Ok, bear with me for a minute.
If the gasket in the dipstick where you check the oil is bad and not sealing...it will vent the "Negative crankcase compression" (some people inappreoprately call this vaccum)
and this interrupts the normal process of the compression and decompression of the inside of the engie. This is what the valve breather does..regulates that compression and decompression.. This will cause oil to be forced into places that it does not belong....one of which is the air filter.
When a carburetor is leaking fuel because of a bad float valve or bad float. The carburetor overflows. The gasoline, then goes into the various different passageways that are usually reserved for oil to pass through. It also will pass through the breather tube and enter the inside of the engine crankcase....contamination the oil with gas. This gas , thins the oil and then that oil can go into places that it normally cannot get into because the oil is normally too thick.
Good compression has nothing to do with keeping the gas from getting into the oil or the cylinder or the air filter for that matter.
in addition, you might replace the actual OIL breather. This is usually located in fromt of the valves. If this is not working properly, then you can also have the problems you are having.
I hope this helps clearify my answers.
I know it may be a little confusing.
As engines sit or get older, fuel that is left in the carburetor can turn to gum and varnish and cause this and other problems.
Also, any gasoline that was left in a gas can for a period of more than 30 days must be discarded because it also has begun to turn to varnish.
More than 70% of all of our repairs in our lawn mower business are due to this same issue. You most likely have dirt, gum, varnish...etc in your carburetor plugging up the small passageways and jets in the carb. It Must be removed from the engine, cleaned very well, blown out with compressed air and reassembled using a NEW carburetor rebuild kit. ALWAYS clean the fuel tank and replace the fuel line when doing this repair or you may have to do it all over again. The inside of the fuel line disintegrates over time and these small pieces of rubber will plug up the carb too. Dirt and water from a dirty fuel tank will also plug up the carb. Find the Model, type and serial or code numbers off of the engine and take them to your local dealer to get the carb kit.
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