Changed the radiator on my xr3i (efi) this morning was running fine!

Started her up now and it only just started and it was spluttering and had to keep reving it to keep it running. Turned it off now it won't start!

Took the plugs out and they are black! Gave them a clean and still no luck!!!

Noticed the 4th plug has very slightly bent, what could course this?

Help please