Confirmation that the decompressor on my Victa Lawnmower had failed and this was the reason that It wouldn't start. Comparison with a good valve from a 2nd hand lawnmower confirms the correct operation of these valves, which are operated by the inlet manifold vacuum. My solution to the problem was to simply dispense with the valve completely and replace it with an old spark plug - just as a convenient way of blocking up the decompressor hole. The engine operates fine without the valve. Maybe the starting torque is slightly higher on the pull-cord but this is hardly noticeable.
Remember that you'll need to seal up the vacuum take-off that is normally connected to the decompressor valve diaphragm via the plastic tube. I put a blob of silicone sealant on mine and let it set overnight . Simple, effective and easy to remove later if necessary.
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