Hmmm I have been fitting iridium spark plugs to my bikes for years and before iridium's it the Platinum tipped even in the old British bikes such as AJS18cs Norton Dominator and Matchless G12CSR where there is a marked difference over the standard plug.
Anyway I fitted a new Iridium plug to my newly acquired XTR. started her up no problems then I took her out for a spin.

I noticed that putting the engine under load at low revs the engine would "pink" at first I thought I was imagining it and sure enough the engine pinked again and the engine felt just not its usual happy self.
After a good spin I got back and replaced the plug from another bike of mine where I have had such problems only for the engine to continue to Pink, I double checked the gaps etc everything was set correct.
In the end I refitted the original standard plug and no more pinking and the engine was spot on, refitted the iridium spark plug only for the engine to pink again I tried the same plug in my XTZ and no trace of any pinking what so ever no matter how hard I tried.
Its as if by fitting the Iridium spark plug has some how advanced the spark a fraction which in turn causes the engine to "Pink"

So now I run the XTZ with a Iridium plug and the XTR with a standard plug