1968 340 AVS 4425S carb problems

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Joe1972dart

Joe1972dart

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How's it going guy’s, a while back I posted a thread about wanting to adjust the carb on dads GTS when there were no problems. Now upon starting the car the rpm varies a decent amount (more when warm and at night when the lights are on you can see them getting brighter and dimmer, you have to power break to put it gear to keep it running) and when trying to accelerate a little more it will stumble/hesitate and can sometimes can stall out. The engine was blueprinted (To oe specs as far as we know) awhile back when his buddy owned it, and the carb appears to have been cleaned and rebuilt. From the previous thread that I posted we now know the fuel air mixture screws are down low recessed and can freeze into place (picture posted below). We are going to check to see if there are any vacuum leaks soon, but if there is no leaks should the mixture screws be adjusted (we are not sure how they were set from the rebuild). Also does anyone know if we would be able to put longer adjustment screws like on the carb pictured below? If so it would make it easier to adjust the screws and every other carb we have worked on has had that style of screw. If we have to free up the screws using fire we are going to rebuild the carb to ensure the float is good and the level is set correctly (probably would do it even if fire was not added). We would prefer not to change the carb out for it is original to the car.

I was reading this thread, now makes me think it can be the same screws set wrong.

Rough idle Carter AVS

Thank you guys in advance for your suggestions anything would help.

Joe

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The 1968 AFB originally had the one idle mixture at the top (another brilliant idea for smog control). When delivered they idled like crap! So we knocked out the lead plugs to reach those little idle mixture screws that you see. I don't think that hole is threaded all the way, at least threads may not have survived the process. Originally they would then adjust well. I have a '68 AFB that I have restored and yes- those little screws can become frozen. I had to get another carb as I wanted to be original.
 
I hope that the screws aren’t stuck to the point that we have to get a new carb. If we have to get one should we get a 1969 avs carb with the screws that are not recessed?
 
Well, just to add to the confusion- I now have three 1968 AFB's. All are stamped with the correct factory number but one of them has the standard idle screws. I was there in 1968 and have never seen one like this one. Obviously there was some variation at the factory. A 1969 carb will be a little bit different as it will have provision for a vapor recovery system to return vapors back to the gas tank. So it really depends on you- how correct do you want to be?
 
We would like it to be as close to original as possible, he really wants to run return lines. He likes to keep everything stock if he can help it.
 
At this point- check for vacuum leaks and try to adjust the carb. Top screw- bottom and then back out 3/4 turn, then adjust two bottom screws as normal. What does this tell you? How did it adjust? Did anything change? Lets deal w/ original carb until we find a problem.
 
One possibility since it ran fine in October.
Have you put fresh fuel in since June 1st?
Winter fuel can do that because of its high RVP. When its hot, take the air cleaner lid off and if it gets even worse - its the fuel.

Description suggests lean condition at idle. So it should get a little better when testing as Murray suggests.
If the adjustments don't do anything, then throttles are too far open (unlikely - you haven't changed that) or fuel is not getting through. So at that point check fuel level and passages.
 
We checked for vacuum leaks yesterday and didn’t find any. We got gas a couple of days ago with no improvement. I did take the air cleaner off and covered the top and the rpms went down like it should, this makes me believe there is also no vacuum leaks we did not spot. We are going to buy a rebuild kit and make sure all is good on the inside and try and free the screws for I think they are frozen in place. When we do rebuild and try and get it running better (hopefully) I will give an update.
 
I reading this post and I see discussion on the 1968 Carter AFB.
Do you really mean AVS?
After 1967, the only Chrysler cars to have AFB's were the Hemi engines and they had the two "fuel" idle mixture screws.
When the Carter AVS come out in 1968, it had the upper mounted single, left hand threaded "air" idle mixture screw. The two lower mounted factory set "fuel" screws were behind the lead plugs.
 
So good news guys the screws are free, but we have run into another problem. The passenger side fuel air mixture screw is not going down as far as the drivers side, they are both bottomed out and the screws are not pointed the same direction in the carb. We made sure the screws were the same height and they are. Do the screws have to poke out the holes the same length or is it fine the way it is just adjust little by little and tune with a vacuum gauge? I posted a pic below to show what I mean the left side is not as low as the right.

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I would think you are OK if you can confirm that both airways are indeed closed when bottoms. Difference can be in the depth of screw hole or in wear in the seat. Congrats in not being frozen.
 
The area where the needle contacts the carb main body is weak and can crack. Look closely. I never could get the original AVS to run as smooth as a Holley 750 on the 383 in my 68 GTS. I drilled out the idle mixture screws which were hopelessly stuck and replaced them with the typical mixture screws to no avail. The car would run okay, however it idled rough and the tuning window was very short. Some day I will figure it out. My next step was to lead the air idle air bleeds and re-drill to richen up the idle circuit.
 
Well we cleaned out the carb, there was some varnished over gas on the floats and a little dirt at the bottom. We put everything back together and turned the screws 2 1/2 turns out as a starting point and the air bleed 1 turn out and the car started right up. It was running rough but with the vacuum gauge I found the highest reading and the car now idles more smoothly and did not vary in rpms as much, only once or twice after stopping. It is still not 100% there but it is getting closer. I will play with it some more soon and see what I can do then get back to you guys when I have any updates.
 
Sounds like progress.
Doesn't take much to clog up a idle circuit. Well alot compared fuel injector. LOL

Careful not to put too much emphasis on the vacuum reading, especially in neutral.
At a given rpm and conditions, yes higher vacuum indicates stronger.
The condition in gear is much higher load than in neutral.
The engine needs a richer mixture under high load. At idle the engine makes little power so slipping it into first (or Drive) is a relatively high load.

A typical idle mix procedure without a CO tester was to lean the mix just to the point rpm was about to drop. Then make it a little richer.

As you know, for early CAP and CAS, they purposely ran the idle mix leaner than ideal. To make up for that, the idle rpms were increased a little.
 
Where that arrow is pointing, under that plug is adjustment for off idle adjustable air bleed. At normal curb idle, does nothing closed,as more off idle slot is exposed it begins to function.
 
There is a screw under that plug that adjust the opening size for air bleed hole, adjust as needed a little does a lot, mark your starting point. Tension of spring not adjustable. There is feed hole on side,plug it with finger for testing.
 
Thank you bugman, sorry for the late response a lot has been going on recently. With the shorter screws that are recessed are those idle air bypass passages meant to be open (1968 carb with recessed air fuel screws vs the 1969 style that you pictured above with the air fuel screws and springs not recessed)? Also the off idle adjustable air bleed is unplugged, and did not know there was an adjustment there for we did not see it. The carb was rebuilt a while back and we do not know who did it and how long ago. With the adjustments we made the idle has had less variation and preforms better. With the plugged air bypass drilled out would the car run better and/or preform better?

Thank you
Joe
 
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