"Cylinder leakdown" is probably one of the most useful, yet misunderstood tests for engine diagnosis, and doing it right is extremely simple. All you need is an air compressor capable of developing 100 PSI, a pressure regulator with a gauge, and a way to apply the air pressure to the cylinder. The same quick connect adapter used to hold the valves closed while changing valve seals also works for a leakdown test. With a manual transmission vehicle, just put the transmission in high gear, chock the wheels front and rear, and pressurize the cylinder with both valves closed. The simplest way to do that is to loosen both rocker arm adjusters so both valves are closed with the piston close to the top of its travel on the compression stroke- - - - -not necessarily right at top dead center. Adjust the air pressure to 100 PSI with no air flowing, plug the air line onto the cylinder, and look at the pressure gauge. For instance, an 85 PSI reading would be an 15% leak rate. A recently rebuilt engine should test in the mid to high 90% range. Below 80% is a disaster. Listen for intake valve leaks at the carburetor, exhaust valve leaks at the tailpipe, and piston ring leaks at the oil fill cap. It's not "rocket surgery"!
Jerry
