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How To Find Vacuum Leaks



Vacuum leaks are difficult to detect and can cause a numerous symptoms. Our Moss Motors Tech department will give you some tips that will make finding a vacuum leak quick and easy.

When air enters your engine it should go through the carburetor. That assures we have the proper air/fuel ratio and the engine will run as it should. Sometimes you can develop a vacuum leak. When that happens, air enters the engine through an unplanned portal and without the gasoline it should have. The motor will run lean. The idle will be rough.

Traditionally, mechanics used a spray can or starter fluid or carb cleaner and sprayed the areas where a leak might occur. The idea was that if there was a vacuum leak, the engine would pull the spray in through the leak and the motor would respond by changing its idle speed. For various reasons, many of these products have been reformulated to be less combustible. That makes them less effective for this test. Instead, you can use a propane torch. You turn on a soft spray of propane without igniting it. As you pass the end of the spray head near the places where a leak might occur the motor will pull in some propane and change its idle speed. That’s how you will know you’ve found the leak. You want to test the gaskets between the carbs and the manifold, the gasket between the manifold and the head, the bushings at the ends of the throttle plates of the carbs, and all vacuum hoses.