David,
The problem is that would be repairmen think squirting oil or CRC etc on that solenoid shaft is going to help, and it only tends to make matters worse. You're more than capable of taking that coil assembly apart, but you may not have to in this instance. First things first, make sure the clapper moves without resistance. If there is any interference, resolve that first. Next, get a can of no residue contact cleaner and flood the shaft and solenoid hole. Let it flash off and use a Q-Tip to make sure the shaft is clean. Now use a 164-16 conical spring and wrap it around the shaft - yes it takes finesse. You may have to trim a turn or two off the spring - just check its compressed length.
I'd give you a recommendation on the contact cleaner, but I'm home at the moment doing my at-home kidney dialysis.
