My 2100 carb wants to stumble/die just as I leave idle. If I lift on the pedal or if I go harder on the pedal it will recover. Seems to get a little better after the engine is fully warmed up.


I've adjusted the idle mixture screws for max vacuum at idle. The idle rpm in gear is about 650-700. Idles steady and seems to have plenty of power just stumbles when you ease it off idle. The accelerator pump is not leaking and the spray shot looks good.


Any ideas what is causing the stumble?



72 F250 with a 390 & C6

 

Have you checked for vacum leaks?

 

It doesn't have an EGR, does it?

Does it have the original points distributor? Does the vacuum advance work?

If you have points, get a dwell meter, hook it up and activate the vacuum advance by hand (mouth). As you apply vacuum to it, does the dwell change a lot? Could be a bad distributor upper bushing causing the points gap to change as you apply vacuum advance.

Or, the vacuum advance isn't working at all...

 

If your accelerator pump is ok, not clogged, I would look for vacuum advance problem on your distributor. Maybe a loose/sloppy vacuum advance, or its sticking.

 

Have you checked for vacum leaks?


- No vacuum leaks that I can detect.




It doesn't have an EGR, does it?
Does it have the original points distributor? Does the vacuum advance work?
If you have points, get a dwell meter, hook it up and activate the vacuum advance by hand (mouth). As you apply vacuum to it, does the dwell change a lot? Could be a bad distributor upper bushing causing the points gap to change as you apply vacuum advance.
Or, the vacuum advance isn't working at all...


- No EGR.
- Distributor was rebuilt with new points, condenser, cap, rotor and wiring.
- Dwell is normal, 28 to 29 degrees.
- Vacuum advance works.


I'm leaning towards the idle mixture screws and/or idle RPM setting. Going to play with them again soon and see if it makes a difference. Thanks for the replies.

 

Have you put a vacum gauge on the truck and what's rhe reading?

 

Have you put a vacum gauge on the truck and what's rhe reading?

Fully warmed up, ~20 inHg at idle.

 

Had the exact symptoms you described. My initial timing was set at 6 btdc.
It is now set at 17 degrees btdc with a total mechanical advance of 38 degrees. This 390 never ran so good.

 

Ditto... the factory specs of 6-8 btdc make these motors run like a dog (it's an emissions/warranty kinda thing)

Even in slightly juiced up form these motors like 16-18 degrees of initial.

Just keep an eye on the mechanical....don't exceed 38 total.....but FE'S like their timing up front, not on the back end.

 

I wonder if he got this figured out? I got the exact same symptoms.

 

I had the same thing on my carb. What I figured out is when I barely push on the butterfly on top of the carb while the truck is running it'll fix the idol problem and it'll run fine. If I let go of it it shortly dies. I gave up and bought a whole new carb.

 

Nope.
Still stumbles a little.
Going to try a new carb this spring/summer.

 

I'm going to go try and find a carb today at the boneyard and rebuild it myself. The carb on it, is new..... but I've checked every other thing it could be and CARQUEST wont give me another. It doesn't have 10 miles on it but whatever (before I go into a rant).......no more of my money will go to Carquest

 

Well that's not good I just ordered my carb from carquest.

 

I found in the long run, with my engine anyway, it was cheaper to just buy a new updated carb from Holley. Hardly had to make any adj. out of the box to run perfect. Save the original if I ever sell it.