Some background info would be helpful too, for diagnosis. Is this machine new to you, or is this a fault that has developed while you've had it?
BTW, valve clearances for this engine are 0.004" or 0.10mm for both valves, with cold engine, and piston at TDC on compression stroke.
I was thinking that this engine may be prone to the same decompressor issues [similar decomp design] as the B&S Intek, which grumpy has confirmed.
