clean the idle jet and gallery and while ya on do the main jet and emulsion tube -- u will need a alkaline based cleaner and make it undiluted -- so far since november i have had 4 bikes thru with that same prob
Hope this helps

Scuse me for poppin me 2 penneth in but ah BET its an air leak either from perished inlet rubbers or loose clips on the air box rubbers --just fettled 2 bikes with identicle probs -- however also the idle jet could be varnished up so its carbs off and clean the gallerys with a good solvent -- also if you are running a freeflow zorst and or a K&N air filter it would pay you to raise the carb needle one notch ------hope this helps
WoOk 

Chris i spent some of last year gallopping around Eire for a week 2 up with panniers on me 10 plate XJR --we did 2k miles in a week on pilot road 3's --BELTING rubber only down side is the DO slow the stearing a bit -- however wear rate was about average got 5k out of rear and I DONT ride easy -- even in the wet I was able to grind the auld hero blobs on the pegs    [no ah didnt fall orf lmao ]
Prior to this I have always ran metzler Z6 or * lately and they to are a CRACKING hoop with a better wear rate -
The only tyre i would NOT advise to anyone is the MAXXIS diamond -- Frelling disaster !!!!!!¬
As for mix matching tyre -- have done it off and on for 30+ yrs and aint EVER had any worrys --
TTFN
WoOk 

Wotcha Mush    well done on the test --ya NEARLY  geordie    am just outside Whitley Bay

Yayy thanks to all nutter ?? a bit --oracle --sometimes pmsl -- eddjit -- usually -- willing and able -- ALWAYS