G'day Could it be that the engine is a bit "gunked up" with old oil? And as a result, the carby is partly clogged and the ring is stuck in the piston and so not sealing properly? If the engine has been sitting, perhaps what it needs is a good clean out of the carby and possibly the ring needs to be freed up? Bearings too could be clogged up. If it were mine, I think I would start with a good cleanup and even possibly a new ring even though the "old" one is not all that old and possibly even some new bearings. Castor is great stuff if you use an engine often but not so good if it gets to dry out in an engine in storage. I have had little blockages in a carby from castor after only a few weeks storage.