Well, we had our first official cold snap here in the Houston area. It got down to 39 degrees.
I replaced the exhaust manifold with another one that had a working exhaust "flapper" a few months ago.
The slant ran okay in the cold weather, unlike last year when it ran like kr@p.
Hmm...
Could there be something to this intake manifold heat?
Of course I had to learn the hard way.

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"Ja, Ich fahre ein altes auto."
'78 Volare 225
'67 Charger 318

Manifold heat vaporizes the fuel from the carb and thus evenly distributes it through the intake runners. Much more efficient to spread out a gas than a liquid (in fact gases naturally expand to fit their container). If it doesn't properly vaporize, and the intake cools from evaporation, it can ice and fluid gas can run down runners unevenly, puddle in the intake, etc. You could have overly lean or rich conditions at different cylinders simultaneously.

OK, you might wonder how many folks use headers or otherwise no heat to the intake. Since the exhaust manifold is on the same side and underneath the intake, under most conditions it's pretty darn hot in the engine bay. But, as you noticed, when it's cold outside, that may not be enough.

I have an issue with too much heat at the intake, my Webers boiling over in traffic and flooding the engine with gas. That's a lot of cast iron in those exhaust manifolds, a very large thermal mass that radiates a lot of heat. I've built heat shields which manage the problem (not solved, just managed). They allow me to stay in traffic longer than I otherwise could. I also installed a cold air intake that helped cool the carbs from the inside.

So, yes, there is a balancing act. Not enough heat is bad, too much heat is bad. Definitely the heat riser provides smoother performance, more evenly distributed fuel mix, better gas mileage due to the efficient vaporization and distribution of fuel.

The way around all this is fuel injection....someday....someday....

brian

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https://tinyurl.com/yynpj4v2

Interesting that you brought up about your Weber "boiling over" in traffic. I have a Weber 38DGES on my slant six, and in heavy stop and go traffic, it seems to not run as well. I may try to rig up a cold air intake of some sort.

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"Ja, Ich fahre ein altes auto."
'78 Volare 225
'67 Charger 318

My Weber History:

I have dual 32/34 DFT's 2 bbls.

Initially, I was using a stock mechanical fuel pump, but it was putting out way too much pressure (8-9psi) for my Webers. It worked OK, but sometimes if I took a right turn too sharply, they would flood. So, I put a pressure regulator on there and got the pressure down to 3 psi, much better for Webers.

Still, I had the flooding problem if I sat in traffic too long. So, I installed a heat shield - that helped a lot, gave me many minutes more in hot stand still traffic, but didn't solve the issue.

Then I figured out that part of the issue was, at least I think, when sitting in hot traffic heated air sits underneath the hood (for some reason the air circulation in the a-body engine bay is not so good and I have a fan that pulls a ton of air, I tried electric fans once, great for the engine cooling but the carbs boiled over pretty quickly, air flow wasn't right). So, I figured if I had cooler air piped in while in traffic, I could cool the carbs from the inside. This seemed to add an additional benefit. I measured this by increased traffic time before boil over but also, when I stop after cruising and check the carbs they are noticeably cooler to the touch than before the CAI was installed - its doing some cooling.

Anyway, since then I've also switched to a recirculation system for my fuel supply, so the carbs always have cooler fuel to draw on (also insulated my gas supply line in the engine bay). Since I did this, I've not tested it in dead stop traffic to see if that added any additional time or, let us pray, solved the problem. I doubt the problem is solved, but these are the measures that I've taken thus far to address the sensitive-to-heat Webers.

Brian

P.S. Otherwise I love the simplicity, instant response, progressive format, and "set it and forget it" dependability of the Weber.

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https://tinyurl.com/yynpj4v2

For street-driven carbureted engines to run properly, a hot spot under the carburetor is essential. If the carburetor percolates from a hot intake manifold in heavy traffic, the better solution is to add insulation between the carburetor and manifold.

see Intake Manifold Heat

see Down the Gasoline Trail (7:57 video)

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1965 Plymouth Barracuda,
225 engine, Quadrajet, HEI, Dutra Duals, 904 Torqueflite, 2.76:1 axle, Addco front bar
Rods & Relics - Fort Erie, ON / Collector Car Tech