Make sure you are really picking up #1 lead with the timing light, and that the distributor body is not too loose while you do adjustments, and that the balancer hasn't slipped. I set by ear, turn back until the engine cranks like crazy but runs very weakly, adjust advanced (with distributor vacuum port plugged) until engine runs smoothly and let it warm, then adjust again until the first hint of warm cranking resistance shows up. Back off a touch from there if necessary to eliminate pinging. Then take it for a test drive after snugging the distributor clamp. If all is happy, mark your balancer for later reference.