Carter AVS Lean Spot

[bennydodge]

One recommendation I can make that helped me a bunch was getting a FACTORY service manual. Nice section on AVS's in there.

[bennydodge]

Some good reading/reference material here-April 1990 Mopar Action. Hopefully you can blow this up and read it.

Not sure why these pics rotated on me..

[kawahonda]

Fantastic material there. That's stuff you can't even find online. I'm saving that for sure.

Now I'm excited to remove my air horn to see what secondary jet sizes I have. It should be .098.

Yep, I got the FSM and also a carb/edel tune book.

[734406PK]

Quote: Perfect, I'll grab them both.

Not sure if I'll take the carb apart quite yet. Eventually. If I can solve this "cruise mode" lean spot with a metering rod change, then it's time to actually dial back on the accelerator pump. Probably will put the .043" one back in and reset it to the center pivot. Once that's set, I may consider advancing my timing again by a degree or two.

Great Idea!

Interesting note about the additional idle fuel jets--I'll maybe look into that. Is that a strong recommendation?

- Yes, Very Strong. Most of the carb issues I see are caused by dirt etc plugging things up. The idle passages are most effected due to the small size (.031" in this case).  I would drill the idle jets out a few thousandths and readjust the mixture screws, just don't over do it. They should be evenly synchronized in a perfect world. You should be able to have full adjustability from too rich, at say 3 turns and too lean, at say 1 turn out (+-) Perfect at, say 2 turns. Now you will know that the idle circuit can provide more than enough fuel than needed. Check your hot idle compensator or plug the passage. Its probably original 50 yrs old...

Wouldn't only changing out metering rods still require trimming on the mixture screws?
- Possibly but very slight, the idle jets are in play here.

Thanks Bennydodge for that info. Looks like the No-ECS is the same jetting with a slightly thicker top portion meter. No evidence so far that the ECS models were "leaned out".
-Great Info! Both carbs were likely "leaned to the max" to meet emissions. Fuel was much different formula back then also.

Is it true that Carter only produces 3 "tall" jets for the 3-step roads?
- Edelbrock bought Carter years ago and use AFB 2 step rods and jets on their AFB/AVS series. No listing for tall jets or 3 step rods on their site. It appears there were only 3 tall jets available from Carter. You could ask Uscarburetors if you're getting the 3 step rods from them. Old school place!

[bennydodge]

Unless it's a misprint you should have .096 size in the secondary.

[734406PK]

One recommendation I can make that helped me a bunch was getting a FACTORY service manual. Nice section on AVS's in there.

There is a 70 FSM available for download on this site under the "Resources" tab. That's where I found it.

[kawahonda]

Benny- yep, I was just eyeballing exactly that!

Sounds like a .098 move is a definite.

Where do I find this idle jet online, contact usecarbs?

[734406PK]

Benny- yep, I was just eyeballing exactly that!

Sounds like a .098 move is a definite.

Where do I find this idle jet online, contact usecarbs?

You can ask UScarburetor about AVS Tall Jets, the idle jets are part of the primary boosters in your carb already.

[bennydodge]

Benny- yep, I was just eyeballing exactly that!

Sounds like a .098 move is a definite.

Where do I find this idle jet online, contact usecarbs?

You can ask UScarburetor about AVS Tall Jets, the idle jets are part of the primary boosters in your carb already.

Nice photo there. I think he's referring to the main secondary jets. These would be standard AFB/Edelbrock stuff.

[kawahonda]

So I should inquire about boosters that have the larger jet installed since its kind of a one piece ordeal. I don't see a lot on eBay, and without a part # is it going to be anyone's guess as to what size the built-in jet is?
Question here: Are they all .031, or are certain carters .033?
Another question: Are the AFB boosters interchangeable with the AVS? Probably not.
I'd hate to drill out my originals here...would rather get another set that has the correct size, or to drill a "spare" set.
If they are all .031", then it's probably best to attack this last and only if needed. That's my thinking.

I'll definitely ask/obtain about the .098 jet.

I don't think I'll need to mess with the main jet for now since I have two needle sizes to try on the way!

[734406PK]

So I should inquire about boosters that have the larger jet installed since its kind of a one piece ordeal. I don't see a lot on eBay, and without a part # is it going to be anyone's guess as to what size the built-in jet is?
Question here: Are they all .031, or are certain carters .033?
Another question: Are the AFB boosters interchangeable with the AVS? Probably not.
I'd hate to drill out my originals here...would rather get another set that has the correct size, or to drill a "spare" set.
If they are all .031", then it's probably best to attack this last and only if needed. That's my thinking.

I'll definitely ask/obtain about the .098 jet.

I don't think I'll need to mess with the main jet for now since I have two needle sizes to try on the way!

Trying to find a pair .033" boosters is going to be tough without a part number IMO. The idle jet size,air bleed etc will be application specific but ask Uscarburetor, you never know...
The idle jets can be filled/crimped and drilled back out to.031" if needed but i doubt you will have to do that, you have plenty of adjustment in the idle mixture screws.
If you zoom in on the second photo that bennydodge posted, there is a Carter suggested carb tune for a modified 340, pretty interesting, staggered jetting.

[bennydodge]

So I should inquire about boosters that have the larger jet installed since its kind of a one piece ordeal. I don't see a lot on eBay, and without a part # is it going to be anyone's guess as to what size the built-in jet is?
Question here: Are they all .031, or are certain carters .033?
Another question: Are the AFB boosters interchangeable with the AVS? Probably not.
I'd hate to drill out my originals here...would rather get another set that has the correct size, or to drill a "spare" set.
If they are all .031", then it's probably best to attack this last and only if needed. That's my thinking.

I’ll definitely ask/obtain about the .098 jet.

I don't think I'll need to mess with the main jet for now since I have two needle sizes to try on the way!

I can't definitively answer any of these questions but I wonder if the emulsion in the booster clusters is different between Federal and ECS carbs-idle jets and air bleeds being different. Never measured any of them. I don't know of anybody selling or modifying AVS booster clusters.

Modern fuel has less energy(BTU's) than older fuel and, of course, these old carbs were calibrated using the older fuel. This means that stock calibrations of old carbs are going be even leaner than back in the old days. So if it were mine I would get the jetting closer to optimal(richer) before I did anything else..

[kawahonda]

You guys rule. Really love the help. Over the past 24 hours, I've learned a hell of a lot about these so far!

That is an interesting staggered setup...both in the rods and in the jets. That's kinda going in a little deeper than what I'd like to do, at least for the time being.

Thanks for continuing along on my journey. I'm pretty sure I'm going to solve this real soon!

I'm kinda thinking in the future, but it seems like the accelerator pump gets blamed for a lot of things when many times it's probably not the issue. I wonder if my .043 and .046 have really just be compensating for this lean issue all along...wondering if I may even put back the factory .033 squirter (eventually).

You should always do one thing at a time, but I'm going to break that rule in this case. Removing air horn today.

1) Will replace secondary jets from .096 to .098. This is the spec for non-ECS carbs.

2) Will put install "large" accelerator nozzle (.043) from the "extra large" (.046) that is on there right now.

3) Considering to replace the boosters with larger ones, or may just "drill".

4) How to test that metallic piece that is in there for proper sealing?

Retune/retry carb. Likely issue will be fixed. Position back pump to center position and recheck. If no change in the transition part, then keep it there.

Then move to advance timing back up and see if I've unlocked some new potential there.

At this point if all is well, you can consider carb tuning done. But knowing me, I just "may" go back to the .033 pump if I remove the air horn  again and retest. I don't think there's anything wrong with leaving the .043 on there if not needed. Probably slightly worse fuel economy?

Tuning is fun.

BTW, UScarb doesn't have the larger boosters. So my only option is to drill. I'm still kinda "iffy" on that. That graphic you posted shows that it will help with my issue, so kinda makes me want to try it.

[734406PK]

I would take it off and plug the passage. There is another 4937S for sale on eBay. I did find the Mopar part numbers for the boosters but no luck on a web search. 3514889 and 3514888. Drilling jets is considered taboo by many,but as time goes on its the only choice sometimes. Also the idle jet is responsible for fuel metering in the transition slot just above idle, but make a jet/rod change first and drill out the idle jet as a last resort.

[bennydodge]

To much pump shot is actually a bad thing=sluggish throttle response and tip-in. You want it as lean as possible without a stumble or bog, this will make response crisper and snappier.

You know I've never had problems with the hot idle compensator-even on big cam engines. It's designed to increase idle speed and therefore help cool the engine under certain conditions. The port under the strip goes straight to manifold vacuum-essentially a controlled vacuum leak. It should only open under extreme heat conditions, such as A/C on, 100 degree temps, stop and go traffic-all 3 of these things at once could make it open. I've seen guys fashion block off plates to get rid of this device. I guess if it fails in the "open" position it would be problematic.