Unter dem Youtube-Video welches Dirt in dem Beitrag von jondoe verlinkt hat, habe ich folgenden Kommentar gefunden:
The oil system in the xr's is very simple where a pump pushes oil against two flow orifices that provide backpressure, and due to their size the oil is distributed accordingly. One orifice is located in the cylinder head supplying oil to the camshaft, the other is located in the crankpin supplying oil to the bigend bearing.
Because there is no pressure relief valve in the system honda had to decide to size the flow orifices to avoid excessive backpressure at startup when the oil is cold and thick, but ensure enough backpressure remains as oil temp increases and viscosity drops, to guarantee flow to both parts of the engine.
So as I discovered there's another component that determines the pressure(or lack thereof) in the system and that is the fact that the orifice that controls flow to the crankshaft is actually located in the crankpin and is subject to centrifugal force of the rotating crankshaft which changes the flow characteristics of the orifice and, unduely increasing the flow, causes the pressure to drop to the point where oil flow to the camshaft is reduced or stopped altogether.
A simple way to prove that is to place an equivalent size orifice upstream of the crankshaft into the clutch cover, effectively taking the influence of the centrifugal force of the crankshaft out of the equation, and having done that .....voila ....there's pressure to spare, which means oil flow to the camshaft is virtually guaranteed.