Mick, you can inspect the positive crankcase ventilation valve if you want to: it is built into the cover over the valve chest, on the side of the crankcase. You can find it by back-tracking the breather hose that goes into the carburetor. Here is the instruction on it from the Briggs manual:
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If the valve is clear, and the valve chest is clean, I think you can be fairly sure it is a blowby issue.