There are several methods to detect vacuum leaks of unmetered air into the fuel delivery system
One method is to use soapy water in a spray gun.
The real issue here is how are you going to detect a vacuum leak and how do you know you've found it.
What is a vacuum leak in this context related to a fuel delivery system?
Unmetered air in a fuel system is considered to be between the butterfly in a throttle body or a carburetor and the cylinder head. The components involved are:
Throttle body or carburetor
Gasket between the throttle body and carburetor and the intake manifold
Intake manifold gasket between the intake manifold and the cylinder head
Vacuum lines to operate various components that require vacuum such as the brake booster
This is a pretty big list and finding an efficient method to test all of the joining points plus the accessories that may require a vacuum can be daunting. Efficiency in troubleshooting this issue is important.
Symptoms of a Vacuum Leak
Issues with Air/Vacuum Leaks
Unmetered air in a fuel system adds oxygen to the mix. This changes the air fuel ratio the components are trying to control after the control mechanism, the throttle body or carburetor. The additional oxygen changes the air fuel ratio making the car run poorly. The damage that unmetered air can do to the overall system includes:
Increased combustion temperature due to more oxygen
Increased temperature of an exhaust valve due to more oxygen
Burned valves due to overheating
In rare cases, points in the combustion chamber and piston that become molten during high temperatures leading to complete failure of the engine
Soapy Water Test
The Soapy Water Test has you putting water in a spray gun with a bit of soap on areas of the intake system that are suspected air leak points. You may need to occasionally rev the motor a bit to keep it idling or increase the idle RPM while troubleshooting to prevent stalling if that is one of the problems you are experiencing.
As you spray the soapy water on suspect points the idle with flatten out and become normal as the soapy water blocks the air leak or crack temporarily. Continued spraying on the suspect points to temporarily block the unmetered air will ultimately reveal the culprit of the air leak.
Once the failing component or vacuum leak is discovered you would fix it accordingly and then repeat the tests to ensure success.
Tools Necessary to Complete Troubleshooting