Has it been wet sumping while it's been stood?...If it has there will be more oil in the crank case than the return pump can cope with when you start it initially and it will come out through the breather until the crank case is cleared..Ian
I've found there can be wide variations in how long it takes for a meaningful amount of oil to get to the crank case due to wet sumping...Some engines will do it over an extremely short period, just a couple of days, others can take weeks or even months...
How long is your bike standing for under normal circumstances?...Ian
