Cooler plugs?

With 3 ranges cold I can't imagine it runs well at all. It is probably carboned up from un-burned fuel. I would change the plugs to the 14yc's, make sure it runs on all 6 and drizzle some sea foam down the carb with a warm engine at 1/3 throttle or so. My cold plug issue made the 273 run rich at idle enough that you couldn't stand to be an where close to the tail pipe. It would burn your eyes.