Hi Ken
If the GS911 does not pick up any faults it means that the problem is mechanical.
From your description, it certainly sounds mechanical.
Possibilities are:
Collapsed oil Filter (Remove and check/replace)
Blocked Valves (See 1 below)
Stripped Oil Pump Gear (See 2 below)
1: Once you have removed the oil filter, remove and check the 2 valves behind the filter.
Note that an impact driver is required to remove them without damaging them. If all appears clean, replace them or clean them if necessary.
2: The 650 motor has 2 oil pumps, the pressure pump to pump oil around the motor, the other to scavenge the oil from the bottom of the motor (sump) and transfer it to the oil tank. In the photo below, the red arrow is the pressure pump, the green arrow the scavenger pump.
Both these pumps are driven by plastic gears. It is possible that the pressure pump gear has stripped, or is at least missing a few teeth where the drive gear is now engaging.
You can check the operation of the pump. If you turn the upper gear (Can't remember in which direction) the oil should flow out of the small hole directly between the two valves in the top photo.
These pumps can be removed from the left hand side of the motor. Both however, are located behind the clutch basket, which needs to be removed first.
Clutch cover removed with basket in place.
Note too that before the clutch cover can be removed, the oil return pipe needs to be removed. While the cover is off, it is worth checking the water pump gears, which are located a little forward of the oil pump gears. These are also plastic and tend to loose their teeth in old age as well.
Water pump drive gear (Blue Arrow) The driven gear will be on the impeller shaft located in the cover.
