Carter AVS Idle A/F Mixture

[69L48Z27]

I've searched here and the Web but still not finding what I need. I have a 68 440 AT carb, 4429 I think, and it's been through a rebuild. Everything is looking good but I hit a wall looking for some info, service manual was no help.

At the base of the carb there are 2 screws that are recessed in the base. Service manual says they are no adjustable. Well, they came out when the carb was rebuilt. Then there's the single screw right above these, LH thread.

My questions are:
- What is my base setting for the screws in the base?
- What is my base setting for the LH screw right above that?

Car runs and drives well, a little fat at idle. I plan to do all the vacuum guage and idle stuff but need a base line.

Thank you!

 

[eldubb440]

rule of thumb is turn them in until idle slows, then back them out to max rpm, go back in 1/8 - 1/4 turn..... can't picture the third screw

 

[69L48Z27]

rule of thumb is turn them in until idle slows, then back them out to max rpm, go back in 1/8 - 1/4 turn..... can't picture the third screw

Thank you. I'm aware of that part of the process but I'm looking for information on where to start, screw in/out to find the sweet spot after that. Usually base screws are 2-1/2 - 3 and the one above it is 3/4 - 1 turn out. I cannot not find a reliable source to confirm. I'm also trying to understand why the these screws are not serviceable according to the service manual.

 

[lewtot184]

the bottom two screws are idle fuel mixture. elbrock carb screws can be substituted. the top left hand thread screw is idle air mixture. all this is part of the emissions built into the carb.

 

[440beep]

My "Rebuild and powertune Carter/Edelbrock Carbs" book states to "Set the idle mixture screw by adjusting the screw lightly against its seat (all the way in) and then back off 1 1/2 turns as approximate initial adjustment." The idle mixture screw is referring to the single, left hand thread screw.

Note: for the AVS left hand thread screw, turning the screw clockwise (outward) makes it richer and counterclockwise makes it leaner. The idle passage is fully open at about 3 turns off the seated position. At that point further adjustment will not change the idle A/F ratio.

Thank you. I'm aware of that part of the process but I'm looking for information on where to start, screw in/out to find the sweet spot after that. Usually base screws are 2-1/2 - 3 and the one above it is 3/4 - 1 turn out. I cannot not find a reliable source to confirm. I'm also trying to understand why the these screws are not serviceable according to the service manual.

 

[lewtot184]

the left hand threaded screw will lean the idle mix when turned clockwise and richen idle mix when turned counter clockwise.

the bottom mixture screws may have to be backed out significantly due to the idle jet size. on some of the avs's the idle jet is only .031" due to the emissions calibration. i've found that somewhere between .035"-.037" to be more ideal.

 

[440beep]

So the book is wrong regarding that single left hand threaded screw? Guess it wouldn't be the first time

 

[lewtot184]

So the book is wrong regarding that single left hand threaded screw? Guess it wouldn't be the first time

the only reason i can see for this is that the direction the top air screw and bottom fuel screws are turned give the same results. maybe less confusion for the mechanics,......?

 

[69L48Z27]

Thx everyone. I'll take a stab at it this weekend.

 

[OldToys]

I have the same set up and my lower mixture screws also have the caps removed. Wondering where to start with adjustment? Im at 3.5 turns out and still having issues with idle.
When starting off if I lightly add throttle the carb. really hesitates to the point it will die.
I have the upper screw 1.5 turns out.

 

[oldbee]

If I remember right those lower sealed screws had a shorter "needle" on them than an OEM mixture screw. You're just gonna have to play with different settings but the head of the screw was usually 1/8-1/4"? recessed inside base of carb.They were probably set a little lean from factory for emissions, hope this helps.

 

[OldToys]

If I remember right those lower sealed screws had a shorter "needle" on them than an OEM mixture screw. You're just gonna have to play with different settings but the head of the screw was usually 1/8-1/4"? recessed inside base of carb.They were probably set a little lean from factory for emissions, hope this helps.

Thats good info thanks, I bet It will be 5 or 6 turns out to get the setting you mentioned? At 3.5 turns I am so far in its hard to find a screwdriver narrow enough to get to the head of the screw.

 

[lewtot184]

Thats good info thanks, I bet It will be 5 or 6 turns out to get the setting you mentioned? At 3.5 turns I am so far in its hard to find a screwdriver narrow enough to get to the head of the screw.

you probably have the small .031" idle jet. that will need perhaps 5 turns or more out and still won't work real good. just open the idle jets up to .035"-.037" and things will noticeably get better.

 

[oldbee]

Oldtoys it's time for one of those pocket-clip screwdrivers and a little grinding on the sides to fit or an eyeglass screwdriver kit(they're really small)

 

[oldbee]

Also those baseplate screws weren't set up with springs for tension they were cut/slotted for tension on the baseplate threads so ur kinda screwing up the aluminum threads if you go in&out Willy-nilly.

 

[69L48Z27]

I forget I started this thread. Over the winter I had the engine apart and I had issues last summer that I was chasing assuming it was the carb. It wasn't.

The adjustments I made for mine are as follows:

Lower embedded screws are 2 1/2 - 3 turns out. And upper is around 2.

Car runs good and smells slightly, I mean slightly, rich at idle. When it's really hot I get a very slight hesitation from idle to primary circuit if I go really slow on the throttle.

I think 5 turns on mine would have it really lean. Keep in mind the upper screw is the gate keeper for the lower two, not really sure why they had the two lower ones. They were sealed from the factory and not accessible.

Also make sure your chasing a carb problem...

 

[oldbee]

69L old age is setting in! LOL

 

[OldToys]

I tried two full turns with the upper and is worse. I went back to 1.8 on the upper and moved the lower to 4 and 4.5 the lower screws are about 1 to 2 threads inside the housing. Its better but still has a slight hesitation.

 

[OldToys]

BTW my timing is 10 BTDC with vacuum advance, is that a good setting on the timing?

 

[69L48Z27]

Every car is a bit different, depends on the cam, fuel, altitude, etc. I have just a bit more base timing in it, stock type cam but modern cut. Has 4* built into it where the stock has none.

What do you mean 'with vacuum advance'? Base timing should be set without VA hooked up, should be blocked off.

If it's doesn't ping at WOT try throwing some more base timing in it.