It's not at all unusual for bikes that get parked for a long time don't run well upon return to service.
This is because the gasoline can turn gummy and start clogging idle fuel passages (jets).
More recent gasoline is even worse in this regard with the ethanol attracting water and causing corrosion.
Symptoms are consistent with fuel flow problems such as rusty particles in the carb bowls or clogging the fuel petcock outlet. Could be as simple as the gas tank vent is closed/clogged so the gas can't get out of the tank.
I'd remove the fuel line from the tank to the carb and drain some of the gas into a clear glass jar and look at what's in there. While draining look for strong consistent flow. If poor flow, undo the tank cap and try again. If opening the cap allows flow - it's the tank vent not working.
If it doesn't flow, should remove petcock and see what kind of rusty gunk is in the tank.
If tank checks out good, reinstall with fresh gas and apply to the carbs. Carbs often have drain screws so you can drain the bowls without removing the carbs (Both my Hondas, my Yamaha, and my Suzuki had these). Drain into the clear jar again. (should come running out some small diameter rubber fuel lines at the bottom of the bike (try to follow them from the carb to be sure they are there). Rusty flakes, water or other junk might be from the ends of the lines but could also be crap in the carb bowls. Sometimes just flushing the junk out is all that's needed.
I put 80% odds on the issue being old/bad fuel rather than broken parts. Carbs are elegant for their simplicity but they don't like bad fuel any more than any other fuel delivery system.
BTW- white smoke out the exhaust at cold temps (such as 15F) is just water vapor. Burning gasoline produces CO, CO2 and H2O. You can see the H2O.
Current: 1988 BMW R100GS (the 'numberplate' model)
Past: 1987 Yamaha XT350
1983 Honda CB900F
1980 Honda XL185S
1979 Suzuki GS425E