To clean out the thread in the casting go to the hardware store and get a 3/4" x 6" bolt (or correct metric size if differant than USA) and run it in and out with some oil. You can take a 4" hand grinder with a metal disc and cut a couple of grooves in the bolt and it acts as a tap. When trying to get a good grip on those screws I have heated up a large screwdriver and sunk it in perpendicular to the factory slot. If you get it so the screw goes in well and you run out of adjustment then maybe you do need a new spring. Make sure you lube the cams with waterproof grease or spray regular with silicon. Good Luck.