How can you tell definitely that the center crank seal has gone? What are the classic symptoms?

My MC21 is hard to start and seems very lean at idle. It has stock slow jets, each carb has a K&N filter so it's basically running open carbs. It has the HRC black box and stock pipes.

Once it's warm it will idle steadily at about 1250 RPM but it takes some revs to get moving. Once going, it runs well in the mid-range and top end.

The bike sat idle for a coupld of years but once I cleaned the carbs I got it running right away.

Thanks in advance for the advice.

Skunkbait