Nah Rosco,
There is an O-ring that goes behind the main gear on the crankshaft sealing off gases sneaking between the crank and the gear.
The bike performed well at full tilt and low down - still crap midrange and now (suddenly) a prick to start. Once started all is good (unless you leave it too long).
414 and Billet YZ have suggested a few things to look at - just a process of elimination really
. I know it isn't crank seals now.