Is it a misfire on only 1 cylinder, or more? Never heard of a pink plug before, but I suspect you found your problem cylinder. I'll hazard a guess and say it's due to the incomplete burn of the propane. How did you set your mixture?

Check the usuals for the misfire, wires, cap, change the plug. Chuck out the vacuum delay, it's not needed. If it persists, then try richening the mixture a little.