This may be a completely off track train of thought...
It's probably too silly a question to even ask, but, your car doesn't happen to have one of the carb or injector cooling fans does it?
Just happened to think that depending on how things were aimed and or shrouded, it might be possible that there might be some drafts of cooler air wafting onto some sections of the header.
No idea if it could be enough to cause a reading that is *that* different even if a fan was blowing on a pipe or two, or even if you car has the fan or a shroud directing airflow/etc.
Like I said, my thought is probably totally irrelevant, it just happened to occur to me that sometimes what seems like it is the most likely, logical from one way of looking at things, can turn out to be completely the wrong direction because sometimes causes that are completely unrelated can end up causing the same the same, or very similar, surface symptoms.
Like, just for the sake of an example...
Once a very long time ago a guy I was worked with was having some problems with his truck. he'd had his carburetor rebuilt at *least* twice replaced fuel filter and pump, had several friends and some mechanics go over things trying to solve his "fuel starvation problem".
It was some sort of great unsolvable mystery or something as to why this truck just kept dying out in various situations.
Anyway, I don't remember what all he described as far as symptoms go and such (this was maybe 14 years ago, so it's been a while), suffice to say, fuel related problems could have explained most of what he told me about it.
So I'm like, you checked the vacuum advance, right?
And he just stares at me like I've said something that was incomprehensible or something. So we go out to his truck and popped the hood and I did exactly what I'd learned the same way I'd learned it by watching my dad and grandpa when I was a kid.
I pulled the vacuum line off the carburetor and sucked on it, with the intent of building a bit of suction then putting my tongue over the hole to see if it would hold the vacuum I'd created.
Well, it didn't quite turn out that way because what happened was that I sucked, and suddenly found myself with a mouth-full of dust and grit and stuff because the advance diaphragm wasn't *weak*, it was like, *nonexistent* I'd just sucked straight air and whatever dirt and junk had accumulated on the floor of his distributor.
Anyway, I was like, Ahem, yeah. you need to replace your vacuum advance.
lol.
he still looked at me like I was an alien or something so I attempted to explain how/why having no ignition advance could cause a truck to start dying out when it got up around 45 mph, because the timing couldn't automatically adjust to keep up at higher rpms.
Anyway, he ended up sending in his carburetor rebuilt again instead. lol
After all, all his other friends and mechanics already *knew* it had to be a fuel starvation problem.
Anyway, that story isn't directly related to this one, and I'm barely even a mechanic anymore, I'm just telling the story as an example of how sometimes mechanics can get so accustomed to figuring out, looking for, dealing very complicated problems, that it's possible to sometimes end up with some blind spots at times and if they aren't careful, can end up locked into a sort of "tunnel vision" to the point where they can miss some things that might seem too simple to even consider.
A simpler example was back when I was about 20, and some of my mechanic buddy's track me down to ask what I thought about a problem they couldn't figure out.
They'd been banging their heads against a wall trying to figure out why the heater had stopped working in an S-10.
I couldn't keep up with all the stuff they were saying they'd tried. it was a spiel that was like, everything under the sun but rotating the tires and/or facing the car into the wind.
So I was like, you've already made sure there's water in the radiator?
And they just stared and me for a bit, then both jumped up and went running outside.
A minute or two later they came back in, red-faced and laughing their buns off.
It was bone dry. lol
It's not like I was some sort of "super mechanic" or anything.
It's just that they'd already obviously been attacking various possible complex solutions, so the most logical thing to do was try to think of what could be the simplest *alternate* solution.
How does a heater produce heat? by using heated water from the engine. Therefor, simplest possible solution to no heat, is no water.
It happened to be the only shot I took, but it was really also the only shot I had off of the top of my head. if it'd had water in it, I didn't have a second possible answer handy.
Anyway, It's just a sort of fundamental thing that I had always tried to remember when it came to troubleshooting various kinds of complex systems. If results to complex tasks or tests don't yield the sorts of logical results that are expected based on what is known, take a step back and look over the situation from the most basic perspective to verify that there isn't another much simpler alternate explanation, or unconsidered/unexpected variable interfering with results.
So yeah, don't even know if you have a fan blowing into that area on your car, but that was my one shot at the simplest alternate explanation that I could think of.
Some pipes colder than expected or colder than others... Cold air blowing on some of the pipes?
Anyway, good luck with your project, and have a Happy Easter!
