A poor return to the tank is the problem....simple. The actual cause will also be simple, but finding it won't be.
The return or "scavenge" system is separate from the pressure "feed" system. This confuses people because both go through the same oil pump, though the flow and return are completely apart.
If the tank is emptying the pump must be working, so the question is "Why does the oil not return? Plenty on the Forum already, so first easy check is the filter gauze in the sump (which needs to be the right size mesh, clean and undamaged to do its job) along with the little ball valve you can see, make sure the ball is not stuck and the housing is not gummed up with carbon. This is common on bikes that have been left standing. The ball should be free to lift a couple of mm and return when released. Give the valve a good spray of carb cleaner.
Well worth taking off the oil tank and giving it a good clean out, check the return pipe in the tank, any blockage will send more oil to the engine in a continuous loop via the rocker feed, and this results in an ever increasing volume of oil in the sump which the pump is never able to clear back to the tank. With a sump full of oil the return should be a good steady stream at anything above idle engine speed.
A fracture of the pick up pipe in the sump, or a poor seal of this pipe to the crankcase will give an air leak and cause this problem, as will a loose oil pump with a leaky gasket.
Try the easy stuff first. Then have a look at this link, you will see it's a common problem.
https://www.a7a10.net/forum/index.php?topic=14612.msg121892#msg121892 Swarfy
