Use a multimeter and check the resistance of each lead, mine were all under 7k ohms when i last checked them. The problem is the connectors used on the 909 leads (the ends that go on the dizzy), they fur up over time, to the point where the mutimeter can't get a reading!

I had an old set of Ford(beru) leads, one of which was broken, so i took all the connectors off both sets of leads and put the ford connectors on the 909 leads, job done!