A little more about the two ball valves.
As bergs says, the scavenge pipe non return valve should be in view, pointing downwards, when the sump plate is removed. It extends below the face of the crankcase, and as such is often bent upwards when cases are stored without the sump plate to protect the pipe and valve. If it's not there, there is something seriously amiss. A correctly operating valve consists of a cylindrical housing for a loose ball bearing which can be pushed up and move freely upwards to allow oil flow back to the scavenge side of the pump, yet not obstruct the flow when fully open, and drop down and seal when the engine is not running. Without the pipe, which runs almost horizontally into a drilling in the crankcase on a Swinging Arm engine, the scavenge will work to a degree, but with a higher sump oil level, and if I understand correctly this could be an explanation. The valve should be easy to identify and prominent with the sump plate off. Earlier Plunger engines are slightly different in pipe design, but the principle is the same.
The other valve, the so called " anti wet sumping valve" is a spring loaded ball in the oilway from the output port of the pump to the timing bush. This is only accessible for cleaning or replacement from inside the crankcase, and the blanking plug behind the spring and ball can only be reached by splitting the cases. The operation of the valve can be crudely checked with a matchstick down the oilway, pushing the ball and feeling it return under spring pressure. A loose or missing blanking plug means poor oil pressure (loose and leaky) or no oil pressure (missing) and the oil tank contents pumped directly into the sump.
To endorse Slymo's excellent and often overlooked observation, the pump to crankcase mating face has to be well oiltight, as it experiences both pressure and vacuum in very close proximity. Leakage means a tendency to wet sump when standing and low oil pressure at the bearings when running, and a lack of suction on the scavenge side as air rather than oil can be drawn up.
Pump gaskets are notoriously variable in accuracy of manufacture and quality of material. Whatever you have, try it in different positions to get the best match of oil holes and gasket holes.
From experience, oil return on start up, at tickover, is a strong continuous stream into the tank as the oilpump scavenges oil from the sump. This lessens to gulps of oil and air, still on tickover, as the scavenge successfully drains the sump. Under power, the return oil stream is strong and continuous.
Swarfy
