Ok this is going to be hard to explain but when I took my second hand pair to the local guy who does all the servicing etc he said to screw them down to tight then start them, slowly release the tension nut until you hear the motor pick up. I have found if the tension id the least bit wrong mine won't clip anyway. Good Luck!
I've had Wolseley Swifts for years & nicnag is about right with the tensioning. Also keep them well oiled & give them a break when they get hot, let them cool down naturally, don't use anything to try & quicken the process.
its the little knob on the top, what you do to get it right is wind it quite tight and turn it slowly until you hear a drop in pitch in the sound then they are right, to tight and you will burn them out