Climatic choke operation - musings
 

Currently in the Dad's Truck Build thread, a discussion regarding ways of supplying 12 volts [key on or engine running] to the electrically heated choke is going on.

This caused me to pause and think and I believe I have come to understand more how the engineers designed this climatic choke to function and how the often made owner modifications and or inoperable original components affect this system[usually for the worse].

First, let me say that my experience is based on my 1986 F150 with 4.9L engine which has been converted to Duraspark 2 ignition and a Carter YF carburetor.

For discussion sake, to me the climatic choke is one which adjusts based on two heat sources. There is a metal tube which originates at the top of the carburetor, where it gets a supply of filtered make-up air, goes through a section of red silicone vacuum hose to another piece of metal tubing which connects to the bottom side of the exhaust manifold. On the topside, a second piece of the metal tubing [insulated with a white fabric covering] exits the exhaust manifold and goes to the circular choke housing. [The metal tubing is connected to a passage through the manifold, so air passes through, not exhaust gasses.]

Inside the choke housing is a bimetallic coiled spring which is attached to the choke plate shaft. This spring uncoils and coils based on temperature, thereby adjusting the choke plate opening. When cool, the choke plate is more closed and as heat is applied to the spring, the choke plate opens.

There is a small vacuum port found in the choke housing. This vacuum draws the air through the tube. It has been stated that this type of carburetor is jetted differently to compensate for the minute vacuum leak.

The Ford engineers task was to take into account the warm-up characteristics of this rather large and heavy piece of iron [engine] and have the engine be fed a rich mixture for start up and lean out as the engine reached operating temperature. What they did was provide a heat source [described above] which quickly ramped up as the engine ran. Added to that was a supplemental source of electrical heat provided by a heating element in the circular black plastic choke housing. The source of the electrical current was directly from the "s" stator of the alternator.

The "s" stator provides current only when the alternator is being turned, so if the operator turns the key to the "on" position, but does not immediately start the engine, the choke does not begin opening prematurely. Pretty smart on their part!

According to the current discussion in Dad's Truck Build thread, aftermarket choke heating coils, like ones used on Edelbrock carburetors, are rated for 12 volts. The voltage supplied @ the "s" stator" is ~7 volts. Some of our members have stated that they have switched to 12 volts with the original Ford choke heating elements and have had no "burn-out" problems, but have noted that the choke opens more quickly.

The Ford engineers must have been able to achieve the desired results by using the exhaust heated air along with less supplimental heat than would have been provided with a full 12 volts to the choke heater. When operating as designed [i.e. my son's 1984 F150 with 4.9L and completely stock and well maintained], the system works beautifully... almost like fuel injection!

Well, often times, by the time we owners get our vehicles, things have rusted off, fallen off or have been taken off or modified by previous owners or by ourselves. :o As a result, things might not work as designed.

I will use me as an example. My original feedback system was in such poor shape when I purchase the truck from the original owner that it ran poorly, belched black smoke and returned 8-11 mpg. I did the normal Duraspark 2 conversion and replaced the carburetor with one taken off a 1982 F150 [Carter YFA]. The metal tubing supplying the heated air to the choke was missing. I capped the port at the top of the carburetor and capped the threaded port at the choke housing. So, the choke was receiving "supplemental @ 7 volts" heat from the electrical heater only. Well, this worked fine in the summer [when a choke is not as necessary], but gave me fits in the winter. The choke then would be slow to open and the truck ran poorly until fully warmed. Keep in mind, too, that I capped the port at the choke, so if indeed these carburetors are jetted differently [richer?] to compensate for a designed vacuum leak, I had upset that balance.:o

Next, due to a worn throttle shaft, I decided to replace the carburetor with one from a 1970 F350 [pre-emmission]. This carburetor came with provisions only for the heated air, not the electrical part. I swapped the "innards" and operated like that successfully, but did not realize again that the ambient temperatures were mild and everything worked great. Then winter came and again, poor cold engine performance.

The 1970 Carter YF does not have the stepped fast idle feature that later carburetors have. As a result, the fast idle was @ approximately 1000 RPMs, but quickly dropped to 600-700 with a still cold engine. Quess what... @ 600-700 RPMs, with a stock 70 amp alternator, the alternator is NOT charging. And when the alternator is not charging, the choke heater is not receiving 7 volts so what would be a slow opening now becomes a "no" opening. Once the engine warmed, the truck ran just fine.

I could have repaired the original tubing or could have switched to a 12 volt source [read the other thread to get ideas regarding wiring and use of relays]. If I went directly to 12 volts, I would run a vacuum line directly from the threaded choke port to the upper carburetor throat port which would supply the air needed to balance the jetting of the carburetor.

What I did, prior to reading the conversation in Dad's Truck Build was to install a manual choke. This has worked quite well, but most people I know would be hard pressed with starting and driving my truck [due to the inexperience of most with manual chokes].

Forgive me for the length of this post, but I thought it might help others. ;)

I agree! I was completely shocked at how well a carbureted vehicle could run with a functioning hot air choke and the stock thermostatic air cleaner. My own truck came to me in much the same condition as yours. Since I am too young to remember vehicles with carburetors, I just assumed this was normal for an older engine. I was certainly proved wrong! My truck now runs just as good as any modern vehicle with EFI, and it actually starts faster than any other vehicle I own. You just have to remember to push the gas down once in the morning to start it. :-X22

The "electric assist" portion of the stock choke system found on the later stock carburetors does not work on its own like an aftermarket electric choke. In fact, it doesn't have to work at all in order for the choke to be effective. It is only there to "assist" the hot air choke in temperatures above 60 degrees, where it helps the choke come off sooner for cleaner emissions. It doesn't work at all when the weather is colder than 60 degrees. It connects from the choke cap by a wire that connects to the back of the alternator. This part of the choke is secondary, meaning the engine will run fine without it, but the engine will not run well without the thermostatic "hot air" choke system.

The thermostatic "hot air" chokes works MUCH BETTER than the fully electric chokes found on all aftermarket carburetors in that the choke opening corresponds with the engine temperature. The engine gets the right amount of choke it needs, and because it uses hot air generated from the engine, it doesn't choke the engine when it isn't needed. By contrast, the fully electric chokes are run off a timer and needs a 12V power source. These come off much too soon and will always reset anytime the engine is turned off. Which means the choke will turn on again every time the engine is started, (or the key is turned "on") even when the engine is already hot and doesn't need it. :mad:

I wasn't around in the glorious age of carburetors either. I was simply amazed at how much easier and better my truck started when I put on my manual choke kit from Dorman, even though I have no fast idle cam at all. The choke rod running from the butterfly valve to the fast idle cam had broken off before I got the truck, so all I have is an "idle" cam now.

Not having to pump the gas is great, in my opinion. :-X22

Fully electric chokes from Ford may run off a timer, but aftermarket ones usually don't since people like me do the wiring. In my case the relay is pulled in using the stator voltage off the alternator so the 12v is there as long as the engine is running.

And, not all Ford electric chokes were 6 volt as someone, Jim I think, pointed out to me. IIRC, the 4180 on the 351HO's and 460's had 12 volt heaters in them. And it is probably those that were on a timer, although I think that timer gave them 12v initially and dropped back to 7v after the prescribed time.

Also, just because a Ford vehicle is equipped with an electric choke mechanism doesn't mean it was connected from the factory. Dad's truck had the hot air tube to the 2150 carb, but it was rotted out so I was going to connect the electric choke. However, there was no wiring. Nothing in the harness at all. So I ran a wire from the stator terminal to the choke and it worked pretty well that way.

When my 70 Chevelle was totally stock it had a 2bbl 307 with points ignition. I had everything tuned up and working like GM intended and it was one of the easiest cars to start in any weather. As long as you knew what you were doing... :D

I went to a friends house and he wanted to start it. He's older than me and as he got in he started pumping away at the gas saying "I remember these old carbureted cars..." By the time I screamed STOP! he had already pumped it 6-7 times... When I finally got it started I limped home on 5 cylinders... The other plugs were just too fouled to run...

Normally warm you just touch the key and the engine is idling... People used to be amazed at how fast it started compared to today's cars.

I hear that, f100beatertruck. As soon as someone finds out that your vehicle has a carburetor, the first thing they assume is that it is going to be difficult to start every time and you must pump the gas repeatedly before you even try. Few people seem to understand that a properly functioning engine (and that means with a WORKING choke!) with a carburetor should start right up for the day with only a single pump of the gas, and then 1/4 pump or none at all throughout the day.

I wonder why it is that almost everyone, even people who are much older than me and who actually grew up with carburetors, love to talk about "cold start problems" with carburetors. Did everyone back in those days throw away their stock air cleaners and disable their chokes or something? :-huh

Here is great example one of our fellow FTE members (Nathan Plemons over in the 1973 - 1979 forums) made on how a properly functioning carbureted engine should start:

Because, they get old and dirty and would fail. All that linkage out in the open was prone to get gummed up an sticky, and then it would not work right. Also most of the choke pull-offs had rubber diaphragms that would go bad, and also as everyone mentioned in previous posts, the tube that goes into the exhaust manifold liked to rust off. You know most people drive their cars till they won't go and then trade them in.

Yes, a properly-adjusted choke can work quite well. Even an all-electric choke can work well, as on Dad's truck. I drove it today after it sat outside overnight. It started immediately and didn't hiccup whatsoever during warmup - or after for that matter.