Also, just because the header is hot does not mean it's lean. If you can lean out the cylinder and watch it change in a given situation with an EGT sensor it is more meaningful.
When I built my first headers a long time ago, I made some major screw ups like pairing the wrong cylinders. I built a program on the computer to look at EGT's and it was very confusing.
When I ran the engine on a test stand I got some headers running 2x the temps at others:
At first I thought the gauges were wrong. I swapped sensors and the sensors were accurate.
Then I thought the hot cylinders were the lean cylinders. They were not. After looking at the inside of the headers I discovered that the hot temperatures were way rich:
Conventional wisdom says richer - cooler but there was a fault to this belief here.
In THIS situation, I really screwed up the pairing of the headers. The lean cylinders had back pressure. The rich cylinders had no back pressure!
The rich cylinders had pulled the air out of the cylinders before the fuel was done burning so the fuel was still burning in the headers causing a much larger temperature in the header. This did not mean the cylinder was hotter!
I found this all interesting. It points to the fact that it is all not as simple as it seams. It also points to the fact that specific numbers for heat lean or rich are irrelevant unless you can only change mixture in an exact scenario without changing anything else.