Alwyn,
I notice the same on my Mark 3 Sports. I attribute it to the loss of drag on the gears as the oil warms up. I run straight 50W, so the problem may even be worse for you running 30-50W. It seems to be better when I was able to get straight 60W. I don't think it is a case of significant clutch drag, as it does not go 'crunch' when engaging first from a standstill. But it does seem to need something to slow all that rotating inertia down a little when changing up. (Remember the gears are spinning faster than in a normal bike with a primary chain reduction.) I just shift it like a Burman heavyweight 'box, a slow and deliberate change, sort of 'roll' it into the next higher gear.
The plunger for the cam plate should have a air relief to prevent that from making the change stiff. But that would not gradually get worse. I wonder if something has moved in the box and has increased friction. But then I would expect you to notice the engine labouring harder. With only 17hp to play with, you notice when it is loosing 1/2 a pony!
Keep an eye on it but you must make the rally!
-Doug
