The function of a carburettor is to regulate the engine's fuel air mixture. If the mixture is too rich, meaning the air content is too low, increased carbonisation of the engine can occur. It can also decrease engine performance and increase the chances of smoking. If the mixture is too lean, or the air content is too high, there is less lubrication to the engine which could cause cylinder scoring. Maintaining the proper fuel air ratio is necessary to avoid these problems. This can be done by adjusting the fuel air mix. For most carburettor models, special tools are required to adjust the carburettor. These tools adjust carburettor-specific screws which are now used on most models to prevent adjustment by non-EPA-certified mechanics. Five different carburettor adjustment tools are currently on the market.
Single "D"-shaped Tool
The single "D"-shaped carburettor adjustment tool is used to adjust the tamper-proof adjustment screw found on some carburettor models. Both the tool and the tamper-proof adjustment screw are "D"-shaped. The adjustment of the screw changes the amount of air that is allowed into the engine. By turning the screw in, the amount of air allowed into the engine is reduced, creating a rich fuel air mixture. By turning the screw out, the amount of air allowed into the engine is increased, creating a lean fuel air mixture.
Double "D"-shaped Tool
The double "D"-shaped carburettor tool is used to adjust the double "D"-shaped tamper proof adjustment screw on small engine carburettors. This is done while the engine is running and has had a chance to warm up. This is because the carburettor settings will change once the engine has warmed up.
The hex-shaped carburettor adjustment tool is used to adjust the tamper-proof adjustment screw on both small and large engines. The tamper-proof adjustment screw is generally blocked with a cap by the manufacturer to prevent adjustment of the carburettor by non-licensed professionals. Special tools are required for the removal of the cap. The distribution of the cap removal tools are also regulated by the EPA.
The pac-man-shaped tool is used for the tamper-proof adjustment screw on some Walbro and Zama carburettor models. These tools are also referred to as V-notch carburettor adjustment tools. They are offered to certified mechanics through MTD distributors.
The splined carburettor adjustment tool is generally used to adjust the tamper-proof adjustment screw on Poulan and Weedeater carburettors. These tamper-proof adjustment screws have four sets of splines each spaced 90 degrees apart.