I'll go along with most of what Dirtybird related. The engine mounting lug may not be a good enough ground. In testing the wire polarities of the new ignition on a T-52 I noted that grounding at the engine flange bolt did not provide a good ground. That's prolly what's happening with yours. Grounding to the case directly did provide a good ground, but there is no means of attachment. You could also drill and tap a small hole in the top fin of the cylinder to attach the screw. Go slow.