Hi all, my first post here. My daily driver is a 1981 Ford F-150 that my brother gave me about a year ago. It's got a 300 straight six, and a 4 speed OD transmission.

I'm having some trouble, and am hoping to get some help. I'm new to working on vehicles, but am learning a lot over the past few years since I started driving, especially because everything I drive seems to break down often. So hear is the story:

Early last year I had a local shop rebuild the carb on this truck. It's a Carter Single Barrel, YF. Or is it a YFA? Not sure how to tell. They rebuilt the whole carb, and machined the throttle body, as the throttle shaft had a lot of play in it.

Right away it seemed to run fine, I've just always had problems with the choke. I'd hit the gas once to set the choke, start it, and it would be idling WAY to high until I kick it down. I don't have a tach so I'm not sure how high. Anyway, I had them adjust it a couple times, but it was still too high. Long story short, I didn't end up taking it back for an adjustment until a month of so ago, because the warranty only lasts a year. After there adjustment, for a couple weeks it was great! I'd hit the gas, start it, and it would idle just fine and warm up fine. Then the cold idle started to creep back up to being to high, so I took it back to the shop again. They messed with it for a couple days and said that they backed down the cold idle as far as it would go, but it was still at about 1800 RPM.

I got it back and it still is too high, feels higher than 1800. So I called them and asked if it could be something else. They said it could be a vacuum leak. I got out there today and sprayed starter fluid around the intake manifold gaskets and the base of the carb, and around the EGR. No change in idle. I let it fully warm up and hooked a vacuum gauge to it. It registered about 17, and the needle jumped around a lot, between 15 and 18. I revved the engine up slowly, and the vacuum increased, but was still real jumpy.

So my question is: Is the fast cold idle and the low/jumpy vacuum two separate problems? Or are they linked in some way? Does the vacuum reading indicate a valve train problem? I have been hearing a ticking noise from the engine the last few months while it's cold, it eventually goes away once it's warmed up though.

One more thing: the EGR is not hooked up. Not sure what effect if any that could be having...

Any help will be greatly appreciated. This is my only transportation right now, and I'm going out of my mind trying to figure out whats wrong.

Thanks in advance!

 

Man, it's been a while since I fooled with a YF, but fast idle is usually controlled by a second idle screw that engages the throttle only when the choke is closed. Eighteen hundred rpm is too high, but not ridiculous - 1,200 - 1,500 is probably stock (check the emissions sticker under the hood.

Here's a link to a vacuum diagnosis chart:

Reading your vacuum gauge

I'd start by checking all the plugs, plug/coil wires, distributor cap and rotor. A lot of "carburetor" problems stem from the ignition system.

 

Yes it is controlled by a second screw. It's adjusted as far down on the fast idle cam as it will go. That's what the shop told me anyway, I will double check that.

You're right about the stock fast idle range. I would prefer it be around 1200 though. I had no problem with this before they rebuilt the carb, so it's either something to do with the carb or a new problem.

Thanks for the link. I'll check everything you mentioned. How could I go about testing the coil? Could I test it with a multimeter?

Thanks for the help!

 

By the way, welcome to the forum!

The fast idle screw is located to the right and lower than the screw which controls the "curb" idle. Reading what was written above and highlighted in red, to lower the speed, the screw should be backed out rather than tightened as far as it will go. Or if you meant that it was on the last step of the fast idle cam, the placement of the steps on the fast idle cam are controlled by the position of the bimetallic spring located in the choke housing [black plastic and round]. Once the choke is completely open, the fast idle adjusting screw should not even touch the cam, as the cam will have rotated out of the way.

If the choke is out of adjustment, the fast idle cam will be in the wrong position and can cause the engine to idle fast, even when at normal operating termperatures. If this was the case, I would imagine that the engine would be surging and that would cause a fluctuating vacuum reading on the gauge.

I find that where I live [we have 4 distinct seasons] the choke has to be adjusted 4 times each year. Is it possible that your carburetor choke adjustment is off? Do you know how to adjust it [easy!]?

Hope this helps or perhaps causes someone else to come up with the answer!

 

Thanks!

Yes that's what I meant, the screw is on the last step of the high idle cam. I had no idea that the choke adjustment would affect the fast idle cam like that. I don't really know how to adjust the choke.

I've thought about installing a manual choke. It seems like it would be simpler. What would be the pros and cons of a manual choke?

 

The "pro" about a manual choke is that you have absolute control over the choke setting; the "con" is that most people would not know how to operate the choke.

Adjusting the choke is an easy task, but may take several attempts over several days. One morning, when the truck has sat overnight, remove the air filter housing. There likely will be one vacuum hose, the PCV hose which goes to the valve cover [provides replacement air to the crankcase] and the flexible tubing which directs hot air from the exhaust manifold to the intake snorkle to disconnect. Make a mental image [or draw / take pictures] so these can be replaced when finished.

With the engine NOT started and no key in ignition, press the throttle slowly to the floor and release. Then, look down from the top of the carburetor. The choke plate, located at the top of the carburetor opening, should be closed. Some carburetors have a pull-off which opens this plate just slightly as soon as the engine is started. Earlier carburetors or ones which have been altered do not have this feature and then the choke plate is to be adjusted such that there is a slight opening [~1/8"; some people have suggested inserting a 1/8" or 3/16" drill bit as a guide. Be very careful not to drop the bit into the carburetor!!!].

If adjustment is required, first look on the choke housing which is located on the back side of the carburetor. It is round and has a black plastic cover, held in place by 3 screws. Some later carburetors had rivets instead of screws. The rivets can be drilled out and replaced with small bolts. Before doing anything, look on top of the choke housing for some lines used in measurement. Using a sharp pointed object [i.e. nail], make a mark to indicate where you began, then loosen the three screws just a little so you can rotate the black plastic cover. Turning one way will open the choke plate [more lean] and the opposite will close the choke plate [rich]. Note how many marks are passed in making the adjustment which will help in determining the next adjustment, if another is needed.

If the choke is properly adjusted, after the gas pedal is slowly depressed, and the ignition key turned to start, the engine should start and be on the fast idle. As the engine warms, the metal tube which is connected to the choke housing [covered with a white insulating sleeve] transfers hot air from the exhaust manifold to the choke. At the same time, the electric wire attached to the choke housing provides power to a heater inside the housing. This electricity comes directly from the alternator and only when the engine is running [~7 volts]. These two sources of heat cause the bimetallic spring located inside the choke housing to uncoil, thereby opening the choke plate. This way, the engine gets a richer mixture when cold [first started] and leaner once it reaches operating temperature.

Once the choke is adjusted, if the fast idle speed is incorrect, the high speed screw [right side and lower of the 2 screws] can be turned to adjust. Turning to the right will increase the speed. [Be careful of the fan and moving accessories!!!]

As the engine warms, a gentle tap of the accelerator will allow the engine to idle down. Once fully warmed, the choke plate should be fully open and then the curb idle speed can be adjusted by the idle screw [upper left of the two screws]. Again, turning to the right will increase the idle speed.

I hope this makes sense.

 

Thanks for taking the time to write all that, I had a rough idea of how the choke works, but that clarifies it a lot.

I'm seriously considering installing a manual choke. I'm the only one who drives the truck, so as long as I know how to use the choke it'll work fine. I called the local NAPA and they have a manual choke kit for the YF for $10-15 depending on the cable lengh. Am I correct in thinking that it won't require any under-the-hood adjusting once it's installed? Just adjusting from the cab every time the truck is started cold?

Thanks the help!

 

The conversion kits are a catastrophe. There's a cheapo plastic cap with an equally cheap lever that replaces the electric choke. If you want a manual choke then take your carb and swap it for a manual version. It will work much better in the long run.

That kit comes with an ell shaped bracket to screw in under the dash. If you don't want to drill a permanent mounting spot somewhere that's about your only option.

 

Thank you for your input. There's no way I'm replacing this carb though, it was rebuilt recently. I don't mind bolting it under the dash. I think I'll stop by NAPA and check out there conversion kit and see what it looks like. It sure don't seem like it could be much worse than the current setup...

 

This is what is wrong with the manual choke conversion. The exhaust heated automatic choke has a vacumn passage built in to help pull hot air from the exhaust across the choke coil. The manual conversion does not seal off this vacumn, so you have a constant vacumn leak and this will make your idle problem worse.

Try adjusting your timing this can effect idle speed also.

You need to put a new EGR in, Unless you have done extensive rework to the distributor and changed to a non-EGR carb. Un-pluging the EGR will cause the cylinder temps to go sky high (not to be confused with engine overheating) and burn valves and piston rings.

 

True, there is a vacuum port inside the choke housing which draws the heated air across the bimetallic spring, but the original system also is a controlled vacuum leak. Clean filtered air is drawn from the very top of the carburetor throat. What is different is that the air is heated and filtered. From what I understand, it is true that the carburetors equipped with climatic choke are set up differently to compensate for the small vacuum leak.

 

Ideally each cylinder will produce the same amount of vacuum, therefore vacuum gauge reading should be steady. If one or more of the cylinders produce more or less vacuum than the others, the needle of the gauge will fluctuate. For example, if the reading on the vacuum gauge fluctuates between 10 and 17 in. Hg , you need to observe the rythm of the needle. If the needle seems to stay at 17 most of the time but drops to 10 and quickly rises, we know that it is probably caused by a problem in One cylinder. Fluctuating or low readings can indicate many different problems. A low, steady reading might be caused by retarded ignition timing or incorrect valve timing. A sharp vacuum drop at regular intervals might be caused by a burned intake valve.

I would first make sure the ignition timing is correct. Also be sure the valves are adjusted properly. I looked at a vacuum gauge diagnosis chart and seems the closest one I can fine in the fluctuation of 15-18 range falls under "Sticking Valves". Makes sense to me as you said it was a fairly intermittent problem.

Good luck.

 

Thank you all for your help so far.

I went to NAPA and looked at the conversion kit. It looks ok so I bought one.

Anyway I finally got a chance to install it today. The installation went fine, and I got the cable choke working fine. I stated the truck, which was still warm so it didn't need the choke. In my process of verifying the choke worked, somewhere along the line the choke stopped closing at all.

I press the gas pedal down, just like I did before, and pulled the choke cable out, except this time the choke won't close at all. I removed the choke cable, even took the choke cap off all together. I checked the fast idle cam, and as long as the throttle is pressed down, it moves freely, so only the choke won't move. It moves slightly, but seems to get stuck when you try to close it.

I don't understand! It worked fine with the cable at first, then all of a sudden it won't close at all, even with the gas pedal to the floor. Am I missing something? How is this possible???

 

PROBLEM SOLVED!

So I pulled the carb off the truck, and looked inside the choke house and found metal shavings. I could tell that the reason the choke was jammed up was this little counterweight deal that holds the choke open, which has it's own little cylinder inside the choke housing, was stuck.

I called the shop that did the re-build to ask what could have caused the metal shavings. They said either the counterweight was disintegrating, or the metal heat pipe that comes off the exhaust manifold has flakes chipping off the inside. I now remember removing a thin metal plate that covered the opening of that pipe under the choke cap. This was holding back the metal shavings.

So I cleaned out all the metal shavings and got the choke to move, I even took the whole choke plate and rod out, so I could take the metal weight out all together and clean it out. The little weight was in perfect shape. Then I took the heat pipe and cleaned it out, and disabled it by crimping it very tight. I don't need it for the manual choke anyway.

I put it all back together, and hooked up the manual choke, and it works great! I was even able to adjust the fast idle down to the right speed!

In this whole process, I now understand very well how the fast idle and the choke work together.

I haven't put a vacuum gauge on the manifold to see if it has smoothed out, but I'll do that next.

As for my opinion of the manual choke conversion kit that NAPA sells, I think they are great! They have detailed instructions, a nice long cable, and all the brackets and screws needed for the installation. They also include replacement choke caps. The only problem was the L brackets they included were cheap, and I ended up bending a nice sturdy piece of metal, and mounting it through one of the bolts for the throttle bracket. All in all, it's much better than any automatic choke, and as long as it's installed correctly, it works great!

For now, I've solved the immediate problem, I'll put the vac gauge back on there in the next few days, and see if anything changed there. I'll report back once I do that.

Thank you all for your help!