Finding vacuum leaks
If you suspect a vacuum leak you may be able to verify your suspicion by performing a simple check. With the air cleaner removed (carbureted engine) and the engine at idle speed, place a wadded-up rag over the carburetor throat to choke off the air supply. If the engine rpm speeds up, chances are you have a vacuum leak.
There are several methods for locating vacuum leaks. Occasionally, you may experience a vacuum leak into the intake ports from the crankcase of the motor due to the intake manifold/cylinder head interface not being machined parallel. No amount of propane or carburetor cleaner will find such a leak. The best way is to insure the elimination of such a problem while building the engine. See Manifold/Head vacuum leak, internal.
- Get a soldering propane tank and valve attachment.
- Unscrew the end of it so that you have a threaded pipe without the nozzle.
- Attach a 2-foot piece of hose to the end.
- Start the engine and let it warm up a bit.. since vacuum leaks on ANY internal combustion engine will cause a 'loping' idle - the engine will rev up and down a few hundred rpm's. While it's doing this, the propane will be drawn into the engine, but the momentary increase will be hard to detect, with the idle already loping. Either block the linkage, or block the pedal, to get a STEADY idle. Then proceed to the next step.
- Turn on the propane and run the hose all around the vacuum lines and the bottom of the carb.
- If the engine smooths out or revs up a bit, you have found your leak.
Hold bottle UPRIGHT during testing as liquid propane may run out of an inverted bottle.
PLEASE do this in a well ventilated area due to propane gas and exhaust fumes!
Carburetor cleaner method
- With the car running, using a can of carburetor cleaner spray down the hoses and vacuum connections.
- Notice a change in the running of the engine and you might have found your leak.
You may also use engine starting fluid instead, as it does not leave residue.
Vacuum gauge method (internal)
If the above methods fail to locate an external leak, the following procedure may be used to determine an internal leak.
- Disconnect/remove the primary wire between the coil and distributor.
- Adjust the idle screw so the throttle plate in the carburetor is completely closed.
- Remove PCV, breathers, and all vacuum ports on carburetor/manifold and seal openings with plugs/tape.
- Connect vacuum gauge to a manifold vacuum source.
- Have assistant crank engine while you read vacuum.
If gauge reading is 0-2", chances are you have an internal leak between the manifold/head.