Editing Leakdown tester homemade engine cylinder troubleshooting tool

You have an engine cylinder that is not as strong as the rest, and you have determined that it is getting spark, but something is wrong internally. A simple tool that can sometimes locate the fault is the following:     Take an old spark plug that fits the engine and remove all of the porcelan and the center electrode leaving just the outer shell.  Braze an air fitting into the shell.  Now, turn the engine over untill the piston in the suspect cylinder is at TDC on the compression stroke.  Insert the modified plug and apply 60-80 psi air to the cylinder.  You want enough air to follow through the proceedure but not enough to turn the engine.  With air pressure in the cylinder, listen: 1.  If you hear air bubbles it may be a leaking head gasket.  2. If you hear air in the exhaust, it may be a leaking exhaust valve.  3. Air coming up through the carb , it may be a leaking intake valve.  4. Ahissing from the oil fill tube or dipstick tube would indicate a piston or piston ring problem.  This simple homemade device can save a lot of time in diagnosing a problem.