'48 8n front mount, 12 volt, 12 volt coil, original ballast resistor. Has never started good but now won't go without a lot of trying. Good spark (new Echlin points and condenser), new solid core wires. When it runs, it won't rev or take any load without backfiring and missing. No power! Had to replace the distributer a couple years ago because original was cracked. Got it started today by taking out a sparkplug and grounding it to engine. Would run on 3 cylinders. I can hold sparkplug n frame while it cranks but get lifted when it starts. What's going on?