I took this motor for a good run yesterday, and to my surprise after replacing all oil hoses, clamps, etc, again there were big air bubbles in the oil line from the pump, visible in the transparent fuel intake Y-joint. This time I undid the bleed screw on the pump, and ran the motor (on 50:1) with the tank held out of the way to see, and it was pretty clear that air was pumping from the crank case back though the oil pump, with quite a lot bubbling out the bleeder, only possible cause for this was the oil seal on the drive shaft in the pump. For DT9.9C, this is a 16mm (OD) , 9.5mm (shaft diameter), 4mm deep seal. It's not a standard size - nor a part available from Suzuki, but I got one through a car auto-transmission part supplier for a couple of dollars. The old seal popped out easily, levered with a small knife. This seems to have fixed the problem.
