<VV> Oil fouled plugs

Fri May 25 17:38:50 EDT 2012

I rebuilt the engine on my 140, 67 cpe. engine 7,000 miles ago (October 2010) but, some months ago, started fouling plugs with oil.It happened on the left bank only. Must of the time it is the #3 and sometimes #1 or #5. It just happened on city driving or at first start on morning.While on the road, even on 500 miles trips, it runs fine without any trouble and minimal if no oil consumption.Suspecting on broken rings, today checked compression and it is 120 psi. on all cylinders.
What could be happened ? Broken oil rings only ? Worn valve stem seals ?
It looks weird to me but maybe some of you wizards could lead me on the right path.
Daniel Monasterio