About the "thermal tube", the choke will have a tube that is insulated with a heatproof fiber/linen jacket. This tube attaches to the side of the choke unit and goes downward into a "choke stove" that bolts to the intake manifold next to the carb. The "stove" will have a metal tube that ties into a rubber hose that plugs into the airhorn. That is that particular unit.

But if you don't have the choke stove beside your manifold, you probably have a single vacuum line that should be going to some sorce of heated water on your engine via vacuum port.

During the 70's and 80's, Ford used (3) different varieties of chokes: Electric choke; Vacuum choke, and the hybrid Electric/Vacuum choke. The latter sounds like what you have. So yes, I'd tie into one of the vacant ports since this is not really a pollution device but serves actually as a thermal signal for your choke.

(Edit: this anaolgy corresponds to the diagram on Bubba250's web site. What I am calling a "Vacuum choke" is what he titles - probably more correctly - as a "Hot-air choke")

 

In the first place, I was attempting to convey information about 351m and 400 engines; big engines in big vehicles. The tuning on 351M/400 engines had to be altered to reduce their NOx emissions. You're talking about a 78 Fox body with a 302 that used a VV carb and had bathtub combustion chambers on a 5.0 engine. Apples and oranges.

In the second place you're wrong about ping not happening at part-throttle. It's just as common.

 

It sounds to me like you have a carb from another model that had a manifold heated choke. There was a choke stove in the manifold and hot air was drawn into the choke housing from the choke stove.

There is a small vacuum port from the carb that causes positive air flow and draws the heat into the choke.

The rubber tube you describe simply went to a port on the air cleaner, to make sure that filtered air was drawn in.

If your choke is now electric, simply plug it off with a vacuum cap. It's a small vacuum laek, nothing more.

 

I have kind of the reverse question than kjkozak has. I have a 77 f-150 4x4 and I have the strangest feeling that it doesn't have all its smog equipment. And since I need to get it smogged for registration, I need a good reference for what should be there. I have the factory manual I got from motorhaven, but I can't seem to find a list (with pictures) of what I need. Anyone have any ideas?

 

The vacuum diagram is your only help/hope. Do you have it?


As for Smog pump versus egr versus catalyst:

The thermactor system existed before catalysts/catalytic converters. It was intended to burn excess hydrocarbons IN the exhaust manifold by injecting fresh air. I expect that that has the side effect of increasing exhaust velocity, both from the air injection and from the fuel lighting off in the exhaust manifold (think turbojet here)(or rocket engine for that matter).

I don't think it is critical, but I think it should be pointed out that the Thermactor system had been in use since at least 1969, whereas the 351M/400 were designed after that point with the Thermactor system in mind.

The catalyst/catalytic converter explicitly requires the air pump to function correctly:

"The catalytic converter system requires the use of a secondary air source (Thermactor Air Injection System) in order to maintain the converter feed gas (exhaust) oxygen content at a level high enough to obtain maximum oxidation of the hydrocarbons and carbon monoxide in a catalytic reaction." -- Motorcraft Vehicle Emission Control Systems manual volume II (1975)

A catalytic converter is designed to maximize the surface area of the catalyst elements exposed to the exhaust, so it always reduces exhaust velocity, and so it will accumulate deposits from the exhaust, and if that stuff isn't cooked off regularly it's going to clog. (How rapidly I cannot say.)

Given the above, to my mind, shutting down the air pump without removing the converter is probably pretty dumb, since you won't reduce emissions but you will reduce exhaust velocity. On the other hand, the air pump running without the converter is going to reduce emissions and may increase exhaust velocity, which in turn is going to increase the amount of O2 pulled in through the carb. (There's a tradeoff involved - how much power is consumed by the pump pulley versus how much gain you get from running the pump. I don't know the numbers or I'd figure it out.)

This needs a good test.

I'd be interested to know how extra air from the Thermactor affects the O2 contents of the exhaust gasses recirculated through the EGR.


ash
['Complicated interactions.']

 

On 335-series (351C/351M/400) engines, one exhaust port on each cylinder head feeds exhaust gas to the EGR through the intake manifold's exhaust crossover. The "Thermactor bump" in that exhaust port is not drilled for air injection, so no O2 is added to the exhaust gas going to the EGR.

Just as well, since the O2 added to the exhaust gas does create a secondary combustion and additional heat, which you probably wouldn't want in the intake manifold anyway.