62 Tiger Cub Motorcycle

FRS 106, Michael Littman – Spring 2011

Cub Carburetor

The carburetor (aka “carb” or “carby”) facilitates the mixing of air and fuel for an internal combustion engine.

Efficient firing of the engine requires that there be an ideal ratio of air (mostly oxygen and nitrogen) and fuel (hydrocarbons). This ideal ratio is refered to as stoichiometry and is approximately 14.7 pounds of air per pound of fuel. If the engine runs too rich (meaning there is fuel left unburned because not enough oxygen is present) hydrocarbon, carbon dioxide, and oxides of nitrogen emissions rise while fuel economy suffers. At ratio’s leaner than stoichiometry, emissions rise while driveablity and power will weaken.

How it Works

A carburetor is an open pipe through which the air passes into the the engine. The pipe narrows at one section and then widens again, causing the airflow to increase in speed at the narrowest part. Below that there is the throttle valve (which is a rotating disc that can be turned end-on to the airflow to hardly restrict the flow at all, or can be rotated so that it almost completely blocks the flow of air) This valve controls the flow of air through the main pipe and thus the quantity of air/fuel mixture the system will deliver.

Fuel is introduced into the air stream through small holes at the narrowest part of the main pipe and at other places where pressure will be lowered when not running on full throttle. Fuel flow is adjusted through precisely measure divots, referred to as jets, in the fuel path.

The carburetor is connected to the throttle by a cable which helps regulate the positioning of the valve and consequent air-to-fuel ratio. It controls the flow of fuel indirectly by controlling the speed of airflow into the carburetor which, in turn, controls the pressure. The pressure determines the amount of fuel that is drawing into the air stream.

(see Bernoulli’s principle: http://en.wikipedia.org/wiki/Bernoulli%27s_principle)


Our ’62 Cub carburetor is in pretty good shape. It’s a little dirty and rusted and needs a new rubber cover but otherwise it looks to be in good working order. A little time in the parts cleaner should do it some good.