First start, very hot....

The plugs don't look right, but don't seem to tie up with being isolated to a single carb as being the problem so I did not mention it.
On some engines, not maybe Rover, if you deviate from std cam timing you must check piston to valve clearances otherwise you can get piston / valve contact.
But I suspect this is not your problem.
Do you have a digital temp reader ? If so take readings of header temps near the head at idle and see what you get. It looks like some of your plugs are not doing much.
 
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