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Off idle stumble/die. FIXED!

I left the steel check ball out of a carb's accelerator pump well once and it had a horrible "flat spot" off idle..it'd die if you didn't feather the gas pedal on takeoff..it must have fallen out when I had the carb apart for a rebuild..found one in a junk carb and it fixed the problem..

Some carbs use a vacuum piston inside to open a passage when the throttle is opened to richen the fuel mixture--if it doesn't get vacuum it wont richen up when the throttle is opened--(sometimes the carb kit has several top bowl gaskets but only one has the correct holes for the passages too).

Some carbs have no adjustment for the accelerator pump,other than bending the linkage that operates it..I've had to tweak a few to get enough stroke for the pump to work right...
I also had one carb that had the wrong size accelerator pump cup in the kit--it was too small for the bore it rode on,and gas just bubbled up past it instead of making it squirt out the right passages in the venturi..

Another thing I just remembered--many Mopars had the "lean burn" system in the 70's and people would ditch that and just put a regular carb on it from an older engine..and sometimes they just didn't run that great..
 
HankScorpio said:
Working on one of these right now myself. Do you have the 4412 2bbl? Dealing with one on a sandrail. Jetting for altitude first, then working on drivability.
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I think it is a 4412. That number sounds familiar.
 
Yes it is a 4412.
This is the kit I used.

For what it's worth. The car also ran absolutely no different than it does now before the carb was rebuilt... Rebuilt it trying to fix the issue lol.
 
I've seen a weak coil or a distributor cap or rotor with a carbon track or crack cause weirdness like this also...
 
It has a newer looking msd street fire dizzy and control box. Coil to go with it. Guess it's possible that that's messed up, but it works real good for everything except normal take off.

I keep coming back to the idea of vac leak under the intake. But by now I'm grasping at straws. :dunno:
 
I'm going to play with it some more tonight or tomorrow. I'll have enough time to really try and fug it up lol
 
I've had a few bizzare vacuum leaks..

One was on a small block chevy,the thing ran fine cold,but after the choke opened it wanted to idle slower & slower,then sometimes stall,sitting at a red light ..I did the usual vacuum leak tests with a vacuum gauge and spraying carb cleaner at every suspect spot,made sure the EGR valve was closing all the way--even removed it and blocked it off..no change..

I did notice the vacuum would drop a few inches when it began stumbling and wanting to stall,the needle also was vibrating instead of staying steady...adjusting the idle mixture did nothing to help it..

It was like it was way too lean at idle after warming up--if I adjusted the idle speed higher it would run OK for awhile--then it'd start climbing higher all by itself,till it was idling around 1500 RPM..then suddenly start faltering and want to stall again..

I even swapped another known good 2 bbl carb on it and it did the same thing..spraying carb cleaner on the intake gaskets did make some change in idle,so I decided to pull off the intake..

I had seen a few develop a crack on the underside of the intake,where there are exhaust passages cast into it to pre-heat the base of the carb for better fuel atomization when cold (and the heat riser valve was closed)----it had a sheet metal baffle riveted to it,and I took it off ,cleaned out about a pound of black gook that looked like coal,and found there was a hairline crack in the intake under it..
I heated the crack up with a torch,and it opened up a lot wider,almost enough to stick a coin in it..

Evidently the exhaust gases were blowing out of the crack after the engine warmed up and it probably was not only letting exhaust blow into the lifter galley,but also letting air or exhaust gases get in the intake..
I put a 4 bbl intake and carb on it and it never did that again..

An old mechanic told me he had an engine that wanted to stall or stumble taking off ,and he went nuts trying to figure out why--it finally was discovered it had some real loose intake valve guides and air was getting sucked in past them,enough to throw off the idle mixture when it was warmed up..
 
Wes Harden said:
MSD ignition and 2 barrel? I'm why?
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In 79 chrysler was playing with an electronic ignition. And parts of it are still in the car. My guess is they were getting rid of that crap. And I'm starting to wonder if that was one of the PO's attempts at fixing this same issue. Maybe even the carb swap. It has an adapter for the holley to bolt on. I've been scouring CL waiting to find a 4bl intake and carb for it.
He said he had it 2 years and never really drove it much. :thinking:. Maybe we know why lol.
 
It's odd opening the hood of a 79 and finding a plug that looks about like an odb1 connector mounted to the driver's fender pointing at you.
 
What do you have in it for jets and power valve? I was running way rich and after jetting down the drivability improved dramatically. I had a stumble off idle also, its much better now.
 
Another check can be to put a visual spark detector in one of the spark plug wires. Watch the spark as you hit the throttle.
 
Jets are whatever was in it...:dunno:. they could be wrong for sure. I read that the rebuild kit had a 4.5 powervavle in it.
Think that's what they said anyway.
 
Not sure really on spark.... If you ease the gas just far enough to get off idle, then gun it, it responds pretty decent. I really need to see if there's any tricks for timing it with that street fire. All I'm doing is pulling and plugging the vac advance line. Nad really I'm not sure what a 360 is supposed to have lol. I went with 12 btdc because that's what my last sbc was set at lol.
We've turned it both ways and the way it is now was the best so far.
 
Does accelerator pump level move as you ease off idle? Or not till after you have eased off idle.
The pump cam is probably wrong.
 
I "timed" many SBC by advancing the distributor until it just had a trace of spark knock power braking it wide open after it was fully warmed up...then made sure it'd re-start without binding up,if not,retard the timing a tad...most high mileage engines have some slack in the timing chain and the harmonic balancer doesn't always give an accurate reading,they "slip" sometimes..
 
Wes Harden said:
Does accelerator pump level move as you ease off idle? Or not till after you have eased off idle.
The pump cam is probably wrong.
Click to expand...
Lever moves nearly immediately. I've tried gapping it farther, so that it didn't touch right away. And I've tried taking it down to where it was all but touching to get it to activate faster. Sort of simulating a different cam.
Neither one made any noticable difference. Right now it's back to .015.
 
Think it s black cam. But not sure on that.
 
So I don't remember there being an issue with dodge electronic ignition. Lean Burn was an 80's thing.
I know you didn't swap out the distributor. Sometimes hot rod parts are to hot for the engine
 
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