Hi, I have a 69 F100 with a FE390 that is running with a Motorcraft/Autolite 2100 carb. Not a huge problem, but the engine is prone to stumbling and sometimes cuts out until it is fully warmed up. This is only a problem when pulling away from stop lights or junctions or if trying to accelerate from a very slow speed.

For the first minute from cold it is fine, presumably whilst the electric choke is still on, but the choke opens fully very quickly and from then it starts the stumbling. The stumbling gets less and less as the engine warms up, but is still present for around 15-20 mins driving. When fully warm it accelerates very smoothly.

Any suggestions on what I should be trying to address this?

 

You have the answer. The choke is opening too soon. You can put a resistor in series with the wire to the choke to lower the current in the line and slow the choke down, or convert to a manual choke.
To add a resistor: Measure the resistance of the choke coil cold.
13.8V/Coil Resistance = Current.
For half the current, add the same cold resistance as the coil in series.
Power = Current*Current*Resistance
Pick a resistor with a wattage 1.5X to 2X this calculated power (Watts). It will probably be one of those rectangular ceramic jobs.
Or just convert to manual choke. I love my manual choke. The conversion kits bolt right on to where the choke coil goes. The factory manual choke version had a little tang that would also crack open the throttle plates a little bit.

 

I'd check for any air leaks and on a separate issue,,,,, when I had the 390 in my 68 I had various issues but I pulled the points/condenser out....threw them as far as I could and installed one of the pertronix ignition modules. They are awesome.

 

Thanks for the replies.

I thought it might be the choke opening too quickly, but usually that's accompanied by a choppy low RPM idle, but I don't get this....the idle remains smooth, though perhaps a tad uneven.

As for air leaks I did check earlier today and everything seems good. I was able to give the carb to manifold nuts maybe an extra half turn each, but that was it. All the vacuum hoses and connections appear good. I hope it's not a vacuum issue as I did have to remove/refit the inlet manifold when I first got the truck to sort out another issue and that wasn't a pleasant job. I was careful to clean up all mating surfaces and used the proper gaskets with some decent silicone so I'm hoping that's not the culprit. I didn't really drive the truck before doing that so am not sure if the stumbling was there before. But, if it was a vacuum/air leak then why would the stumbling go after the motor warms up?

 

If there are any old vacuum hoses under the hood replace them.

 

From what I can see there are three vacuum hoses. One from the carb to the vacuum advance unit, which I have already renewed, another going from the outlet on the rear of the manifold to the transmission, which I've already changed and one going from the same place to the brake booster. I haven't changed that one but will now do so.

 

That's the one that was leaking on mine. It was cracked right at the elbow on the intake.

 

2X. The choke opening too soon. Sounds like you have a 12V re-wire job going to the choke. Try hooking it up to the Stator terminal on the alternator. Ford did this on some vehicles. The stator terminal will only give about 6 volts.

 

Yes, you got it. Thank you!

When I first got the truck I spotted that the choke wire was attached to the coil, which I've heard is not a good idea, so I hooked it up to one of the fuse box switched 12v feeds. I've now connected it instead to the stator terminal on the alternator that you mentioned. This is supplying between 7 and 7.5v and the choke took a lot longer to fully open when I tested it. This was with the truck idling in my driveway (to make sure the choke would fully open) so I don't know if it will have helped the stumbling. But I guess it will have and I'll find out for sure when I try driving it from cold tomorrow.

 

Sounds good yaba. "It's good to be the King". Let us know on the stumbling also. We like closure.

 

And to be clear, these are not the 'get in it and go' type vehicles we have today. Even well tuned they are a little sluggish when cold. Nature of the beast.

 

Hi, unfortunately the stumbling is still there, so still some thinking to do. I do acknowledge what Rasputin53 is saying about how old vehicles will drive when cold as I've owned 60's cars/trucks and driven them regularly for the last few years. But I'm pretty sure this stumbling issue can be resolved.....just not sure how!

I spotted something on this post of interest....

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

The advice given was to connect together the two nipples shown in the photo. The equivalent nipples on my Motorcraft 2100 both just have rubber bungs over them at the moment so I will try connecting these together with some vacuum hose. I'm assuming this is how it should be?

 

How about a pic? From the big pic in that other post, the lower angled capped off port is where the choke stove pipe (from the exhaust manifold) used to go. That was to supply hot air for the bi-metallic choke cap to operate.
How it originally worked:
1. Clean air comes from the port hanging below the air horn through a tube to the exhaust manifold.
2. The exhaust manifold (or the spacer at the outlet depending) has a passage in it to exchange heat into the fresh air.
3. Hot air is sent back to the choke cap where it meets the bi-metallic spring and causes it to uncoil opening the choke. The pipe would screw on to where the large rubber cap is located.
4. There is a little piston that acts as a choke pull off to meter this action.
5. A small vacuum port in the back of the choke cap housing is the draw for this air flow.
That being said, the piston can stick very easily. If I was to replace the standard style choke cap with an electric one I would do the following:
1. Remove the piston. It now does nothing of use and can jam easily.
2. Block the internal vacuum port. There is no need for it as you are no longer drawing hot air for the choke cap and now it's just a vacuum leak. I use a tap and a set screw. Blow out the chips by putting your air source at the port at the base of the two venturis. That way the chips come out of the carb and not go into it. Don't do this mounted to the engine. (Well, you can't really)
3. Leave the rubber caps in place with the possible exception of the one on the fuel bowl. The one at the bottom of the choke housing will keep the choke housing from collecting crap; the one on top of the fuel bowl is actually a vent for the bowl and shows this to be a later carb than what came on your truck if you have this feature. You can tie that one back to the air cleaner. The correct carb has a small hole on top of a boss and a levered capping device connected to the accelerator pump linkage. Even earlier models (67 and before) just have a small hole. This works in conjunction with the two bowl vents that look like tubes sticking up between the air horn and the fuel bowl top to prevent any vacuum build up in the fuel bowl during cruising or acceleration.

 

Thanks for that info. As per your suggestion I'll take some photos later and post then in this thread. I know the carb I have is a rebuilt one that the previous owner bought off eBay as I have the invoice for it. It was actually listed as being for later model year trucks and I presume he deliberately purchased this one because it comes with an electric choke.

 

Here's some shots of my carb from a few angles.....it's only after looking at the shot of the rear of the carb that I can see what looks like a vacuum port/nipple that has not been either connected or capped off.What is that, another source of ported vacuum?