing with more air and traveling to the engine head.

The pilot jet and air screw are tuned in conjunction to provide the optimal air : fuel ratio at idle. They have the greatest effect on fuel flow between idle and 1/4 throttle.

Needle Jet & Jet Needle

Despite their similar names, the needle jet and the jet needle have different functions.

The needle jet, like the pilot jet, allows fuel to be drawn from the bowl into the venturi. It is a narrow jet that sits inside the main jet. Air drawn through the needle jet is metered with air from the air jet, allowing the fuel to be atomized, before entering the venturi.

The jet needle is a needle attached to the bottom of the throttle valve that moves in and out of the needle jet as the throttle valve moves up and down (as the throttle is opened and closed). The needle is tapered such that, when the throttle is closed, it blocks most of the area of the needle jet opening and very little fuel in the needle jet is exposed to the low pressure in the venturi. As the throttle is opened and the needle is pulled out, the taper causes it to occupy less of the needle jet opening area, allowing more fuel to be exposed to the low pressure in the venturi.

The low pressure in the venturi can be described as an attractive force being exerted equally over the entire surface of any fluid at a higher pressure (ie the fuel/air in the jets/bowl) because P = F / A (pressure equals force per unit area). For a given throttle opening, the negative pressure (and thus the force associated with it) is constant. The amount of fuel that is drawn is thus dependent on the surface area of fluid (gas/fuel mixture in the jets) exposed to the negative pressure.

The jet needle is designed to control the amount of surface area of fuel mixture exposed to the negative pressure of the venturi for any given throttle opening. As the throttle is opened, the jet needle is tapered so as to occupy less surface area and allow more fuel to be drawn.

Around 15% throttle, fuel starts being drawn from the needle jet. Above 3/4 throttle, the fuel flow is mainly controlled by the main jet because the jet needle is pulled high enough to essentially not block any of the surface area of the jets. Adjusting the needle jet and jet needle is thus most effective for 1/8 to 3/4 throttle.

Main Jet

The main jet is the largest jet, and it sits around the needle jet. Around 3/4 throttle, the jet needle is out of the way and the pressure in the venturi starts to be low enough to draw fuel from the main jet. Fuel from the main jet is metered with air from the air jet; they are mixed in the jet before entering the venturi.

Adjustment of the main jet is most effective between 3/4 and full throttle.

Air Jet

The air jet is a small opening on the intake side of the carburetor. Air flows through the air screw into the needle/main jets where it is mixed with fuel and drawn into the venturi.

Adjustment of the air jet is most effective between 1/2 to full throttle.


The floats maintain a constant level of fuel in the bowl of the carburetor. If the fuel level is too high, too much fuel will be drawn into the venturi, and vice versa if the fuel level is too low. Floats move up and down on a metal shaft and are made of a material that floats on gasoline. As the fuel level in the bowl increases, the floats move up. Eventually, the force associated with the pressure from the intake valve matches the force of buoyancy on the floats, and fuel stops flowing. This constant level of fuel is thus maintained in the bowl.

The floats are carefully manufactured to maintain the desired level of fuel in the carburetor, and thus shouldn’t be tampered with or adjusted.

Starter Systems

Various systems are employed in carburetors to provide the rich (low air : fuel ratio) mixture required to start the engine.

In a starter plunger type carburetor, a plunger on the outside of the carburetor can be pushed to deliver a rich mixture to the engine. In this system, the throttle must be closed in order to create the vacuum necessary to draw fuel into the plunger tube.

A carburetor with a choke allows you to close the choke and limit air intake through the venturi, thus richening the mixture.

Finally, some carburetors have a fuel enrichening system which has a lever on the exterior of the carburetor that opens a different set of jets and allows more fuel to flow.