Why is gas leaking into the oil of my Briggs and Stratton 25HP model 445777 filling the crankcase with gas when the mower sits?

Answer 1

From looking at discussion threads, it seems that B&S engines are notious for this. The carb needle valve is dirty and fails to seat properly, thereby allowing fuel to leak into the cylinder and down into the crankcase. Have your local small engine shop clean and rebuild your carb and install an inline gas shutoff valve.

When the mower is idle, simply turn the valve to the off position to prevent any further fuel leaks.