Check your intake bolts - carb base bolts - bowl screws etc. They may have dried out/shrunk over the winter enough to create a low vac conditon /
oil from valley etc or cause your PV's to stay open at low speeds.
Should clarify the shrinkage thing a bit- what really happens in a cold climate is the alloy parts shrink-allowing the bolts that retain them to relax there torque values.
Jac Mac