I am slightly embarrassed to admit that mine is like it too!!
Run it every few days~it starts by opening the garage door. Leave it for a week and it sulks like a *****.
Had many thoughts about this during; this year and last and replaced all the usual suspects.
Came to the conclusion it was down to hot soak and evaporation.
I seem to be on a personal crusade to find out what they have done with the pixie dust that used to be in petrol. It appears now, to be water with a few 'E' numbers in it.
Fine if you use it every day but rubbish if you leave a small quantity of it in an 'open' float bowl for a week.
Looking at the 'hot soak' and evaporative emissions test results from The US EPA and others; it appears that the vast majority of VOC's are lost within the first hour after shut down.
http://www.epa.gov/otaq/models/mobile6/m6evp007.pdf
That said; i left half a gallon of petrol in an open container in the garage, with dirty carbs in soak over a week during the summer and nearly all (approx 80 - 90%) disappeared.
I might do some flame testing of new and week old petrol to see what happens. If you dont hear from me again....................
Was also reading that 'Premium Unleaded' what is referred to by some as 'the good stuff' is possibly 10 - 15 % ethanol (this may apply to the US only though)
Ethanol is known to damage some older type carb components/seals and hoses made from natural materials.
It also has an affinity for water.
That was my final though after 'hotsoak' and 'evaporative losses';............. condensation in the carbs.
Ring any bells anyone?
AL
why. The bike starts without the need for choke the jets are all correct, fuel heights set, is this common 
