Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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The oil is not overfilled?
Since it seems to only happen once the engine gets warm or hot then I would say that you most likely have a head gasket that it blown. This is allowing come of the compression from the cylinder to get into the crankcase and create an internal pressure inside the crankcase which will cause the engine to begin to blow oil back out through the breather assembly.