Mysterious Oil Leak

[Brendan W]

If it's gear oil it will smell of cat pee. The main route for gear oil is through the vent hose although JIm R looks to have some sort of quantum tunneling going on.
If the tensioner and or pressure switch are leaking oil, it can end up almost any where back there. In fact a leaky tensioner can show at the front of the block on the ledge at the bottom of the chain tunnel.
The main culprit for engine oil in the starter cavity is the o ring seal at the sharp end of the starter motor.

 

[Brendan W]

Ah. No. I take the oil level screw out and fill from the fill hole on top and watch the oil come out of the hole where the oil level screw goes.

Thinking sideways here. When you open the filler is there an upper chamber that might also have the vent and could the pressure head of the upended oil bottle be enough to send some out the vent as you are filling ?

 

[Johno]

I have traced the leak to the oil pressure switch under the carbs.

The oil is running into the starter motor well and exiting via the drain hole.