Oil & filter change about 500 miles ago, also new air filter same time new plug 2 weeks ago. Valve clearance? (not fully up on engines, sorry) How is that done? You'd need a manual for the bike if you haven't already got one.....calling Spanner...... I'm guessing on that bike it will be set with tappet adjustment. Basically it's adjusting the gap between the cam and the valve stems. If the gap is not set correctly you'll either have valves not closing fully (loss of compression) or not opening fully (not enough fuel/air mixture getting into the combustion chamber or exhaust gases not being expelled fully) ........either can cause the symptoms you are experiencing. ......