symon, as far as I can tell from the internet, all of the larger B&S OHV engines (Intek and Vanguard) have the same decompressor, and it operates in much the same way. When people have the problem you are having it seems to be nearly always incorrect tappet clearance (in the few remaining cases it is a defective or failed camshaft or decompressor). I suggest you look at this video, (which happens to show a single cylinder engine) then follow the procedure it shows:
