Carter AVS Lean Spot

[734406PK]

If this is the thing that leads to that "larger" exterior port on the left side, then yep, there is a plug on her already.

I'm wondering if my jetting is just a bit too lean to begin with. I wonder if this has nothing to do with the accelerator pump. I would think that the pump "tuning" would come more into play for "quick' transitions when the pedal really moves, not for slow (but "purposeful") 10% foot movements.

Again, I'm no expert. But a .046 bored nozzle is getting pretty damn up there--it's larger than Edelbrock's largest nozzle that they sell @ .043. My engine is only a 340, it ain't a big block.

Is it common that people go even larger as one way to solve this?

I would love to see what AFR my rig is pulling. If I ever went that route, I'd go for a temp install. It isn't cost-effective for a temp install.

The hot idle compensator valve is in between the secondary venturies, where a secondary acellerator pump nozzle would go. It kind of looks like a feeler gauge blade. The part on the left side you mention is probably the bowl vent valve, part of the ECS.
Your on target with the acellerator pump theory. It only pumps fuel when you advance the throttle and not in a "steady state" as you've noticed. The metering rods/jets/spring combination takes care of steady state fueling. The engine may be burning off the pump shot and then falls off due to lean condition. You can drill the nozzles out some more but I think you have hit the point where the engine is getting all of the fuel the acellerator pump has to deliver.
Here is an Amazon link to a stand alone AFR gauge that may be of interest:
https://www.amazon.com/Innovate-Motorsports-BASIC-Digital-Wideband/dp/B001S7W836/ref=pd_sbs_263_1/138-4682770-8667205?_encoding=UTF8&pd_rd_i=B001S7W836&pd_rd_r=0fc16d81-773c-4a2f-ae92-290caf89e590&pd_rd_w=xGqa9&pd_rd_wg=PUEnJ&pf_rd_p=7cd8f929-4345-4bf2-a554-7d7588b3dd5f&pf_rd_r=VEB9AGCYSCDJ382EN7M3&psc=1&refRID=VEB9AGCYSCDJ382EN7M3

[kawahonda]

Here are my plugs

[734406PK]

 Looks like its running lean to me. The plugs should be tan not white. White means the plugs are running hot which can cause the pinging under load.

[kawahonda]

If I "jet up", could I run into the case where I could actually get away with more advance? Seems logical if so....my dizzy recurver set mine up to run on 91 blended fuel and 15-17 initial advance! I'm back down to 13 advance to keep it from pinging at full load with passengers in the car...

That might just be the final key to this puzzle...it's running lean, and it probably isn't a accell pump thing...

What's the next jet size that I should try? Figure going one step up would be good. What do you guys think?

Current jets: Front 489, Rear: 120.

Could use some help to figure out what to get next. In my opinion, all signs are pointing towards to jet up a level....

My research tells me that:

Primary 489: .089" - confirmed
Secondary: uknown, my dad only wrote "120" which doesn't tell me anything
Metering rods: 16-574 (.065 x .060 x .053)

I do have a spare AFB that I can use to convert to Edelbrock stuff (requires new metering rods and jets and flat covers. I don't want to put TOO much time/money/effort into this carb because the engine will be replaced soon, but I'd like to keep it all as an assembly with the right tuned OEM carb to go with it. Let me know what all I should do here.

Perhaps my AFB, if the jets are interchangable, may have some jets that I could use, if AFB and AVS jets are interchangeable?

As always, double-check my work/research.

[Chryco Psycho]

This is another issue with AFB / AVS carbs , up jetting is not simple , Yes the jets interchange BTW
You need to change a combination of jets & metering rods to step up 1 size & them the metering rods cange change 2 areas such as criuse & WOT when you only want to change 1 of the 2
Holley is easier as you can just change the jet for one that is a .001 bigger with no effect on any other circuit .
Anyway , you could just drill the jet .001 -.002 if you have the right drill bits

[kawahonda]

I just removed my jets/metering rods from my chevy AFB.

Front (main): 120-400, which tells me it's 1.00.

Rear: can't tell, but it looks like 120- X95, where "X' is hard to make out. It's probably a .095 jet.

Metering rods are 16-W8500, 3-step rods. From a corvette forum, this shows as .0725 x .0470

At least that's what I have on hands now, at least.

Not sure if I should use any of it, or parts of it. Tell me what to do. I'd rather buy a couple jets or a set of rods if that's all I need. Sounds like I  need to pull my rods and figure out what they are.

[Chryco Psycho]

do you have a caliper handy ? if the rods are thinner try them

[kawahonda]

Go your PM. Thanks Crycho.

Let me get the airhorn off of the 4937s carb first, just so I can accurately say what jets I have. I confirmed the primary and the metering rods already, but the secondaries I need to check because my father just wrote "120", which is not the size (just the first few digits).

I texted my engine builder about this idea of jetting up 1 level, and he responded "or 2 levels".

What I absolutely have no idea on is how to "jet up" to solve my issue. Not sure if I should be focusing on mains, secondaries, or rods, or all 3. That's where I'll look towards your guy's expert opinion.

I'll probably start tomorrow on getting the air horn off (again).

At least I got the AFB dissembled. Who knows, maybe there's something to use from it, maybe it's way off.

[Chryco Psycho]

I would richen the primaries first so thinner rods or larger jets , usually a combo of both to move 1 size

[kawahonda]

Interesting.

Looking at this Carter page:

https://board.moparts.org/ubbthreads/ubbthreads.php/ubb/download/Number/170980/filename/8258711-Metering_rod_and_jet_sizes.jpg

My metering rod is listed (16-574) @ .065 x .064 x .055.

My main jet is listed in the "AFBS-AVS Primary High Step Jet for staged metering (Not interchangable w/ low step jets)". Not sure what this means, but it's listed there as 120-489 @ .089. Is that just a fancy word for 3-stage rods? Seems like there's only 3 "compatible" jets listed there.

Anyhow, let me know what part #s you would recommend by looking at that. I can ring you tomorrow as well.

EDIT, the mains and metering rods are not interchangeable between the AFB that I have. The AFB that I have uses 2-step metering rods, which has the "shorter" jet height and the "flat top" covers. The secondaries are probably the only thing I can use from it, or use the flat top covers and get edelbrock kit, but I'll try to avoid that for the time being.

This is an ECS "California" Carb. Curious what the non ECS 340 carbs used. That may be the reference point.


Here's a set of rods on fleabay. (.065 x .062 x .053) in sexy carter packaging. That will richen stage 1 and 2 by a couple notches. Snab it? Or is this too minimal...

https://www.ebay.com/itm/PAIR-OF-CARTER-AFB-AVS-CARBURETOR-THREE-STEP-METERING-RODS-065-053-062/402061352591?hash=item5d9cb95e8f:g:P1gAAOSwoXRZjdSu

There are others available on eBay as well that are smaller. Let me know if I need to jump on any of them.

[734406PK]

That was the Carter info I was looking for! As you can see, there aren't many 3 step metering rods available anymore, I would get the eBay sets just to have on hand. There are 2 sets you could try: ,065x.062x.053 and .063x.060x.051. That's your one and two step increase suggested. If you look at the Edelbrock website for Thunder AVS carb in 650 cfm you'll see the out of the box jet/rod/spring setup. .095 primary jet with .065x.047 metering rod equals .030"/.048 rod to jet clearance. Uses 5 InHg springs. You have .089 jet with .065x.064x.055 m rod so that's .024x.025 x.034" rod to jet clearance by comparison. Not exactly the same carb but a significant difference in clearance.
   https://www.edelbrock.com/thunder-series-avs-650-cfm-off-road-carb-with-manual-choke-in-satin-non-egr-1825.html

If you are going to take the carb apart I would check the idle fuel jets on the bottom of the boosters. The FSM states .031" stock jet size, maybe increase it to .033" or so and trim off with the mixture screws.

[bennydodge]

This is another issue with AFB / AVS carbs , up jetting is not simple , Yes the jets interchange BTW
You need to change a combination of jets & metering rods to step up 1 size & them the metering rods cange change 2 areas such as criuse & WOT when you only want to change 1 of the 2
Holley is easier as you can just change the jet for one that is a .001 bigger with no effect on any other circuit .
Anyway , you could just drill the jet .001 -.002 if you have the right drill bits

Yeah, we get it, you love Holleys, blah blah blah...How the hell does this help the OP?? What's hard about changing a set of metering rods?? It literally takes 1 minute to do and no messy gas spills. It's also very EASY to understand. The thinner the rod the more fuel flows through the jet. If you need to, take a few extra minutes and take the top off for a jet change. This is not rocket science... Anyway....

1970 Carter AVS 4934S no ECS

Primary jet-.089 (120-489)
Metering rod-.066/.064/.055 (16-546)
Secondary jet-.098 (120-398)

Going richer might solve your issues. You probably know this already but the factory Carter AVS primary jets are a different(high step) design than standard AFB jets. If you change to AFB/Eddy jets you will have to run all Eddy/AFB stuff in the primary side: metering rods, metering rod covers, jets.

Based on your plug reading I would go a little richer on the jets/rods.

[kawahonda]

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.

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

Wouldn't only changing out metering rods still require trimming on the mixture screws?

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".

Is it true that Carter only produces 3 "tall" jets for the 3-step roads?

Edit: Sounds like a good plan is to remove air horn and identify secondary jet. Jet up to .098.

[bennydodge]

Interesting.

Looking at this Carter page:

https://board.moparts.org/ubbthreads/ubbthreads.php/ubb/download/Number/170980/filename/8258711-Metering_rod_and_jet_sizes.jpg

My metering rod is listed (16-574) @ .065 x .064 x .055.

My main jet is listed in the "AFBS-AVS Primary High Step Jet for staged metering (Not interchangable w/ low step jets)". Not sure what this means, but it's listed there as 120-489 @ .089. Is that just a fancy word for 3-stage rods? Seems like there's only 3 "compatible" jets listed there.

Anyhow, let me know what part #s you would recommend by looking at that. I can ring you tomorrow as well.

EDIT, the mains and metering rods are not interchangeable between the AFB that I have. The AFB that I have uses 2-step metering rods, which has the "shorter" jet height and the "flat top" covers. The secondaries are probably the only thing I can use from it, or use the flat top covers and get edelbrock kit, but I'll try to avoid that for the time being.

This is an ECS "California" Carb. Curious what the non ECS 340 carbs used. That may be the reference point.


Here's a set of rods on fleabay. (.065 x .062 x .053) in sexy carter packaging. That will richen stage 1 and 2 by a couple notches. Snab it? Or is this too minimal...

https://www.ebay.com/itm/PAIR-OF-CARTER-AFB-AVS-CARBURETOR-THREE-STEP-METERING-RODS-065-053-062/402061352591?hash=item5d9cb95e8f:g:P1gAAOSwoXRZjdSu

There are others available on eBay as well that are smaller. Let me know if I need to jump on any of them.

Those metering rods, if they are that size (.065/.062/.053) are good. Jet up on the secondary to .098 as well.

[bennydodge]

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.

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

Wouldn't only changing out metering rods still require trimming on the mixture screws?

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".

Is it true that Carter only produces 3 "tall" jets for the 3-step roads?

Yes, I believe there's only 3 high step jets .089,.098,.101. My 1970's AVS strip kit has those 3 sizes only, but there are 4 different sized metering rods and a multitude of secondary jets.

The non ECS carbs are richer but only in the secondary side. I can't verify if the ECS carbs have air bleeds and fuel restrictors that are leaner but it stands to reason they would be.

Edit: Sounds like a good plan is to remove air horn and identify secondary jet. Jet up to .098.

Sorry, my response got mixed up in the middle of you last response...