Minto. Years ago we all went by the book and observed the BSA recommendation to leave that ball valve well alone. In normal regular daily use the small amount of oil draining down overnight was easily cleared on start up. The problem comes with standing unused for longer periods. Oil accumulates in the engine crankcase, this is termed "wet sumping". The oil pump is really two pumps, side by side. The scavenge side has greater capacity , so returns oil to the tank to leave a "dry sump".
Terryg is right, the external aftermarket valves to prevent this wet sumping always have a degree of uncertainty about them.
So, if and when the time comes, it is worth checking that all is well. It's just a ball on a spring, but probably a bit gummed up.
The valve is in the oilway between the oil pump output and the timing bush. When the pump is taken off, you can feel the ball with a matchstick deep in the oilway. It should move easily and return to its seat. The spring has to hold the ball valve closed at rest, but open under oil pressure to allow oil flow to the bush and the rest of the engine. In a way it impedes flow, so some people remove it and put an external tap or valve in the feed to the pump, again fraught if you forget to turn on the tap or the external valve sticks, as outlined above.
So, your options are to just clean it with a good blast of carb cleaner, or dismantle and replace ball and spring. The only access is from inside the crankcase, carefully drill away the locking dimples and unscrew the grub screw, spring and ball will lift out. The grub screw is under full oil pressure, with the engine running, so ensure an oil tight seal on replacement, a good dose of Loctite is usually used, pessimists also reinstate the locking dimples.
This also explains the popularity of nice sump plates with a drain plug...very convenient and a lot less hassle.
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Swarfy.