Carburetor adjustment is critical on chain saws and other two-cycle engines. An adjustment that is too rich will cause a saw to smoke, have insufficient power, result in additional carbon build up and may damage the engine. An adjustment that is too lean will also produce insufficient power and is more likely to damage the engine through lean seizure. The correct carburetor adjustment will allow the saw to produce maximum power, extend the duty cycle of the engine, produce minimal smoke, idle smoothly, rev up nicely and give trouble free service.
An over rich carburetor adjustment is when the proportion of fuel in the combustible air/fuel mixture is so high that the fuel does not burn well. The burn does not produce much heat, so the power stroke is weak. The partially burned mixture is expelled into the muffler and exits the saw as smoke. An over rich condition causes carbon buildup and will plug the fire screen and cylinder ports if run for a period of time. Outside of carburetor adjustments this can also be caused by having too much oil mixed with the fuel, and/or old stale gasoline.
A lean carburetor adjustment is when the proportion of fuel in the combustible air/fuel mixture is so low that there is not enough fuel to burn. This also makes a weak power stroke and causes the saw to have insufficient power. In addition to low power, a lean condition causes the cylinder temperature to rise, which often leads to seizure. A lean condition also allows for excessive RPM which often leads to big end rod bearing failure. Other causes for a lean running condition include lack of oil in the fuel mix, and when the fuel tank runs empty. Always stop a saw before the tank runs completely dry. Be sure to use the correct gasolineil
mixture ratio recommended for your chainsaw.