If the valve is gummed up you can detect it by seeing if the push rod is out of position or even maybe bent. The best way to clean this is to remove the whole thing and clean the entire valve guide and stem.
If the push rod is bent or curved it is better to get it replaced by the company. Does your lawnmower still come under warranty? If yes, then this will not cost you anything at all. posted by
wonderman (201)
Always do an adjustment on a cold engine, remove the valve cover and gasket and the spark plug. Then position the piston at TDC. Use a feeler gauge to proper clearance specifications. Then adjust tightning mechanisms on the rocker arm until the drag is felt. Check or replace the valve cover.