Excerpt from "How to Service and Repair Small Gas Engines" by Lindsley
The following quote is in part why I chose to replace all gaskets and seals in hopes of fixing the engine oil from coming back through the breather and contaminating the carburetor. This and what I learned from users experiences on multiple forums and the Briggs Tech support site.
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There are two reasons for maintaining a vacuum or reduced pressure in the crankcase of a four-stroke engine.
First, the combustion process introduces combustion gases, water, and sometimes raw gasoline into the crankcase. These contaminate the oil and can contribute to acid and corrosion. Since the oil normally runs above 212 deg F, those combustion by-products tend to remain in vapor form and can be drawn out of the crankcase by negative pressure through the breather system. The system works much like the positive-crankcase-ventilation or PCV system on your car. The wast products are normally discharded to the carburetor intake where they are taken in and burned or passed through the combustion chamber to the exhaust.
The second reason for crankcase vacuum is to prevent or reduce oil leaks. When the piston comes down, it compresses the crankcase vapors and without an escape passage the compression would force oil out around the crankshaft bearings. With negative pressure the oil is induced to stay in the crankcase. Some leakage of the seals will introduce fresh air past the crankcase seals. If the crankcase seals are worn to badly, the engine might also ingest dirt-most of which will be trapped at the lip of the crankcase seal, where it will act as an abrasive on the crankcase seal, and increase wear on the seals.
Conditions other than bad crankshaft seals may cause a low crankcase vacuum, and that's why the vacuum check is important. Excess blow-by from bad piston rings, cylinder and piston wear, leaking valves, a sticking or defective crankcase breather valve, or a poor seal on the dipstick cap are other common causes of low vacuum and oil leaks.
The following quote is in part why I chose to replace all gaskets and seals in hopes of fixing the engine oil from coming back through the breather and contaminating the carburetor. This and what I learned from users experiences on multiple forums and the Briggs Tech support site.
----------------
There are two reasons for maintaining a vacuum or reduced pressure in the crankcase of a four-stroke engine.
First, the combustion process introduces combustion gases, water, and sometimes raw gasoline into the crankcase. These contaminate the oil and can contribute to acid and corrosion. Since the oil normally runs above 212 deg F, those combustion by-products tend to remain in vapor form and can be drawn out of the crankcase by negative pressure through the breather system. The system works much like the positive-crankcase-ventilation or PCV system on your car. The wast products are normally discharded to the carburetor intake where they are taken in and burned or passed through the combustion chamber to the exhaust.
The second reason for crankcase vacuum is to prevent or reduce oil leaks. When the piston comes down, it compresses the crankcase vapors and without an escape passage the compression would force oil out around the crankshaft bearings. With negative pressure the oil is induced to stay in the crankcase. Some leakage of the seals will introduce fresh air past the crankcase seals. If the crankcase seals are worn to badly, the engine might also ingest dirt-most of which will be trapped at the lip of the crankcase seal, where it will act as an abrasive on the crankcase seal, and increase wear on the seals.
Conditions other than bad crankshaft seals may cause a low crankcase vacuum, and that's why the vacuum check is important. Excess blow-by from bad piston rings, cylinder and piston wear, leaking valves, a sticking or defective crankcase breather valve, or a poor seal on the dipstick cap are other common causes of low vacuum and oil leaks.
