86 Sprint running poorly

86 Sprint running poorly when cold, much improved when warmed up but still down from a couple of months ago, have noticed considerable loss of mileage (down 15mpg) over the last couple of tanks of fuel and appears to be slowly geting worse. Cap, rotor, plugs, fuel filter have not changed situation. This one is carburated so I am questioning condition of carb, age, possible choke sticking, etc.... Anyone out there have any experience rebuilding a carb on one of these? Suggestions?