Hello

There is a very large class of people that own 65/66 Mustangs that, as far as I can tell, anyway, have been, for the most part, ignored entirely. They don't really want that 100 pt. show car that is so nice and was soooo expensive that they're afraid to drive it, they also don't want to make their car capable of achieving warp factor three. They just want this car that they dearly love to be able to cruise around smoothly and reliably, without having it dump them out on the side of the road or have it start making weird noises or belching out big clouds of funky-smelling smoke. And I think, truth be told, that this is by far the largest class of Mustang owners. They take their car to some technician when what they actually need is a mechanic, and this, frequently, does not work out very well at all for the owner. They don't want to re-engineer the entire car, they just want someone to fix what broke. These are the people that I am trying help out with this blog. Some problems require a little bit of back and forth, as in, "Try this." "I tried that and it didn't change anything."
" Oh. well, you probably need to try that." " I tried that and it helped, but it still isn't quite right." "Now you need to try this...." If you go to http://www.allfordmustangs.com/ and then go to the classics forums, you will be able to do that with a pretty hefty gathering of some very knowledgeable people that also happen to be very friendly. None of that ridiculous one-upmanship, no flaming or abuse, none of that stuff. Just good, solid advice from people that know what they are talking about.

Wednesday, April 23, 2008

Carburetor common repairs and adjustments

This first picture is of the accelerator pump diaphragm housing, held onto the front of the carb with four screws.If fuel seems to be leaking from that hole towards the bottom of it, that means that the diaphragm is ruptured and needs to be replaced. The ruptured diaphragm will also make the car behave very badly coming from a dead stop. First, you remove the four screws and pop the housing cover off and swing it out of the way. What you will see when you do that is shown in the sixth picture. That is the diaphragm itself. Pull that thing off. When you do that, what you will see is in the fifth picture. That spring is what pushes the diaphragm back outafter the arm cam pushes it in. If this is all that you intend to do to the carb, just put the new diaphragm on, making sure that it goes on facing in the same direction as shown in the picture, with the spring underneath it, replace the cover, snug up the screws and you're good to go. Do not overtighten the screws because you could strip out the screw holes and the carb will leak like crazy from that day forth.


If fuel is gushing out of the vent tubes on the top of the carb, that means that your needle and seat assembly is not working correctly. This is commonly called ' float is stuck'. To replace the needle and seat assembly, remove the top of the carburetor by first removing the little clip holding the choke flap arm onto the choke linkage. It is shown (sort of) in the third picture. It is that tiny little clip just to the left of the dirty brown plastic thingy. Next, remove the four screws from the top of the carb, along with the stud that holds the breather assembly wing nut on and remove the top of the carburetor. What you will see is shown in picture #11. In picture #10 you see the float clip popped loose. pop the clip loose on yours and lif the float out of the carb. There will be a little brass thingy with a rubber tip hanging on the end of it, as shown in picture #9. You will then remove the brass seat that is screwed into the carburetor, shown in picture #8. Use a 6 point 3/8 socket wrench for this. When you install the new one, be sure to put the new gasket underneath the seat assembly. Replace the old needle ( brass thingy with rubber tip) , reinstall the float with the needle inside the seat, without dinging it up, reattach the clip holding the float in, and again, you are good to go.



If, when you remove the top of the carburetor, yours is all funky and crusty-looking, like the one shown in picture #7, you need to clean that stuff up before you put the carb back together. Those two brass things that look like screw heads with a hole in the middle of them are your main jets. You remove them with, that's right, you guessed it, a big flat blade screwdriver. That other thing in there with the brass tip is the top side of your power valve. This is replaced by flipping the carb over, assuming that you've emptied the fuel out of the carb by now, and removing the housing cover, as shown in picture #15, by removing the four screws and popping it off of there. What you will see is shown in picture #14. That is the other end of the power valve. It unscrews in the normal way with a 1 inch open-end wrench. Unscrew it and install the new one, again, making sure that you also replace the gasket underneath it, then replace the cover, again, with the new gasket, and that is that.
Autolite carbs, with the exception of the K code 4bbls, have thermostatically actuated chokes. How the choke is adjusted is by loosening the three screws on the choke thermostat housing, which is the round black plastic thing on the passenger side of the carb, shown in picture #3, and rotating it slightly, then tightening up the screws. You will notice that there is an indexing mark on the carb to help you keep track of the adjustments you make. Rotating the housing clockwise makes the choke open sooner, counter-clockwise makes the choke open later.

The idle speed is adjusted by running the screw on the throttle linkage on the driver's side of the carb, as shown on the right hand side of picture #12. The fast(cold) idle speed is adjusted by the screw on the choke linkage over on the passenger side of the carb shown kind of right in the middle of picture #14. Nothing to it.







Basic carburetor function

Here's how your Autolite 2100 or 4100 carburetor does what it does. Understanding this is essential to diagnosing and correcting any problems that it might be having. The carb is responsible for delivering fuel to the engine. How it does this is by a couple of basic physical principles. One is that air moving through an hourglass shaped tube is at it's highest velocity and lowest pressure at the skinny part of the tube. This was discovered by an Italian physist named, that's right, you guessed it, Giovanni Battista Venturi. Your carburetor's fuel bowl has a vent hole on top of it. The pressure at the top of the fuel is normal atmospheric pressure because of the vent hole. The fuel has a few different routes that it can take to get into the venturis, but at the discharge port, the air pressure is lower, because of the venturi effect, than it is at the top of the fuel bowl. That creates a 'push' on the topside of the fuel. An Autolite carb has three basic fuel circuits. One is the idle circuit. This is the path that the fuel follows when the throttle plates at the bottom of the venturis are closed. The discharge ports for the idle circuit are below the throttle plates. This works by straight-up suction, or engine vacuum, so that when the car is at idle speed, fuel is delivered to keep that going. Next is the transition circuit. Here, the discharge ports are slots that are partially below and partially above the throttle plates and works partially by the suction of the engine and partially by the pressure differential of the venturi effect. Last, we have the main fuel circuit, which works entirely by the venturi effect. The discharge nozzles for the main circuit are the ones that are inside the booster venturis, or, the little hourglass tube shaped thingies that you see inside the throat of the carb. The float that is inside the fuel bowl is responsible for keeping the fuel at a certain level. The correct level for the fuel in bowl is slightly below the main discharge nozzles. If the level is too low, the venturi effect won't be strong enough to push the fuel uphill and get it into the carb. If it is too high, gravity will pull the fuel out of the fuel bowl until the two levels are the same, and then the siphon effect takes over and drains the fuel bowl entirely. Where that fuel goes, when the motor isn't running, is straight into your oil pan, degrading the lubricating ability of the oil and making you have to crank on the starter until the fuel bowl is filled up again so that the car can start. You also have an accellerator pump to deliver a little shot of gas to keep the motor running while the carb changes from one circuit to the next. This is also what the power valve does. It is held closed by engine vacuum. As you open the throttle plates on the carb, engine vacuum drops temporarily, allowing the power valve to open and let some more gas into the engine after it has burned the little shot provided by the accellerator pump. The engine vacuum recovers pretty quickly and the power valve closes and the motor is running on the main fuel circuit entirely at high speed cruising speed. Normally the motor is running on a combination of two circuits, depending on what you are asking the car to do, and most round-town driving is down on the transition circuit. Puddles of purely liquid fuel do not burn very efficiently at all. That is why the carb is designed to vaporize the fuel, and you hear people talk about the ' air/fuel mixture'. Inside the carb there are these little thingies called emulsion tubes. These create a mixture of highly airated, but still basically liquid, fuel. When this air fuel emulsion goes out the discharge nozzle, the process of vaporizing the fuel is finished by the shearing and velocity change of the mixture, so that it is vaporized as effectively as possible when it gets into the motor. On the four bbl carbs you have two primary venturis and two secondary venturis. The 4100 has vacuum secondaries. What that means is that the change in engine vacuum caused by hard accelleration or wide open throttle is what causes the secondary venturis to open, allowing even more fuel into the motor, much like a great big power valve. And that is how that works.

14 comments:

Anthony said...

Hello Veronica, I'm sorry if you mentioned this in the post, but I was a bit confused with the terminology because I'm not sure of several titles of things. I have a 66 mustang 200 6cyl with a C4tranny. I was trying to start it today, but the flappy thing on the top of my carburetor which lets the air in won't move at all when I hit the gas. It won't let the car start if it's open, and it won't stay on if it's closed. I turn the car on and no air is getting in because of that metal flappy thing. I'm not sure what it could be or how to adjust or if maybe nothings wrong with it?ha Might you please help me?

Veronica said...

That flap is the choke. If you look at it, you will notice that it is sitting at a slight angle. Take your thumb and push on the low side and see if it opens and then closes when you let go of it. If it pushes open easily and does not close by itself when you let go of it, then the problem is probably inside the choke thermostat housing, which is that round black thing held on with three screws holding a clamp. Remove that and, inside, you should see a coiled spring like a great big watch spring that has a loop on the end of it, and, there will be a little lever thingy inside the part of the housing that is part of the carb itself. That lever should be inside the loop. Make sure that this is what is actually happening inside yours, and then twist the housing in the direction that tightens the spring to where it will hold the choke closed, but you can pretty easily push it open,and it will close itself when you let go of it. Try that and let me know what you find out.

turbo2015 said...

Veronica,
Thanks for the great information. My '66 Mustang carburetor is leaking at the bottom of the accelerator pump system assembly, so I plan to replace it. My question is this: considering that there is fuel in the carburetor, do I have to worry about fuel spilling out everywhere once I unscrew the four screws and pull back the diaphragm? I'm a bit concerned because it seems dangerous to have a bunch of fuel go all over a heated up engine.

Veronica said...

A bit fuel will come out, but, it won't be much. If it was me, I would go ahead and pull the carb off, though. It isn't a big deal at all, and can be done, quite literally, in a couple of minutes. That way, you could also go ahead and give it a good cleaning. Assuming that you got the new diaphragm in a kit, you could also replace the base gaskets, power valve,etc... will you have it off.

turbo2015 said...

Thanks! I'm pretty nervous to do any work on it other than very basic work, in part because I don't generally know what I'm doing under the hood, and in part because I don't have a good work space (I keep it at a storage unit which doesn't want me tinkering with the car there). I'll try to wedge a small bucket under it to catch any fuel that runs out, and I'll just unscrew the old one and screw on the new one. Seems fool proof, but that's exactly the sort of thing I'd say right before screwing something up. Thanks again. -Turbo

Karim Belhacene said...

is it you on the magazine ?

Veronica said...

No, the media people haven't exactly been beating on my door to do a feature article. I haven't heard from Jimmy Kimmel or Charlie Rose for the longest time.I guess they must have lost my number.

Rosemarie Colagrossi said...

Hi Veronica, I am having precisely the problem you mentioned with regards to gas gushing out of the vent hole on top of my carburetor. I have a 1968 Mustang Coupe with a 289 V8 and I'm running an Autolite 2100 rebuilt carb on it. I had a family friend rebuild it and he's had quite a lot of experience building this particular carb as he had a '68 Cougar with the same engine and carb in it (so he really knew what he was doing). He said the carb looked to be in great shape when he sent it back to me. I installed it and after about 1 - 1.5 min running it starts to bubble gas out of the vent hole on top of the carb. I've done a number of troubleshooting measures before taking the top plate off and double checking his work and inspecting the bowl. I double checked the pressure of the fuel pump I replaced the one I just bought to ensure it was not defective and I checked that the PSI was not rated too high (6.5psi). I replaced the fuel filter and blew out the lines all the way back to the tank to ensure there wasn't any debri running through the line causing a bad seal at the pin when the float shuts off the gas flow. I double checked for any vacuum leaks and found none. I replaced the gasket below the spacer that sits under the carb on top of the block to ensure all gaskets connected in any way with the carburetor were in good condition and had good seals. None of those fixed it so I popped the lid on the carb and replaced the needle and seat assembly. I also checked the float level to insure that it measured appropriately at 1/2" below the top of the bowl. Since the carburetor had just been rebuilt, it was clean showed no sign debri build up in the bowl and inspected it for any visible cracks at all of the entry and exit points for the gas and hardware. The float also operated appropriately seeming to indicate no holes or faults. Any other steps for troubleshooting you can recommend? I'm almost out of options and am tempted to just order another carb and shelve this one while I walk away to ensure I stay in love with my car! It sure seems like it has something to do with the needle or seat not stopping the gas flow as I've ruled out the pressure rating. I also adjusted the idle up and down to see if changing that would affect how fast the gas was fed out to no avail.

Veronica said...

Sorry this took so long, I've been a little under the weather. There is a way to determine if the needle and seat assembly is operating the way that it should be, but, you have to be extremely careful when you do this. Pop the top off of the carb, and then start the car. Be ready to shut the car off instantly. if fuel starts running out over the sides of the carb, then the needle and seat assembly are not closing off the fuel flow. If that assembly is working correctly, the fuel level can't exceed the level that the float dictates. It's pretty much just that simple. If the carb holds te level dictated by the float, then the needle and seat assembly are working fine, and you have something else going on. But, whatever that is, it will become immediately obvious with the car running while the top of the carb is off. Again, be very, very careful when you do this.

Dusty Smith said...

I have a 66 Mustang 289 V8 with a 2 barrel on it that the choke seems to be doing the opposite of what it should be. The car sits over night and the choke is all the way open. After driving the car and pulling back in the garage the choke is closed. Any suggestions where to start? Thanks Dusty

lou's 66 said...

My 66 I6 200 ci. 1 barrel carb. automatic has problems staying on when its in drive and reverse. It idles fine, revs-up fine, goes into gears fine and coast fine in drive and reverse. When I give it gas in those gears is when it dies. Also, if I stop to quickly in those gears, it dies.
2 weeks ago it ran fine. I replaced the transmission modulator, pcv valve, fuel and air filter and tested the Distributor Vacuum Control. I checked for vacuum leaks, nothing. I even shook the car when it was on, in drive, to see if the carb. would choke and cause the car to die, but it idled fine thru the shaking.
I recently read that I might have the wrong carb. this whole time. Its a single barrel with the Dashpot in the front and one accelerator pump on the driver side.

Bryan Rowley said...

OK, I can't find any information on this and it just bothers me alright? Anyway, I stumbled on a '66 autolite 1.08 while going through my pile. Where this carb differs from my others is the two vacuum secondary tubes instead of one. Also, the second vacuum tube is on the secondary side going into the booster just like on the primary. What is the purpose of the second tube when all of the others function just fine with one? Wouldn't this create a vacuum leak for the tube on the primary side and not open the secondaries? Ford engineers clearly know something I don't. A few experts seem to know as well but they ain't tellin'.

Bryan Rowley said...

I don't know if my comment posted so I'll try again. I have a '66 1.08 4100 with two vacuum secondary actuation tubes. All of my other 4100's have only one tube on the primary side. But this one has an additional tube on the secondary side as well. What is the purpose of this extra vacuum tube. Since it's on the secondary side, wouldn't this cause a vacuum leak for the primary side tube and not open the secondaries? This seems unique to '66 mustang 289 non hi-po only.

Michael Leider said...

A few years ago had a mechanic rebuild the 4100 carb on my 66 Mustang GT while doing a new head gasket and a valve job and electronic ignition. Got the car back and noticed really poor MPG - like 6-7 MPG. I rechecked the carb myself. Put in a new power valve and accelerator valve just incase - although they looked okay. The new floats looked fine also. Down jetted it a little but had same problem. Seemed like gas was disappearing while it sat in the garage. So I filled it - went on a long drive and filled it up again same day and the MPG was super good 16.6 to 18. Repeated that 3X with the same good MPG results. But again after many short drives, mileage sucks 7-8. Put a glow green-under-a-black light leak detector. Only thing that glows is around the hole on the top back of the carb. I put the dip stick under the black light and I had a green hue - not exactly glowing - not sure if it from the leak detector. I've put a new gas tank, fuel pump, filter. It's a little hard to start but it runs so good when warmed up that I'd hate the change the original carb. I just read in another blog on a similar problem, the answer suggested the accelerator rod was too short. Do you have any suggestions.