I'm having a bit of trouble with the 2100 on my '74 F100 with a 390 and C6. I adjusted the idle screws and idle speed on the carb today and this problem has started since then.

Every time I touch the throttle it immediately hesitates and kind of bogs down, before it picks up and accelerates fine (great, actually). It does this whether I'm sat stationary and move off, or if I'm already coasting then come onto the throttle.
Once the engine is restarted when hot, after sitting for about 20 minutes, it also seems to be idling really rough. Not sure if these problems are linked.

The carb was rebuilt 6 months ago, and since then I've fixed all the vacuum leaks I could find and replaced a bunch of things.
Timing is set to 10 degrees initial then about 36 total with vacuum advance disconnected. Vacuum advance is new and works and connected to ported vacuum source.
I adjusted the mixture screws to the point of highest vacuum (5 turns out, 19 in. vacuum) and set the idle speed. I did this with a hot engine and it was running as smooth as silk at idle at the time, even in gear. With the mixture screws only 1 turn out, the engine wouldn't run at all which seems to be a good sign.
I've checked the wet float height which seems to be fine.
The accelerator pump seems to give a good squirt when I pump the throttle with the engine off.

Before all of this the truck was running very rich at idle which (in my opinion) was probably disguising this issue if it's due to leaning out somehow.

Where should I look?

 

Could be squirting too much or still not enough. Try experiment with accel pump most have a linkage to adjust size of squirt. My guess is still not enough squirt

 

I wondered the same, but I don't think I can increase the size. On the throttle linkage end, the accel pump rod is on the top hole, and at the carb end it's the innermost hole.

 

Sounds like you've done everything right.

I'd set it to best rich idle and see if that fixes it.

If you cover the air horn with a rag does it speed up or slow down?

I'd put a new 4160 on it and forget it if you can't find the problem. there is a reason they sell lots of new carbs every year.

 

My concern is that the idle already seems to be set fairly rich - I had to turn both screws 5 whole turns out which seems like quite a lot, especially as everybody else says something about 2 turns. Any more than 5 turns and the vacuum started to drop ever so slightly.

I've not tried covering the air horn yet, will try next time.
I'd rather not replace the carb, and I'd certainly like to make absolutely sure that everything else is 100% before I think about that.

 

2100 two barrel AUTOLIGHT Carb? POWER VALVE diaphram is probably deteroiated
Power Valve on Bottom side if Fuel bowel should be replaced. repair kits getin scarce!
Vacuum leaks at base carb gaskets, vacuum leaks at intake manifold gaskets
Power Brake booster vacuum leak in Diaphram or hose, grommet to it. If need look at good info:
https://www.hotrod.com/articles/rebuilding-autolite-2100motorcraft-2150-carburetor/

Just some info on trouble shooting. Good Luck!

 

If you don't have a vaccum leak then it nearly has to be a lean condition in carburation. setting to best rich idle may not be the fix but it will indicate where you need to look to find your problem.

If you have a problem in your carb it can be fixed if you want to go to the effort. but if it's a normally worn 40 year old carb your really need to do it right and bush the shafts and everything else it needs. checking the power valve would be a good idea too. .

The only other thing I can think of is if you have your advance hooked up to manifold vacuum it will drop out when you open the throttle and the engine can try to die. but if you have 10 initial that shouldn't be an issue. manifold vac is fine as long as your distributor and settings are set up for it. but it can cause problems if they're not. you might check to make sure.

 

 

I agree with Tedstser9, most carbs I've ever worked on Edelbrock, Quadrajet, lawn mower and motorcycle etc. only require 2 1/2 or so turns out at the most..

 

Why do you have a 2V on a 390 in the first place ? was that even an option ? I'd get a 4V on it in any event.

 

Thank you all for the helpful replies. I'll try my best to reply to everyone!

13mm, the carb was recently rebuilt so the power valve should be good. It certainly doesn't drip out the bottom and the truck won't run with the idle screws right in. If anything, I think I'm actually having trouble with it being too lean, but only around idle.
I'm guessing I probably might have some more vacuum leaks that I've not yet discovered.

440 sixpack, I know a 2bbl is small for a 390 - this truck was originally a 360 but had the engine replaced in the '80s. It looks to me like all the original 360 accessories were swapped over. In any case, I just want to get what I've already got running right. The truck has plenty of torque and power for how I want to use it, and fuel is incredibly expensive over here in the UK!
The vacuum advance is on ported vacuum with delay valve, and works great (confirmed with timing light).

Tedster9, the plugs were previously indicating that it was running rich which is one of the reason I've been in and adjusted everything. I've probably not driven it long enough yet for the plugs to be showing an accurate reading for my new settings.
This trouble actually didn't begin after using the vacuum gauge - before I adjusted it the screws were actually about 7 turns out and it ran terribly! After adjusting it it now idles really, really smooth until it gets good and hot.
The hesitation problem is new but apart from that it drives loads better.

jade79, thank you for confirming. I think I'm now confident that my idle screws are further out than they should be, indicating to me that I've probably got air getting in somewhere else!


The thing I think I've concluded from all this is that I probably have some vacuum leaks, meaning that I'm having to wind the screws right out to get the mixture right.
Does that sound possible to you guys?

Thanks again for all the help!

 

 

Thanks for the reply Tedster9, I appreciate the help. I don't mind you double checking that I've done things right at all - I'm new to this so there are plenty of mistakes I could have made!

I will get out to the truck in a bit and check the casting on the carb bowl. I would like to add that the truck runs absolutely great once I'm off idle so I'm not too concerned that the carb isn't the right one.

What I forgot to mention is that the 19" of vacuum was taken at idle in Park. Should I take it in gear instead? I believe it dropped to about 17-18" in gear from memory. All readings seemed steady, but I've not managed to get a reading at the times when the truck is running rough. When I adjusted the carb the engine was warmed up nicely with the choke open, but it wasn't heat soaked like it is when it runs poorly.

The advance mechanism was previously completely seized and the vacuum advance not functioning. I've now freed the mechanical advance up completely and replaced the vacuum advance. That is actually the main reason that I was now re-adjusting the carb!
To measure the 36° total mechanical advance I marked the harmonic balancer where 35° would fall and the light shows that I'm just past that. Basically the same as using timing tape I guess.

The wet float height was checked by running the engine then immediately removing the top of the carb and measuring down to the fuel level. Initially I found it was much too high so I drained the bowl, adjusted the float and tried again. Immediately after running the engine the fuel level is now the exact diameter of a US quarter from the machined edge at the top, which is where I read it should be.
After this the truck did seem to be running much better in every way, apart from the hesitation.

EDIT: I forgot to add that I found yesterday that the hesitation wasn't nearly as bad if I hit the throttle hard. From a standstill if I mash the throttle the truck will happily smoke both rear tires.

 

 

I did actually try similar when I first adjusted the carb. I started with the screws one whole turn out, but like that the engine wouldn't even run. I moved them to 1.5 turns out and at that point it started and idled. The idle got progressively higher (as did vacuum) until 5 turns out at which point it stopped increasing.
At 1.5 turns it was idling nice and low, by 3.5 or so turns it was idling so high that the mechanical advance was coming in! I then adjusted the idle back down again and continued.

The point I'm getting to here is that my engine idles by far the best at 5 turns out which must be telling me, as you said, that something is out of whack.
Would I be right in saying that a couple of other small vacuum leaks could cause exactly this?

Thanks again for the help!