I've never changed the plugs because they are a pain to get to and you have to remove the intake to get at them.
this should be in "The Excuse Book" lol.
Whenever something is beyond my ability, I take to someone that can. It may not be your plugs, but I'll bet you'd get better mileage with some new ones. IF it were mine, a new set of the recommended plugs gapped correctly, new wires (not the best, but the better set than stock, new Dcap with brass contacts if possible, rotor button; and get that running, From what you've said, my guess is it most likely needs that, regardless.
There may be, should be a solenoid that keeps the throttle open some to prevent the engine from stalling. But again, if it were mine, I would attack the ignition system first, based on what you've said. Simply because a poor ignition would only exaggerate a problem with that solenoid.