Bushlander splutters when I open the throttle - advice please

On the weekend I noticed my Bushlander wasn't pulling like it always does.  Going up a hill, there was a noticeable power loss and when I geared down and opened the throttle, the motor stumbled and then ran very rough at those higher revs.  It has continued with this problem for the past few days.  This afternoon, using petrol, I cleaned the foam air filter which was very dirty, sprayed BelRay filter oil on it and replaced it.  Then it refused to start.  I removed the filter again and started the bike and it still ran rough when revved a bit, even without the filter in.

Any suggestions on wjhat to look for.  I could probably find it if I had the time to start at the beginning and trace the problem.  I was hoping the fundis here could shorten the fault finding process.