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Burnt oil and carbon can decrease the clearance in the piston ring groove to the point where the ring "sticks". Then the ring is no longer free to follow the contour of the cylinder wall and becomes "stuck" in the ring groove on the piston.Remember combustion gases in the cylinder act on the rings to promote sealing. This is where the carbon/burnt oil comes into play. Remember the bevel on the rings? Its there for a reason.Low Compression in a cylinder can be caused by a number of things:Worn Cylinders.Improper ring clearancesStuck rings.Burnt Rings (Hot combustion gases leaking by rings and piston)Valves improperly adjusted (too tight)Valves not seating (worn seats, Burnt valves, etc)To Name a few.On Aircraft engines we do a differencial compression check. This is different then a "Pump up check" in that it measures the pressure retained in the cylinder reference what is applied. For example: 80psi is applied to the cylinder and the cylinder holds 70psi. This gives an better indication as to the state of the cylinder. You can then listen at various points and tell where the problem is. Air escaping out the exhaust indicates an exhaust valve problem. Air escaping out the valve cover or engine crankcase breather indicates a piston or ring problem, Etc.Oil slipping into the combustion chamber is just an indication that fuel/air is also getting out (thus lost horsepower). Unfortunately, Oil bypassing the rings contaminates the fuel/air mix and when in the extreme, fouls out the spark plug.Keep in mind that oil burning is not always rings. Worn valves, valve guides and seals also have caused thier share of oil burning.Hope this helps.
Leak down is a really good test BUT do be sure you are exactly on TDC or that engine will turn over at a high speed when you put the air pressure on and the spanner you left on the crank will "fly" till it finds something to hit which by Sods 6th law will be either the most expensive item you possess OR the the most vunerable part of your body!!!
1 of many. GM makes an item called top engine cleaner, works like a charm. It is made to decarbon engines. The spray makes carb cleaner look like water. I do not know if it is sold to the public now, for years it was not .