The color of the exhaust smoke is a symptom that reveals problems with an engine.
White or blue smoke?
This is caused by the burnt oil in the engine. While a certain amount of oil is consumed and burned under normal circumstances, blue smoke clouds indicate a problem. Old engines emit smoke as the cylinder wears out (motor burnout) and the piston and rings no longer fit as well. Excess oil is added to the cylinder and burned. Fractured piston rings can allow a large amount of oil to enter the cylinder.
The oil in combustion can also be caused by a defective vent that allows the oil to be expelled from the sump and then sucked into the engine through the carburetor (if there is a pipe connecting the vent to the carburetor).
However, another cause is an overloaded sump. On one occasion, my neighbor and his helpful friend decided to fill the lawnmower with oil and, being generous, decided to fill the oil to the edge of the oil filler cap. The result? Thick clouds of white smoke I could see in the distance! Yours really had to save the day and drain some oil from the pruner!
Black smoke?
Black smoke is an indicator of burning too much gas. This can be caused by too rich a mixture (adjust the mixer screw), a defective diaphragm or punctured in the carburetor, a choke that does not open when the engine starts or a dirty air filter.
