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

I ran my XTR with the iridium plug CR7EIX for a tank full and the fuel consumption increased compared to the standard plug and the engine just did not feel right compared to the original plug I swapped the iridium plug from my xtz and tried it in the XTR no change the engine just did not feel right compared to using the standard plug that was fitted, when checking the plugs I realised that the original plug fitted in the xtr was a CR8E not a CR7E as recommended in the owners manual.
I refitted the 8 back into the XTR and the engine is running much better now got another CR8E plug and replaced the CR7EIX that was fitted into my Tenere and B****r me now she is running better than ever.
I have never come across this where a plug one grade hotter or colder made such a difference, the CR8E is a slightly cooler running plug
I am recording my mileage on both bikes on the Fuelly web site
I am going to stick with the 8s and I wonder has anybody else come across the above

Forgive me, but I am wondering if it's the iridium spark plug gap at fault. The iridiums come with wide gaps, I think.

I have an iridium in my TTR250 and it runs really well with it, but I did have to close the gap a fair bit.

I've been wondering whether to try one in the Tenere on the strength of the TTR's result. But now you have me wondering!

Regarding using an 8, I will do that this summer as I think the 660 is on the edge with the 7. It doesn't do much town work anyway. Come to think of it, it doesn't do much work anyway! I hadn't ridden it for 3 months until last week. Been away...

...and typically i've just orderd an iridium CR7EIX


Oh well, will see how it goes now, very strange though for you to having the problems you've had

I doubled checked the plug gap and it was spot on.
Both bikes start and run ok with the 7s in them but with the 8s they seem to run better noticeably so much so that I will be sticking to 8s from now and the 7s that I have I will keep in the tool kit as get me home if in the unlikely event the plug in the engine packs up.
I will be updating the fuel consumption on the Fuelly web site

Hmmm. I think I'll copy you and try an 8. I'm all for giving stuff a try!