I was thinking of changing my spark plugs to a CHAMPION RC12YCC instead of the N11YC which are currently fitted ( after looking at the spark plug wear guide ) suggesting a hotter plug . Does anyone have any thoughts on this.
What kind of driving do you do, Mark? I have a '72 Series 1 and it burns very cleanly on factory spec plugs, but it does longer runs, for the most part. I expect it might coke up a bit if it did predominantly shorter trips. FWIW.
Hi Paul
Yes predominately short trips for some years now ! The car had brand new carbs and aed about 3yrs ago as it had a slight misfire on idle and where it had been running badly with the previous carb set up some of the plugs were badly carbonated, although the new carbs are much better and the plugs are no were near as bad as they were I think the short journeys are not helping hence why I think a hotter plug might improve things ! And although it does idle much better it still has a slight misfire but the mechanic who done the work told me to put some miles on it and it should improve .
I won’t hurt to go up one grade if you expect your usage will remain as it is. What grade fuel do you use? Higher octane will burn hotter. My cars all get 98(RON) fuel.
I only use E5 which is 99 ,as you say it’s minimal plug difference so worth a try and there cheap as chips anyway.
I doubt if the misfire (in idle only?) is related to the plugs used, Mark. The ‘slight misfire’ in idle may just be the infamous xk ‘uneven idle’ - for which the is no known easy cure…
If so; hotter plugs are irrelevant, but, arguably, hotter plugs may be relevant for your driving - as Paul says. But it may be more relevant to work on the carbs…?
Frank
xj6 85 Sov Europe (UK/NZ)
Hi Frank
I totally agree with yours and Paul’s comments ! I actually had another series 1 which is very smooth on idle which I just sold I should have checked what plugs were in that 
I think I will change to the RN12YC and give it a long run just to see if it changes anything , I tend to use the plug wear guide as it is a good tool to get a good idea what’s going on and what to look for . I also think having a specialist check the carb set up would be wise ! Thing is it’s a bit like going to the doctors each one has their own version of what’s going on so who do you believe.
The one type of plug you shouldn’t use are multi electrode ones, they do not ignite properly in the style of combustion chamber on Jaguars
