So I'm rebuilding the Motorcraft 2100 on my '73 F100 (360/4spd). I downloaded and read over all of the tutorials and things are soaking now.

The choke has never worked right on my truck, and I want to remedy it during this rebuild. The lever that actuates the high idle when the choke is engaged was broken, so I'm replacing that, but once I really inspected the choke and how it operates, I'm a bit at a loss. It doesn't appear to be vacuum or electric, but there is some sort of thermal probe that threads into the choke housing at a 45 degree downward angle. The other end of the probe is down by the passenger side exhaust manifold, and appears as though it should go into a hole in the manifold. It's currently not inserted. There is a hole from where the probe threads into the housing that goes up next to where the choke fork goes. I'm not sure how this probe activates the choke, but I was hoping you guys could help me out.

-How does this work, and how do I make sure it works correctly when I put it back on?
-Is there a cheap and easy upgrade I should/could do while I have everything apart?

Thanks in advance,

-MK

 

It's a thermostatic choke and there is no probe.

The port at the rear facing downwards receives a metal tube with a ferrule and fitting which threads on to it. The metal tube is a "hot air riser" or "choke stove" and is connected to the passenger side manifold. When it heats up, the air inside the tube expands and is channeled to the choke housing (the black plastic cap). Inside the housing is a bi-metallic spring that expands and contracts, actuating an arm and choke piston. If you look at the cap, note that it reads "lean" and "rich", depending on which way it turns.. internal of the cap is a tang which, IIRC, preloads the spring.

The typical issue is that the choke piston gets gummed up with carbon or dirt deposits. Clean it thoroughly and ensure that it actuates easily inside its bore.

Install it straight up and adjust from there.... no tricks to provide except the choke butterfly should have a 1/8-gap at the air horn when cold. I use a 1/8-inch drill bit as a spacer and you should too.

 

Thanks a bunch for the info.

The riser tube is NOT attached at the manifold and simply hanging there. Where does the other end attach to? There appears to be a hole in the manifold for it to fit, but I'm not 100% sure. It didn't fit back in so I'm guessing I might have to ream it with a drill bit and JB weld it back in there. Probably explains part of the exhaust leak I hear at that manifold.

Do the thermostatic chokes work well? Seems like a simple design, but I want effective. I'm tired of playing footsie to keep the truck running when I first start it up in the morning.

Thanks again,

-MK

 

No problemo. Does the thermo choke work well? When it's working!!! Seriously, the one on my 65 'stang works great. One pump to set the choke and it fires right up. After a minute, another stab at the throttle settles it to idle speed.

That hole is where the other side of the choke tube is supposed to be. It usually breaks flush with the manifold. Sometimes, though not often, it can simply pop out. I wouldn't JB weld it... it's an interference fit. Plus if applied incorrectly, the JB weld can plug up the tube. I'd smear it on the tube rather than filling the hole.

If it is broken, just drill it out. Try to vacuum out the shavings the best you can. Get a section of steel tube from your local parts store, cut to length, and fab up a new line. I don't think it even needs to be flared because of the nut and ferrule.

If you're really ambitious, drill and tap the manifold for a fitting.

 

So upon further inspection, it does appear as though the tube has sheared off flush with the manifold. It's in tight to the heater box, and without removing the entire heater box (which I'd rather not do), it'd be pretty hard to get a drill bit in there.

How hard is it to fit an electric choke to my carb? It looks plug and play and I found one at Mike's Carb Parts for $21:

http://www.carburetor-parts.com/Moto...stat_p/214.htm

Is it as simple as attaching the unit and wiring it up?

Thanks again

-MK

 

Electric chokes are easy, no tubes. Once started the bimetal coil is heated by a heat coil and this causes it to open the choke in time. You turn the housing to adjust that time within limits. Not a huge drain, but all while you drive there is a constant 12 vdc drain to keep that coil hot. Mine was hooked in at the alternator.

I once had a choke on my '77 with the tube, I just took a tube and bent it to lay along the length of the exhaust manifold for a foot or so, clamped with a couple stainless steel worm clamps, and that heated the air that was pulled into / through the choke housing by vacum (there is a port there) heating the coil.

My '77 now has a manual choke on an Edelbrock 1405 carb.

 

I might just bend up a tube like you described. Where does that vacuum line attach? The only vacuum line attached to my carb comes from the front of the distributor and attaches low on the passenger side of the carb in front of the choke housing just behind the front mounting ear.

I just need to get some sort of choke rigged up. I have a manual choke now, but the kit I bought from Autozone is a POS and I can't get consistent engagement. Again, the problem was that the arm that actuates the high idle was broken, and now that I have a replacement, I want to make sure I get it all fixed before it turns cold her in Ohio.

For the electric choke, there's a coil you need in addition to the assembly I linked to above correct?

-MK

 

Electric chokes are easy, but they don't work near as well as the thermostatic "hot air" choke you have now.

The problem with the all-electric aftermarket chokes is that work off of a timer. This timer will make the choke come off after about 60 seconds, which isn't really long enough when it is real cold outside. They also re-set and choke the motor every time the vehicle is re-started, whether the truck needs it or not.

The stock thermostatic "hot air" choke works on actual engine temperature instead. Here the choke works in direct relation to how hot the engine is, and chokes the engine accordingly. Using hot air that is provided by the engine itself when the engine is warming up, the choke is more efficient because it will stay on only as long as the engine actually needs it, and doesn't choke the motor if it doesn't.

The stock thermostatic choke used two tubes. The fresh air tube connects from the carburetor air horn to the choke stove chamber located on the passenger's side exhaust manifold. This will filter the air, so that foreign particles won't enter the choke housing and jam up the choke piston. From there, the filtered air from the fresh air tube will enter the choke stove chamber where it is heated up, and then is routed up to the choke cap through the hot air tube, which should have an insulated sleeve on it to keep the air warm.

If you can't get the tube out of the choke stove chamber on the exhaust manifold or if you decide to run headers, then you can buy an aftermarket choke stove kit that will work well also.

 

Thanks for the info. The only tube I have is insulated and routes down by the back of the passenger exhaust manifold. I'm having a hard time visualizing where everything belongs. Does anybody have a picture or diagram? Also do you have a link to an aftermarket thermostatic choke kit?

Thanks again, I hope to get this thing put back together correctly.

-MK

edit: I found this electric choke at Advance Auto. Would this part be all I needed to convert to electric choke or is there more to it? Looks like it has the same bi-metal spring and just one wire that attaches.
http://shop.advanceautoparts.com/web...582-_-10744592

 

There's no timer or any kind of one-shot circuit that controls the choke. It's literally just a bimetallic strip of metal without any kind of smarts. Some chokes, however, are built out of PTC (positive temperature coefficient) resistor material, which increases in resistance as temperature increases. So as the choke heats up, its resistance increases, and it draws less and less current until it cuts itself off. I think this might be what you're describing.

Aftermarket all-electric chokes (like what you'd find on an Edelbrock) start heating up as soon as power is applied. The stock Ford choke, however, requires the hot-air stove to heat the choke spring up to a specific temperature before the electric choke heater kicks in.

 

Thanks for the info. I modified my previous post right as you posted. What about the electric choke I linked to from Advance Auto? Would that work with my application in place of the stock thermostatic choke without the riser tube?

-MK

 

So I went ahead and ordered this electric choke from Mike's Carb Parts that fits the 2100:

Motorcraft Electric Choke Thermostat

Where's the best place to get the 12v lead to power it?

-MK

 

Any circuit that is only hot when the key is on. Except the ignition coil circuit, it has a resistor wire which lowers it's voltage to the coil. The pull from the electric choke will be negligeable so any of the hot In run fuses would be fine, preferably a fuse that doesn't get used though, like the aux tank for instance (on my 79 anyway)

 

Cool, thanks. I might just splice in a wire from the ignition switch and fuse it.

-MK

 

Factory electric chokes are powered from the sta terminal on the alternator back.