I often read with amazement the threads written by our gifted and more knowledgable [than me] members. They tackle the hard stuff, so having just made a seasonal adjustment to the choke on by 4.9L Carter YF [easy stuff], I thought I would share the details, which might possibly be of assistance to some of the newer members.

The choke plate on the Carter YF is controlled by a bimetallic spring. This type of spring is made up by two different kinds of metal bonded together and coiled into a spring. Each kind of metal expands at different rates, when exposed to heat, so the spring literally unwinds when exposed to heat and coils back up when heat is removed.

Heat to carburetor chokes can be supplied by various techniques, depending on manufacturer and era [year of manufacture]. The choke on this 1986 F150 receives heat from two sources. One is a metal tube which passes through the exhaust manifold and then is routed to the choke housing. This tube has a high temperature insulating fabric jacket on it from the manifold to the choke housing. The other end of the metal tube is connected to the carburetor throat, usually with a red silicone hose. Inside the choke housing is a small vacuum port, so the heated air is drawn into the choke housing thereby heating the bimetallic spring, which in turn opens the choke plate as the engine warms up. The port where the red silicone hose attaches is to provide clean make-up air, as the system is a closed loop.

The second source of heat to the choke is supplied by a small heater coil located in the same choke housing. This heating element is supplied voltage from the alternator, so it does not begin heating until the engine is actually running.

The goal of this system is to have the choke close the choke plate when the engine is cold [even in summer] as a cold engine requires/likes a rich mixture when first started and needs a more lean mixture when it approaches normal operating temperatures.

My truck has performed very well during the summer and beginning of autumn, but has not done well recently, with the beginning of cool weather. For 1st start of the day, I would press the gas pedal to the floor and slowly release, as usual, and the truck would start easily, but moments later as I left the driveway, the truck would "fall on its face" unless I really "stomped it". Just about 2 or 3 blocks away, everything would begin to work well. The nighttime temperatures have been upper 30's to mid 40's.

This past weekend, the truck had not been started and I had the time to adjust the choke. I removed the air cleaner housing and pressed the gas pedal to set the choke. I looked down the throat of the carburetor and saw that the the choke plate was completely closed. There should be a slight gap, so a little air can be drawn in. I pressed on the choke plate with my finger to open the flap and found that there was quite a bit of tension to overcome the bimetallic spring. The spring, which is attached to the shaft of the choke plate and was properly adjusted in warmer weather, had now coiled more tightly due to less heat in the atmosphere and now was "slamming shut" the choke plate. I loosened the 3 screws holding the metal ring which holds the choke housing in place and turned the black cylindrical plastic housing to the right [clockwise], thereby lessening the spring's tension and allowing the choke plate to open very slightly. My choke housing has marks indicating how much the housing is moved... this adjustment was 2 marks. The 3 screws were then tightened and the air filter housing replaced and all hoses reattached.

It was that easy and now the MIGHTY SIX again starts easily and can immediately be driven with no hint of bogging down.

I hope this will at least help one person.

P.S. If you own a six cylinder with the Carter one barrel carburetor, always check the mounting bolts when you have access as during this job. There are only 2 bolts and they tend to vibrate loose. You need a 1/2" wrench.

 

Thank you for the writeup. I guess sometimes we forget some people on here are new and maybe not used to dealing with the "old" carb systems.

 

Well said.

Does the Carter have an un-loader adjustment? If so, there is a delicate balance between the bi-metallic spring adjustment and that of the un-loader. The spring should bring the choke plate to the closed position and the un-loader should open it up a preset amount - like maybe 1/8".

 

Mission accomplished - you helped me understand how to adjust my choke. Before reading this, I wasn't sure if the plate should be fully closed or partially open when cold, or what exactly turning the black choke housing accomplished. Thanks!

 

I was just getting ready to start a choke thread of my own, I hope you don't mind me asking my question here instead.

I've got an '80 F-150, 300-6 engine, with the Carter YFA carb. My truck starts great when cold (meaning the first start of the day, not the weather), but once I give it more than a bare touch of gas it starts coughing and chugging. If I back off a bit it runs better, and as it gains speed and starts to warm up I can give it a little more gas, until eventually it runs just fine. If I start it warm I have no issues whatsoever.

My choke was initially set two markings clockwise (from behind the carburetor looking forward). When I fiddled with it this weekend I moved it two more clockwise, but then I couldn't start the truck. I backed it off one (three clockwise), and the truck again starts well. It *seems* to run a little better, but that could be wishful thinking - it definitely still chugs a bit.

If I correctly understand what is happening, I need the choke to open more quickly, right? It sounds as though it's running too rich when cold, meaning it needs more air = a more open choke. But the choke is already open about as far as it can go and still start, so I simply need a faster opening once started.

So how do I do this? The heat riser tube that goes to the choke assembly is in place and doesn't appear to be rusted off or anything. The electrical assist wire is also attached, but I don't know that it is working correctly. Is it even involved in this? If so, how would I test operation?

 

Now we are in the realm of the unloader. It is usually a vacuum-operated mechanism that allows the choke to close fully, but when the engine starts it's vacuum pulls the choke open enough to allow the engine to run.

I'm on a trip so don't have access to the FSM to get the specs, but IIRC the choke should open about 1/8". You can check this cold and with the engine off, assuming the carb has an unloader, by pushing the vacuum module's piston in. Use a drill bit to test the opening.

 

Thanks Gary, good information and something to check - the vacuum unloader. I do not not know if the Carter YFA has it or not.

I have a related question to ask that probably doesn't deserve its own thread: is there supposed to be a removable gasket between the carburetor and the air cleaner assembly? I ask because one of my Ford shop manuals makes a suggestive reference to such a gasket. I suppose it would rest right on top of the carb, where the air filter housing attaches. But mine has no such gasket, it is just metal to metal contact between the carb and air filter housing.

Also PS, does this look the slight opening of the choke plate one might want on a cold engine?

 

I believe the thing attached to the upper right corner of the carb in the picture above is the unloader diaphragm, or the "choke pulldown motor" as Ford calls it in the FSM. If so, when you apply vacuum to the hose it should open the choke a bit, thereby "unloading" the choke.

Just looked and the '81 FSM does not give spec's for the choke nor the pulldown. My remembrance is that the choke plate should close completely when the engine is cold and you press the accelerator down once and then let up. (You have to let up as there is a "dechoke" mechanism that opens the choke 1/8" when you open the throttle completely - in case you flood the engine.) Then, when the engine starts and vacuum is supplied to the unloader the choke comes open a bit - something like 1/8" IIRC. And, if the engine starts to die and the vacuum drops the choke closes some, which raises the vacuum and opens the choke, which may cause the .........

I'll look around tomorrow to see if I might have spec's and let you'uns know.

As for the air cleaner gasket, yes there should be one. Otherwise the air cleaner is going to wear into the carb due to vibration, not to mention the air leak that would occur there.

 

I believe you're right. Rockauto calls that thing the "Carburetor Choke Pull Off". And thanks for the info on the air cleaner gasket, I just bought one online (Fel-pro) for .72 cents, hopefully it will fit. I don't want my air cleaner eating into my carb.

 

Hmmm, mine has no such doohickey...maybe that was a later addition? You have an '86, right?

 

Yeah, mine is an '86 with the "feedback" carb. I don't know when they added the vacuum choke pull off to the carter YFA. But here's a picture of it on a spare feedback carb I have, you can see the valve connects to an actuating rod associated somehow with the choke. Also my choke hot air tube is on the opposit side of the carb from yours.

 

Ok, I have some info on choke settings. These are from a Chilton's 1976-86 Ford Pickups & Bronco manual. And, it should be noted that neither of my 1973-79 Hayne's manuals have any such spec's, so if you are buying a manual.......

Also, this manual has info on a wide range of years awa carbs, so I'll try to generalize the info. But, if you have the # off your carb I might be able to give you specifics.

Choke cap setting: Index for most, which means put the pointer on the index.

Choke pull-down: .230 to .320", with .290 being typical

De-choke: .280 to .330", with .280" being typical

Again, if you have a # (something like E3TE-9510-AMA) off the tag on the carb then I may be able to give you the actual settings for your carb. And, if needed I could re-type the instructions on how to put those setting into the carb - for an extra charge.

 

OK, got it thanks. According to the numbers you provided, the Choke pull-down versus De-choke gap opening distance is basically the same (only one one-hundredth of an inch difference in "typical" opening distance). Thanks for looking those numbers up, and I like your idea of using a drill bit as a measurement tool.

 

I was surprised at the pull-down spec as I didn't expect it to be so wide. But, I'm used to 2bbl and 4bbl carbs with a much larger choke valve, so it stands to reason the 1bbl would have to open wider.

And, FWIW, my theory is that it is better to err on the side of more opening than less on pull-down. My thinking is that you may not get the engine to even run if the choke doesn't open enough, or at least it'll run poorly, and when it starts to die the choke will close more due to the lack of vacuum. On the other hand, if you have just a bit too much opening the engine will drop in RPM and the vacuum will drop as well which will close the choke and bring the engine back to life. Yes, it may then bog again but it should also come back to life again, and this cycle should go on until the engine is warm. Obviously you don't want to take this to the extreme, but I would err on the more rather than less pull-down if I had to choose.

 

myself i went to O'reillys and bought the choke kit there for 12 bucks and hooked it up and now have manual choke,problem solved,was pretty easy to hook up too