Hi Edboy.
She's not buring oil while running, only on start up. As Muskrat says. wet sumping is the most likely cause and with the sump drain plug out she is dripping a little. GRRRRR.
The odd thing (although it might be a red herring) is I am sure its only started since I stuck my finger over the return pipe hole in the oil tank to force oil up to the top end. I could not think why this would make her smoke the next day when I had taken her on a run after doing the oil trick (anyexcess in the top end shold have cleared by then).
So, I am debating two causes for wet sumping starting now:
1. A bit of dirt has got wedged in the anti wet sumping valve seat ( maybe leftover from recent machining).
2. The oil is syphoning back from the spin on oil filter because I've upset it with the back pressure of sticking my finger over the retrun hole.
I think I'll go and disconnect the filter to eliminate the latter.
If it turns out to be acute wet sumping I'll stick thicker oil in her.
PS I don't think its excess crankcase pressure as the problem is only on start up.
But do I win the prize for the best excuse for excess crankcase pressure?
You see when I first put the engine together some years ago it would leak oil everywhere and smoke. I decided to check for a blocked breather and Bingo the breather passageway in the crankcase was blocked. A but of judicious poking with a bit of wire and the cause of the blockage popped out. It was a moth chrysalis! It must have got in there between me cleaning th cases and reassembling the engine.