9.21.
Servicing Carburettors
Carburettors should be serviced only for the routine idle adjustments unless an engine operation problem, positively identified, is due to a carburettor problem.
Adjusting Idle.
This is the most common carburettor adjustment involving setting the idle mixture screw to provide a smooth idle and setting the idle speed screw to provide the specified idle speed on a warm engine. Before idle adjustment is taken up, the entire engine system should be functioning correctly and the engine should be sufficiently warm so that the choke is fully open and releases the fast idle cam. In emission-control carburettors idle adjustments for rich mixtures, required to provide the desired smooth idle, is not possible because the carburettors are fitted with limiters.
Idle settings require two adjustments ; an air adjustment that primarily controls idle speed and a mixture adjustment that controls idle smoothness. Idle mixture is set by idle needle screw, usually located near the base of the carburettor. In some carburettor models this crew is located in the idle air bleed passage. The most common speed adjustment method is to limit throttle plate closing with a throttle stop screw located on the carburettor linkage. Air going around the slightly open throttle plate supplies air required for idle condition.
Carburettor Overhaul.
For major service, the carburettor is completely disassembled, after removing from the engine, except for the throttle plates, choke plates, air-valve plates, and permanently installed caps and plugs. The bowl cover, acceleration pump, discharge cluster, and needle valve and seat are removed. The idle mixture screws that turn freely by hand should be removed. The disassembled parts should be examined to identify any abnormal wear, leakage, or cracks for suitable repair/replacement. Except for float, all metal parts without gaskets or rubber that can be removed from the carburettor should be cleaned using carburettor chemical cleaner.
Carburettor mating surfaces should be checked for correct fit. The needle valve seat should be firmly tightened using a proper tool.
Correct float level is essential for proper fuel metering and engine performance. A number of methods are used to measure float level. The simplest method is to measure from the top of
the bowl surface to a specified place on the float, usually the top or bottom. If the float level is not correct, it is adjusted by bending the float level tang that touches the needle valve. The float level is used to establish the level of fuel in the float bowl. The carburettor parts are reassembled and all the adjustments are made, before replacing bowl cover.
