Small Engines (Lawn Mowers, etc.): overhead valves b/s 14.5 hp, hex screws, overhead valves

Question
oil is being pumped into the carb. valves were adjusted. after 15 min. of use, problem returned. found the adjustment nuts broken in half on both valves of rocker. also damage to the hex screws of the adjusting nuts. adjustments were made by a professional before breakage. any advice why breakage occured and any remedies or checks to be made before repair parts to be replaced (adjustment nuts for the valve rockers)?  thank you very much.

Answer
I can't think of any reason this could happen unless there was mechanical damage elsewhere or the engine hydraulically locked (due to excessively high oil level).  I recommend you replace the necessary pieces (and inspect everything carefully to determine the extent of the damage).  While adjusting the valves and after they are adjusted, you should rotate the engine over by hand to ensure everything works smoothly and re-check the valve lash a couple of times.  Then make sure the oil level in the crankcase is correct.  If you suspect a mechanical stoppage (carbon build up in cylinder), it may be wise to remove the head and inspect / clean before you proceed.  If there is excessive carbon build up and the valves have hit anything, they need to be checked for straightness and replaced as required.  Hope this helps.