Editing Finding vacuum leaks

Here's a method for diagnosing a bad running engine that may have a vacuum leak:

#Get a soldering Propane tank and valve attachment.
#Unscrew the end of it so 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.
#Turn on the propane and run the hose all aroung the vacuum lines and the bottom of the carb.
#If the engine smooths out or revs up a bit, you have found your leak.

PLEASE do this in a well ventilated area due to propane gas and exhaust fumes!!


Another 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.

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 motor. 

Here's how I set up the intake manifold/cylinder head interface to prevent vacuum leaks from the crankcase to the head ports....
Measure the thickness of a new intake manifold gasket. Get flat washers or shims that will measure that thickness. With the manifold off and the mating surface on the cylinder heads de-greased, put a dab of RTV on them and stick them on each corner bolt hole on the cylinder head. Let the RTV set up. Stuff paper towels into the ports to keep debris out. Make up 16 pea-sized balls of modeling clay. De-grease the intake manifold at the ports. Place the balls of clay on the top and bottom of each port, squishing them down well so they will stay in place. You want them to be thicker than the shims/washers that are RTV'd to the heads. With your fingers, coat a little oil on the heads where the clay will meet the head to keep it from sticking to the head. Now carefully place the manifold into place on the heads and use bolts on the four corners to just snug the manifold down until you feel resistance against the shims/washers. Remove the manifold carefully and mesure the thickness of the clay at all 16 positions with the depth function end of your 6" dial caliper. You'll know pretty quickly if the manifold/head interface is square. Record the measurements on the manifold with a permanent marker like a Sharpie. The widest measurement will be the standard to which you will want your machinist to cut the other positions on the manifold to make it square with the heads, thusly sealing the motor up.
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