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BLOW-DOWN Test for Compression Leaks - BE-110
If your engine runs bad, misfires on one or two cylinders, has rough idle, low compression, or may be losing coolant, you can do a compression test. For a stock engine, compression pressure at cranking speed should be in the 120-145 psi range (as high as 175 for flat top pistons). Compression should be the same an all cylinders within about +/-10%.
If you find low compression on one or more cylinders you can do a blow-down test for diagnostics. You need a fitting that will screw into a spark plug port and to which you can attach an air line, and you will need a pressure source, so an air compressor would be nice. You can make the fitting from an old spark plug, break off the ceramic, punch out the material in the center hole, and weld or braze an air line connector fitting to the remaining steel part.
For each cylinder in turn, rotate the engine to top dead center on the compression stroke (both valves closed), apply about 20 psi air pressure, and listen. Hissing at the carburetor is a leaky intake valve. Hissing at the tail pipe is a leaky exhaust valve. Hissing at the oil filler port is leaky piston rings blowing through the crankcase.
The center exhaust port is shared with cylinders 2 & 3. Intake ports are likewise shared with two cylinders, 1 & 2 or 3 & 4. If all spark plugs are removed when doing the blow-down test, and there is a leaky valve, you may hear hissing through an adjacent spark plug port.
Also look for bubbles in the radiator or coolant in the crankcase or on top of a piston that would indicate a blown head gasket.