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Basic Diagnosis: Off-Idle Stumble....

Sacbill

Sacbill

Hard Swinger
Need a diagnosis on where to start to correct the problem of a bad off-idle stumble when my engine is still cold. Car will start up with no problem, but while it's cold (first 5-10 minutes) off idle acceleration bogs down. I also feel a bogging when moving from 1st to 2nd gear.

What I'm running: 302 with Edelbrock 1406 w/electric choke, C-4, 3.00:1 rear gears.

I'd like some ideas on the basic things to look at first, before I tear the entire fuel delivery system apart and install an electric pump. :rofl

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Currently playing on the Rock Show: Lucero - Nights Like These
 
Bill:

In my experience, a flat spot off of idle is often a function of a leaning out of the air fuel mixture as the throttle plate is opened up. An accelerator pump that is either slow to squirt additional fuel or not squirting enough fuel when needed can be adjusted to make up for that leaning out of the mixture. On a Holley carb, the linkage cam that activates the accelerator pump can be swapped out for one with a different profile for a quicker shot and the accelerator pump itself can be changed for one with a larger shot. I am not familiar with the workings of Edelbrock carbs so hopefully someone with that experience will chime in with Edelbrock specific suggestions.

Hope this helps,

Regards, Jeff
 
"Sacbill" said:
Need a diagnosis on where to start to correct the problem of a bad off-idle stumble when my engine is still cold. Car will start up with no problem, but while it's cold (first 5-10 minutes) off idle acceleration bogs down. I also feel a bogging when moving from 1st to 2nd gear.

What I'm running: 302 with Edelbrock 1406 w/electric choke, C-4, 3.00:1 rear gears.

I'd like some ideas on the basic things to look at first, before I tear the entire fuel delivery system apart and install an electric pump. :rofl

----------------
Currently playing on the Rock Show: Lucero - Nights Like These
Click to expand...

I have an Eddy 1407 on my car. Had a similar problem that was caused by a dry rotted plug on one of the vacuum ports in the front of the carb.

The carb still has problems but I'm not putting any more work in it. We went though it again last time Rick was down and it still ain't worth a crap. I'll get one of those Summit carbs coming pretty soon.
 
both are good places to start. If it's the accelerator pump, you can do a visual (not too many times) it should spray a nice even mist, not dribble.
If it's a vacuum leak, squirt some carb cleaner around while it's running, if engine speed races up in a certain area, that could be the leak.
 
"JeffTepper" said:
Bill:

In my experience, a flat spot off of idle is often a function of a leaning out of the air fuel mixture as the throttle plate is opened up. An accelerator pump that is either slow to squirt additional fuel or not squirting enough fuel when needed can be adjusted to make up for that leaning out of the mixture. On a Holley carb, the linkage cam that activates the accelerator pump can be swapped out for one with a different profile for a quicker shot and the accelerator pump itself can be changed for one with a larger shot. I am not familiar with the workings of Edelbrock carbs so hopefully someone with that experience will chime in with Edelbrock specific suggestions.

Hope this helps,

Regards, Jeff
Click to expand...

The Edelbrocks have 3 settings for the pump shot. They come set on the middle setting out of the box. It's a 30 second change to move it to another position.

I agree to check the vacuum fittings on the front, especially the one that blocks off the front PCV port. That's a big plug and doesn't last very long before it rots out and leaks. Cmayna pointed mine out to me a year or two ago, when the one on my '66 was shot.
 
Bill, let me know if you find a cure. My Eddy 1406 does the exact same thing. I haven't adjusted the pump linkage yet but it's a quick way to find out if that's the problem. I had a guy suggest a slight "flooding" so I bought the kit with the different needles, jets, yada but I'm not carb knowledgeable enough to really get into that. The first thing I need to do is figure out why the pump is "hung up" when I first press the gas when cranking. It acts stuck but once it moves, it's fine. I think your problem (and mine) is very common. Keep us posted.
 
Eddy carbs :dumas maybe yawl need to invest in a real carb! A Holley!!!

just messing with ya, I never had an Eddelbrock so I can't really say they aren't any good but I have had many Holley's and I can't foresee ever buying anything else.
 
"tarafied1" said:
both are good places to start. If it's the accelerator pump, you can do a visual (not too many times) it should spray a nice even mist, not dribble.
If it's a vacuum leak, squirt some carb cleaner around while it's running, if engine speed races up in a certain area, that could be the leak.
Click to expand...

First i would spray brake cleaner around the seals of the intake manifold and carb to see if you don't have a vacuum leak.VERY IMPORTANT !
Second , try fix this problem by adding an additive to the fuel that cleans out the carb.
When fuel sits still for quite some time it can clogge a nossle.
accellerator pump operation is the next step.
If it is still a problem , open up your carb and clean it.It is easy on an edelbrock.Holley i don't know.
Vacuum advance ???
Thats what i would do in this situation.Hope this will help.
 
Ah, this is the carb thread.

It looks like you're having this problem while it's warming up, but not once it's warmed up, correct? Sounds like a choke issue. A cold motor needs more fuel to stay running, and still won't run properly until it's fully warmed up. Is the choke adjusted properly? How about the idle mix? Timing?
 
That's what heat risers, exhaust crossovers in the manifold and hot air stoves for the aircleaner are for. The cold air is just so dense it leans the mixture out. When the engine gets hot and underhood temps go up the intake air is warmer and problem solved. A little richer choke adjustment may take care of the problem. :conf
 
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