I am with John - if you have eliminated the easy stuff (Indication issue), you need to look at Fuel, spark and timing.
Some great articles to read were written a few years ago by John Deakin on Avweb, and you can still find them:
https://www.avweb.com/news/pelican/182084-1.html
https://www.avweb.com/news/pelican/182179-1.html
These are just 2 links to get you started - but essential reading to bring focus to the relationship that fuel air mixture and spark have to do with you CHT.
Jeff