I know this is pretty common, as I've searched. Mind you I've gone the procedure listed here ('73 F100, 360/4spd, MC2100):

https://www.ford-trucks.com/forums/7...ml#post6422219

I've adjusted the carb, installed it, and loaded the choke. Truck fired right up and idled perfect on the high idle setting. Once I let her warm up a bit, I blipped the throttle, and the truck won't stay idling by itself at low idle.

-I haven't adjusted the idle mixture yet (because I can't get it to idle)
-Doesn't appear that the choke is opening enough, but that could be that my new electric choke isn't operable (I set the cap at 1 Rich). However even with the choke plate held open, it still won't idle.

Thoughts?

Thanks again and sorry for all the posts.

-MK

edit: it just occurred to me that when I put the accelerator rod back in the throttle linkage according to the rebuild sheet with the kit, it was in a different hole (2nd from bottom) than it was when I disassembled. Maybe I just need to adjust the idle screw at the throttle linkage?

 

How warm is "a bit?" Note that the choke generally shouldn't open until the engine is on its way to operating temperature - otherwise the engine will idle poorly with the choke open. The fast idle mechanism should be engaged the entire time the choke is opening; it should not kick off until the choke plate stands completely vertical. If your choke isn't opening all the way, yet your idle is dropping off, then the index is not set up properly.

To get the idle mixture in the ballpark, gently turn both screws in, then both out 2 full turns. This will be close enough to take them out of the picture (if the engine won't idle at operating temperature with the screws out at 2 turns, then they are not part of the problem). Once you get running you can dial them in using a vacuum gauge.

The choke plate should be opening on its own without any assistance. If this is the original factory choke with electric assist, the electric assist is not intended to open the choke on its own. The choke has to get to a certain temperature using the hot-air stove, and then the electric assist kicks in - giving it an extra "push." This was an emissions mechanism. The primary heat source for the factory choke is the hot-air stove, electric assist or not.

It sounds like you've got a few choke and fast idle issues to work out first, before you can look into why it won't idle when it's warm. It's not possible to diagnosis an issue when other underlying issues still exist, because you can't isolate anything.

The accelerator pump rod you mention is not related to the idle circuit; this rod adjusts the stroke of the pump, controlling the volume of fuel "shot" into the venturis when you accelerate. Just set it to whatever the rebuild sheet says.

 

-So the engine was on it's way to operating temp, but I didn't let the fast idle kick off on its own, I blipped the throttle to take it down. Maybe I just didn't wait long enough. I only waited a couple of minutes.

-I noticed that the choke plate wasn't opening past about 1/4" or so. Maybe I just didn't let it warm up enough, or maybe my choke isn't working properly.

-I originally had a thermostatic choke with no electric assist. I purchased an electric choke assembly from Mike's Carburetor Parts, and wired the single terminal to the back of the alternator. Maybe I should just stick with my old manual choke setup until I get the carb dialed in.

Thanks for your help, this carb was hacked together by previous owners, and I'm just trying to get it dialed in with a working choke.

-MK

 

This is likely part of the problem. The engine is not ready for curb idle until it's warmed up and the choke opens on its own.

Depending on how long the engine has been off (as in overnight) and climate, it can take longer than 2 minutes for an engine to reach operating temperature and not require a rich fuel mixture.

Either one is possible. If the engine hasn't warmed up enough for the choke to open, it's not ready for curb idle.

Is it a stock replacement, or aftermarket fully electric? If it's the stock replacement, it won't open without a hot-air stove.

 

This is the electric choke I ordered:
Motorcraft Electric Choke Thermostat

Not sure if it's full electric or not. Can you point me towards a full electric aftermarket replacement? I'm thinking that's probably the easiest and most reliable solution other than hooking back up my janky manual choke (the autozone manual choke kit SUCKS).

Tomorrow I'll start it up again and let it run for 15min or until it warms up by itself.

-MK

 

So looking at the electric choke I bought, I'm almost positive it's the electric assist.

I purchased this full electric conversion from eBay:
eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

Hopefully that'll clear it up. The choke isn't opening at all once the truck warms up. It's gotta be the choke right?

-MK

 

Yep, the choke shown in the first picture is factory electric assist. Those work great; they just require the factory choke stove to be in place. I'm assuming yours is long gone? Dorman used to make a replacement kit; not sure if it's still available. Some make their own out of brake line.

I don't have any experience with the particular fully-electric shown in the second link, but based on the item's description, it appears to not require the choke stove, which I understand to be your intent.

It's not possible to prove at this point that the choke alone is the cause of your problems; you could very well step through this exercise and still have issues once the truck is warm. However, in describing the problem, you pointed out several troubling observations about what your choke is doing while the truck is trying to warm up. My recommendation is to solve those problems first so that you can step through the problem one issue at a time. Otherwise you don't have any way of knowing if you're having issues because the choke should be on but isn't, or shouldn't be on, but something else is going on (like a vacuum leak).

 

Some other things I've noticed:

There is a vacuum leak somewhere because I can hear it hissing when the truck runs. There was something that was plugged when I took it apart and I forgot to re-plug it. I'm going to try that tomorrow. The truck runs and drives just fine, but has a hard time returning to idle once you let off the gas. It also dieseled for the first time ever today, so I'm sure I don't have it set up right.

I'm going to go back to 2 turns out on both idle mix screws, plug the leak, and start over again.

-MK

 

If the vacuum leak is loud enough that you can hear it, then it's highly likely that it is contributing to the problem. Dieseling can be caused by a vacuum leak (like the carburetor not being bolted down completely) or high idle, but it can also be caused by hightened combustion chamber temperatures - which itself is often caused by a lean cruise mixture or malfunctioning EGR system - things that make the engine ping at cruise.

 

There was no vacuum leak, the cap is still in place over the port I was thinking about.

I set the idle mix back to 2 turn out, and by adjusting the idle speed screw at the throttle linkage, I can get it to idle pretty decent. However the choke plate is still only about 1/4" open. I'm assuming once warm, the truck should idle and run fine with the choke plate completely open (vertical) but it doesn't. It does run and drive great the way it is, however I know it's not right. The return to idle is still a bit funny, and the high idle is probably a little too high. At this point I'm pretty convinced I have no idea what I'm doing.

I think I'm going to take my chances and purchase that electronic choke kit even though my particular application is not listed in the applicable vehicles. If it works for a multitude of over engine and vehicle combinations all with MC2100's and thermostatic chokes, I'm pretty sure it won't be much different for a '73 F100 with a 360. I can't imagine the chokes being that different across the board on the same basic carb.

So essentially, I can get the truck to run, idle, and drive pretty good, but I'm still pretty convinced it's not tuned optimally.

-MK

 

You're correct; once the truck is at operating temperature, it should be able to idle on its own with the choke plate essentially bypassed. If this is not the case, then the choke plate is compensating for something by enrichening the fuel mixture. This is often because of a vacuum leak. With the engine idling, take a can of carburetor cleaner spray and shoot it at suspect areas, like the brake booster check valve, carb base, etc. If the idle speed jumps, then this points to a vacuum leak at that location. Sometimes, to make the test work, you have to lean out the mixture screws to make the engine stumble and just barely run, so that the idle jump is noticeable.

 

I'm thinking about just hooking back up my damn manual choke and going from there. At least then I know that the choke is working. Plus I've replaced the originally broken fast idle cam arm (nylon), so I should get fast idle too.

-MK