Editing Finding vacuum leaks


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 the wiki article: Manifold/Head vacuum leak, internal.
Propane method

   1. Get a soldering propane tank and valve attachment.
   2. Unscrew the end of it so you have a threaded pipe without the nozzle.
   3. Attach a 2 foot piece of hose to the end.
   4. Start the engine and let it warm up a bit.
   5. Turn on the propane and run the hose all around the vacuum lines and the bottom of the carb.
   6. 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!!

Fixing a leak. My old neighbor built circle track cars and after the ran the engine he would shut it off...tape off every opening with duct tape then put a hose over the dip stick and hook up to a vacuum source. He would then get his stethascope and listen in the area where the propane or carb fluid indicated a leak. he used Rtv and calked into the area until the stethascope whistle stopped. too much vacuum will pull too much calk into the hole.
Carburetor cleaner method

   1. With the car running, using a can of carburetor cleaner spray down the hoses and vacuum connections.
   2. Notice a change in the running of the engine and you might have found your leak.