Steve
My feeling is that this is caused by evaporation of the fuel in the carbs. If you switch off the fuel when putting the bike away over winter, the fuel in the carbs evaporates leaving small drops that crystallise and gum up the floats and needles. At first start up I always add some fresh fuel to the tank, shake the bike to mix the fuel, open the fuel tap to fill the carbs, leave for 15 mins, switch off the fuel, drain all carbs from the drain screws, switch the fuel on again and leave for another 15 mins before starting.
Not really sure why this works but the bike usually starts well on all four. Before I did this I found the bike would not run very well until I replaced all plugs. (Cleaning them would not solve the issue). I’ve now been using the same plugs for 3 years with no problem. Although I do plan to replace them this year before my trip to The Bikers Classic at SPA.