Old fuel sitting will leave varnish like residue. Fresh modern high detegent fuel may clean some but not likely all of this varnish. Winter fuel has high RVP and is problematic in hot weather.* Summer fuel isn't as bad but may have cold weather start difficulty. Reformulated fuel has components that may or may not deteriorate some seals and gaskets. Plan on cleaning and replacing seals and gaskets of the carb at some point sooner than later.
