The carb is super lean at idle. This happened to me to the extent that the iron manifolds I was running the engine in with were GLOWING RED like they were under boost from a super charger.
This most likely is a Holley carb and the issue most likely is the idle air bleed on that side has an issue.
A Holley carb off of a production emmissions engine like the 85 Mustang 302 GT was set up 1) that even if you remove the plugs that cover the idle set screws, those screws only control the volume of fuel at idle not the a/f mixture.
2) That is preset into the carb and will give you theoretically an a/f ratio of 14.7:1 at idle and show something like 700F. Enough to make the exhausts glow. No matter how you adjust the set screws, you will not change the idle a/f ratio (with that carb)
You might interpret it as an overheating situation IF you are going by exhaust temps.
Measure the actual water temps in both heads and the block simultaneously to determine if they vary.
If you have and the one head is hot then ignore me. I just need a nap?