Mazda 2 Capella 2005 1.6 Duratec engine. Has headgasket issue but was idling fine. In the course of tracking down the headgasket leak I was disabling cylinder 1 spark plug by disconnecting the wire and using a spare spark plug on it well grounded so as not to stress the coil pack. Car did not like running on three cylinders, then gauges started behaving oddly like the rev counter going full swing and back. Connect back up the cylinder 1 plug and the car won't idle; as soon as i take my foot off the throttle it judders to a stop. I have tried disconnecting the battery negative etc. to reset things but no change. Also there now seems a bit of a lag in response to throttle movements. Definitely won't idle.
Any ideas what's happened and how I can restore idling?