Ok so status update.
The DT starts well and there are no nasty noises coming from the motor. Up on the mainstand it revs as normal.
So I took out out for a short blast. At low speed, negotiating the narrow streets and speed humps around my house, it felt horrible - particularly on part throttle. It squeaks and vibrates and forward drive feels terrible - but crucially the engine feels alright.
Out on the open road, the motor feels fine, all the way up to (briefly) top speed*. In fact the whole bike feels fine. temp guage is good
So it feels like something has broken/snapped/come loose. Like an engine mounting or something in the final transmission perhaps.
A quick visual with a torch didn't show anything obvious. I was going to take the plastic off anyway for a bit of a refurb, so hopefully I'll find out then what is wrong with it. That can wait until its a bit warmer though. In the meantime I have a back up plan in the shape of a second hand Toyota :-)
Any thoughts in the meantime (Karl/Stig) are most welcome.
*Yes, I know it's not a good idea to take it up to top speed if there is something loose.....but it's done now.
