Colder outside temperatures.
Did you ride this bike last year at this time? Weak spark can require more fuel. Dirty air filter. Anything that messes what happens with the fuel air mix, and what makes it go bang, which includes compression, and how the bang exits.
My 79 Yamaha 1100 is one of the two winter bikes. It gets about 40 mp large gallon. It drops well into the 30 mpg range in the winter, and especially when hauling the side car. It jumped from about 35 to 40 mpg when I replaced a bad connection on the ignition side that send power to the coils.
As a side note, last week it only fired on three cylinders. Replaced the plug on the one that was running cool and fixed the problem.
I live in Florida, temp isn’t dropping that much to make a difference. Lived in Alabama last winter where it actually got cold, not fun for me. Fuel consumption didn’t change. Something has gone wrong with scoot. Thanx for your input though
Before you tear into the carb, you might consider running a couble cans of Seafoam or Startron thru it. Seafoam for carbs and Startron for fuel injected. Seafoam just might help. But if you've rebuild carbs before with success then you might be okay. I just read over and over where people tear into their carb and end up with a bike that won't even run after. So I suggest something that might clean up the jet mist for a better bang as Krusty puts it.
A filthy air cleaner alone can REALLY affect fuel mileage:surprise:
Winter blends of gas, especially Ethanol laced gas has much less thermal energy and mileage drops.:surprise:
If a plug reading looks anything other than a light tan color then the engine is running rich so it may be the carb needs work. If the 40mm Mikuni or Keihin CV rubber slide diapraghm (sic?) has a rip. hole or tear in it, the manifold pressure won't allow it to move up and down correctly and that can make the mixture rich or lean.
If the cold start choke system is stuck it will cause a rich situation also.