Several enquires about the use of a vacuum gauge. I think I covered this before but it is important. I mentioned a vacuum transducer hooked to a lab scope, this will display the vacuum in a wave form. It is very useful to locate a bad cylinder and eliminate the need to check compression in all the cylinders valuable time saver in today’s cramped engine compartments. It takes some trial and error to understand the wave but after using it you will find it an invaluable tool. Burn time as shown on a scope is also a useful tool in locating a bad cylinder. Do not short out any cylinders to see what they look like as this can cause a lot of problems.A vacuum gauge can also tell you the condition of the engine check a known good engine to obtain the correct vacuum and anything under this is usually an engine problem. Usually low compression or just a worn out engine. A bouncing vacuum gauge is also a telltale sign of a problem usually valves or one cylinder not performing correctly. Valve timing can also cause low vacuum. With older engines the carb and timing has to be correctly adjusted before checking for low vacuum. Cranking vacuum on a no start can also tell you if you have a valve timing problem among other problems anything above 3 In is usually Ok but here again check a known good engine for comparison. The altitude of your location is the reason for checking for a known good value.