In the following picture, what is supposed to attach to the spot circled in red? On our '73 f350 there is a metal tube that screws here but the other end of the tube isn't attached to anything. It looks like the one end goes down by the exhaust manifold but I can't quite figure out where.

 

Yes, its soposed to be a hollow metal tube, usually mounted to your intake manifold--riveted to, but not going into the manifold... The manifold heats the air in the tube, which rises and heats the heat activated element in that round housing- the choke housing- when the engine reaches proper temp, all this happens and opens the choke.

Some people route them down next to the exhaust for a quicker heat up, since the exhaust manifolds get hotter, quicker, letting the choke off sooner.

Unless you change to a manual choke, or an electric one, you really need it hooked up, or else your choke won't open and you run too rich all the time. In theory the element will heat up without the tube and open the choke, but it would take a lot longer, and honestly I doubt if it would heat enough to open all the way.

G.

 

Isn't that an electric choke thermostat already on there?

 

These chokes are electric ASSISTED. They draw their primary heat from the exhaust.
Depending on the engine the choke "stove" is either located in the exhaust crossover passage in the intake manifold or on the passenger side exhaust manifold.
The fitting circled should have a metal tube that goes to one of these heat sources, there should be a second steel line that goes from the heat source up to the air filter. It will be hooked to the a/f housing by a short piece of rubber hose.

 

I used to have one on my Mustang. Technically, it's called a choke stove. A metal tube with interference fit is pressed into a boss on the right exhaust manifold, goes up, and mounts to that fitting with ferrule. The tube end is supposed to have a slight flare.

 

That is where your "hot air" choke tube hooks up to, and you need to eventually get your automatic choke system hooked up or else you are going to hate your truck when the weather turns cold! This is part of the automatic "hot air" choke which a carbureted vehicle absolutely needs!

There should be a tube with insulation on it that is screwed onto the bottom of the choke cap, (circled in red) and that is what operates your choke. That "hot air" tube should be connected to the top of a tube-like chamber that is made into the passenger's side exhaust manifold. This is called the choke stove. There is second metal tube that goes from the bottom of the choke stove back up to your carburetor. That is the "fresh air" tube for your "hot air" choke. It is connected to the top plate of your carburetor by a short piece of rubber hose.

When the engine is cold, it needs a richer mixture to stay running, and that is why the choke closes. Clean, filtered air from the air cleaner is pulled through the "fresh air" tube and into the bottom of the choke stove in the exhaust manifold, where the air is heated up when the engine is running. From there, the heated air goes back up through the insulated "hot air" tube (attached on the top of the choke stove on the exhaust manifold) that attaches to the choke cap. As the air gets hot, it closes the spring in the choke cap, which allows the choke plate to open up as the engine warms up to run on a leaner mixture.

Your Motorcraft carburetor originally used a "hot air" choke IN CONJUNCTION with an "electric assist" choke. The "electric assist" portion uses a wire that connects to the tab on the black choke cap itself and connects to the back of the alternator. The "electric assist" only works above about 60 degrees, when the engine doesn't really need much choke to begin with. It helps the choke to come off earlier on warmer days for better emissions. I think the "electric assist" was mainly for emission purposes, and your vehicle should operate just fine with only the "hot air" choke properly working.

The "hot air" choke is actually a better, more accurate choke system than the fully automatic aftermarket chokes because it is thermostatic, meaning that the choke opens/closes in relation to your engine temperature. On fully automatic chokes, only a timer is used which sometimes comes off too soon or comes on again when the engine doesn't need it.

When the choke system is hooked up correctly, you will have quick cold starts and much better driveability. With the proper carburetor settings and the "hot air" choke hooked up, you will find that a carbureted engine can run just as well as any modern fuel injected vehicle.

 

Great info, thanks! Does anybody have a picture or diagram of where the tube attaches on the exhaust manifold? I cannot find any place for it to go.

 

If your engine is an FE (360/390) the lines hook to the top rear of the passenger exhaust manifold. In this picture both lines are pulled out of the manifold but you can see where they go.

 

Ah, good to know. Yet another piece the P.O. apparently decided wasn't needed (along with the entire air cleaner assembly). I'll have to do something about that once I get the truck back on the road.

Thanks for the great info, mike & Lariat!

 

Engine is a 390. I don't recall seeing those 2 holes on mine but it is very hard to see because the A/C box is in the way. I'll take a closer look this evening. Thanks for the pic, at least now I know what to look for.

 

Ok got the choke heater tube connected. I have 1 indent on the back side of the right exhaust manifold where the choke heater tube rests in. Choke is working great now.

Thank you all for the helpful replies.

 

I have the same type exhaust manifold. I have rusted out tube stubs that need removal before I can connect the new ones I ordered. Does anyone know what is under these two holes in the rear of the manifold? is it just a passage formed inside the casting of the manifold? Is it safe to drill out the old tubes?
Is there other tubing on the bottom side that I can't see?
Can I use a flexible brush to clean out the passage so air will flow freely from the input side to the hot side that goes to the choke thermostat coil housing?

Any guidance on this would be greatly appreciated.

Regards,
JimmiGee

 

I've seen some diagrams where it is a looped tube. On some, one is an intake tube to what looks like steel wool and then then up to the carb. I suppose the wool is supposed to act like a filter.

 

I have bought the repair kit that has sleeves that slide into the broken off tube and the original tube slides into the sleeve. Many years ago when that stuff was on the HELP rack at a parts store.

 

It's referred to as "choke heater repair kit" or "choke stove heater repair kit"... Dorman PN 76850 and PN 03840.