What color is the smoke, and does it use oil? Oil smoke is blue, and if the rings are bad, the exhaust will puff smoke after coasting for a distance, and getting back on the gas. Fuel smoke is black, and causes a dark black, sooty deposit on the plugs. Oil fouling is black and greasy, and will form a hard deposit on the electrodes which foul it out.
Is the plug fouling uniform from one cylinder to another, or is one or more plugs getting fouled worse? Fuel fouling generally happens to all the plugs equally.
Edit: I just noticed you said the smoke is blue. If the compression tests low, and rises when oil is put into the cylinders, you need a ring job.
Jerry
