Are you sure that your throttle isn't hanging up some way?

 

If you still have part of the computer system hooked up, it has a little DC motor that is bolted to a bracket on the carb and the computer runs this in and out to constantly adjust the idle speed. More computer control. When I was having problems I backed this idle control motor with the adjustment screw way back so it was not touching the throttle arm. I still had the high idle. That's when I checked the timing and found it was way advanced. So what I did was pull the spout connector, instantly the idle speed dropped. The spout connector is what you disconnect to set the timing on your engine, and it takes the timing control away from the computer. That's when I knew my high idle was the computer advancing the timing for some reason.

 

I had a similar issue with my son's 1984 4.9L with feedback system.

https://www.ford-trucks.com/forums/1...expensive.html

 

I tried Scotty's cigar (pipe tobacco) method, and I noticed that there was smoke coming out at the rear base of the carb, in the area circled in the picture below. The idle also drops when I spray carb cleaner in this area. I thought it could be one of the gaskets in that area, so I pulled the carb off and did an inspection. Everything looks fine, and the gaskets are new. I made sure all the screws were tight, and put the carb back on and bolted it down tight. No change, still idling high. Is it normal for there to be a 'leak' back there, around that spring looking thing?

 

I'd like to test this out to see if I'm having a similar issue. What do you mean be 'spout connector'? Is that a vacuum line/hose?

 

The red circled area would be at the end of the throttle shaft, which is likely worn, causing a vacuum leak. The shaft goes all the way through the carburetor. Take your fingers and see if the shaft can be made to wobble back and forth. I bet it will.

 

Spout connector is a single wire connector off the distributor wiring.

 

It should look like this:

 

I just realized something. When I had the carb off, it smelled weird. It was a familiar smell, but out of place. I couldn't figure it out. I just realized I was smelling gasoline mixed with pipe tobacco.

 

Doesn't seem to wobble. Do think this leak could still be the issue? Is there a gasket or something around this shaft that seals it up? I mentioned earlier that when I start the truck cold, the idle sounds fine, but as I give it throttle, it kind of stays up in RPM as I let off the gas. Maybe this is related?

 

There is no gasket around the shaft. The shaft passes through the carburetor base and over time, the tolerances increase [wear]. The fact that spraying carburetor cleaner in that area changes the idle speed, indicates there is a leak. The engine, when started cold, should idle high due to the choke and high idle linkage. As the engine comes up to operating temperature, the idle speed should drop to 550-700 RPMs. When there is a vacuum leak, idle speed will be higher than normal. Vacuum leaks are more pronounced at idle. As the truck is driven faster, the throttle plate opens and the engine vacuum is much less, so a leak is not noticed.

 

The issue I'm having is very pronounced, and happened overnight. Is the shaftwear likely to occur in such a fashion that it would cause a sudden leak? Can I fix this by replacing parts?

 

Shaft wear doe not occur overnight. It is something else... something has been disconnected. Go back and check the last thing you have done and look for something loose.

 

So, I did the same thing as you with this idle control unit (I think). It has a little button that protrudes, and it was way out, so the manual idle screw wasn't anywhere near the throttle arm. I think this was preventing the throttle plate from closing. I backed this computer control all the way out, so it's using the manual screw now, threw the carb back on, and the idle is functioning normally. I had to make some adjustments to the screws as it was now idling low. The only other thing I did was flip over the main gasket, and I'm assuming that isn't what fixed the idle as the gasket looks good on both sides.

Now I'm wondering, can the carb function fine if I disconnect all the electronic controls, but don't actually replace any of the other components like distributer and such? The electronic choke assist has been disconnected since I bought the truck. Can I remove the EGR and other electronic components, and plug up any holes that might create? I'm not required to pass emissions.

I'd still like to convert the distributor/ignition and get rid of the computer, but it might take a while to get the parts together on a budget. I'd like to start deleting everything I can with the current system, and clean the engine bay up, getting rid of as much wiring as possible.

 

You can disconnect most everything, and it will run. It will get lousy fuel mileage and power will be down, but you can get by with it till you get some money saved up. Might as well do this in baby steps in case you have problems.

The computer looks at all those sensors, and makes decision on the timing of the engine. The distributor has nothing in it but a timing wheel and a rotor for the cap. The module made onto the side of the distributor takes the info from the computer and constantly is changing the timing. Lucky for you, if it can't get any signals from the computer, it has a limp home feature and just locks the timing to base timing so it will still run.

The distributor you need to swap in is a older style, that has weights and springs inside it that vary the timing mechanically. That's what you eventually need to install.