When fuel burns, it leaves behind deposits that stick to the combustion chamber, valves and top of the piston. How quickly the deposits accumulate depends on the type of driving done and the quality of the fuel burned. Carbon deposits gradually accumulate in a new engine for the first 5,000 to 15,000 miles, then level off. A state of equilibrium is reached where old deposits flake off at about the same rate as new deposits are formed.
However, infrequent driving, infrequent oil changes or internal engine problems such as worn valve guides, or worn, broken or improperly seated rings that allow oil burning can greatly accelerate the accumulation of deposits. This may cause the carbon deposits to reach a much greater than normal thickness, which in turn raises compression and causes spark knock or detonation problems. The deposits may even build up to the point where they cause physical contact between the piston and head. This, too, can make noise as well as be very damaging to your engine.
To get rid of the deposits, pour a can of "top cleaner" down the carburetor while the engine is idling (follow the directions). Allow the chemical to soak for the recommended period of time, then restart the engine to blow out the loosened deposits. An oil change afterwards is recommended because some of the solvent will leak down into the crankcase and dilute the oil.
If chemical cleaning fails to do the trick, it may be necessary to pull the head and scrape the deposits off. Some shops also have equipment that allows them to blast deposits loose by blowing a "soft" blasting media such as crushed walnut shells into the combustion chamber through the spark plug hole.