AVS2 carb tuning. Anyone with experience out there?

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MoparMike1974

MoparMike1974

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I have owned a dozen carter /edelbrock carbs over the years and they have always worked great out of the box. This one (1906 650cfm) has a bad stumble when you mash the gas. Pulls hard once it recovers. Basically you have to ease the pedal into the power mode transistion or the engine falls flat.
Its basically a stock 73 340 that has been freshened with new bearings, rings, valve job, etc. Got about 1500 miles on the rebuild. Upgrades include air gap intake, TTI exhaust, and thats about it. Cam is a comp with specs comparable to a stock early 340.
Trying to use the calibration chart that edelbrock provides but im a little confused with it. I did try swapping around the step up springs first but that didnt seem to help any. Right now im one stage rich on the chart and honestly it seemed to run stronger with the factory calibration. I did not change the secondary jets and now im thinking I was supposed to.
I was thinking about going back to the stock needle and main jet with the orange spring, and stepping up a size or two on the secondary jet. Will this help my transition stumble from the primary to secondary?
I have moved the accelerator pump to the heaviest setting. I have also tried adjusting the secondary air door all different ways. I have not found any instruction on how to properly set the tension on that air door.
On another note, is this 650 big enough for my 340? I do have an new 800 cfm AVS sitting on the shelf I could try but I really think the 650 should be sufficient for this engine.
 
MoparMike1974 said:
On another note, is this 650 big enough for my 340? I do have an new 800 cfm AVS sitting on the shelf I could try but I really think the 650 should be sufficient for this engine.
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Yes the 650 is big enough. If you can't get the 650 to work right . then I suspect you will have even more trouble with the bigger carb.

I would tighten up the air door two full turns and see what happens

now; What's your TC stall and what gears and I assume you have an A-body, but which one?
Can you give an idea of cam and cylinder pressure. Oh wait I see you said 73 340 and stock type Comp cam. Is it this one?
Brand: COMP Cams

Manufacturer's Part Number: 20-212-2

Part Type: Camshafts

Product Line: COMP Cams High Energy Camshafts

Summit Racing Part Number: CCA-20-212-2


UPC: 036584500124

Cam Style: Hydraulic flat tappet

Basic Operating RPM Range: 1,500-5,500

Intake Duration at 050 inch Lift: 218

Exhaust Duration at 050 inch Lift: 218

Duration at 050 inch Lift: 218 int./218 exh.

Advertised Intake Duration: 268

Advertised Exhaust Duration: 268

Advertised Duration: 268 int./268 exh.

Intake Valve Lift with Factory Rocker Arm Ratio: 0.454 in.

Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.454 in.

Valve Lift with Factory Rocker Arm Ratio: 0.454 int./0.454 exh.

Lobe Separation (degrees): 110

Computer-Controlled Compatible: No

Grind Number: CRS 268H-10

If yes then I suspect your cylinder pressure is gonna be in the basement, so your combo is gonna need some help from stall and/or/together-with rear gears
 
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MoparMike1974 said:
I have moved the accelerator pump to the heaviest setting. I have also tried adjusting the secondary air door all different ways. I have not found any instruction on how to properly set the tension on that air door.
Click to expand...
what is heaviest? If you mean closest hole to the pivot, and re synchronized the starting point then, I get it.

What speed and rpm is it laying over at, and do the tires spin?
 
COMP Cams XE262H Dyno Results
Stock everything on this car so stall is whatever a stock 340 came with in 73. It definitly stalls higher than a regular 318 car would.
Car does not have a tach so not sure on rpm. The bog happens when you stomp it to WOT, even at speed. Compression tested at around 160psi but that was right after the rebuild so number might have changed some since the rings seated.
Accelerator pump is in the closest hole (longest pump stroke).
So I went out after I made this post and made some adjustments. I went back to the stock rods and main jets. Then I stepped up the secondary jets one stage. Changed the step up springs to one above stock but cant remember what color it was. With these adjustments the stumble is nearly gone but not quite. I might try a rod adjustment first and if that doesnt work I may try one step bigger on the secondary jet.
I can live with it the way it is now but I will probably play with it some more to get it spot on.
 
A lean stumble or a rich bog? How can you tell @AJ/FormS form
 
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Lean stumble for sure. Pulled the plugs after 500 miles and they are just as clean as when I put them in. I will read the plugs again after a week or so and see if they get any color to them. It runs great on the throttle once you get past the secondarys opening...and it will idle clean for hours on end. I've got it pretty close, I think with a little more fiddling it will be good to go.
 
MoparMike1974 said:
Lean stumble for sure. Pulled the plugs after 500 miles and they are just as clean as when I put them in. I will read the plugs again after a week or so and see if they get any color to them. It runs great on the throttle once you get past the secondarys opening...and it will idle clean for hours on end. I've got it pretty close, I think with a little more fiddling it will be good to go.
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I am tagging along on your thread. I have messed with AFB's but not AVS's or AVS-2's. I have a AVS2 I would like to put on my 318 with a old school LD4B. Sticking around to learn a few tricks.
 
A/J is right on. The resynchronizing is a step that so often gets overlooked. You can’t just move holes and forget it. Sometimes it will pull the pump plunger too far up and all you get is air for the first part of the pump shot. If that is the case, you’ll have to straighten out the pump rod a little.
 
I've got the same 650 AVS2 with a similar setup. 71 340, stock cam, Air Gap, 2500 stall, 3.55 gears. I haven't tried to tune it because the combo will be changing soon, but I get the same lean stumble if I'm too aggressive in the lower end. Goes like a raped ape after that. Runs really nice. Plugs look good, on the lean side. Just pulled the motor out this weekend and the base of the carb and the intake runners are clean as new. No soot whatsoever. I'm not going to be any help, but am looking forward to how you proceed.
 
Everybody wants their 4bbls to open ASAP, but I think you are trying too hard.

The thing is the primaries are good to something like 3000 at 100% VE, say 3800@80%.
So all that happens when you open them too soon is the airspeed falls into the basement and you get a bog.
Don't be shy to slow the secondaries down.

If you cover the bog with pumpshot, you just end up sucking the tank dry, cuz every time you tickle the gas pedal, you are shooting gas down the holes.
Same for the M-rods; delay the secondaries, and then you can dry up and slow down the power-pistons, for a cleaner PT (Part Throttle) drive, and better fuel economy.

If you want the secondaries to kick in faster, get a double pumper, and have fun spinning the tires.

nice cam;makes great cylinder pressure. Should make a nice streeter after the tune is in.
 
Ok, I will fiddle with the air door again and see what happens. That was my thought previously...to delay the secondarys but it didnt seem to help any. It may react different since jetting up the secondaries a little. One problem I see with the air door is when it opens it slams open. It would be nice if it were more gradual.
 
MoparMike1974 said:
One problem I see with the air door is when it opens it slams open. It would be nice if it were more gradual.
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It's made that way on purpose, but if it really-really slams open, then the tension is quite a bit too loose, hence my recommendation for 2 additional turns.
If you can feel the secondaries opening, then the tension is too loose.What you are feeling is the drop in air speed/flow and the subsequent sag in power, while all the circuits re-adjust to the new airspeed, and the secondary fuel flow comes on line. The secondaries don't actually need to be fully open until peak torque, which is gonna be around 4000rpm with that cam.
The air-door should be tight enough to not be able to open when you slam the throttle open in Neutral/Park at idle. But same action at 3000 should get her started. Try 2 turns tighter; whatever is necessary to get the bog gone; you can always loosen it later, as your tune gets sharper..

Do not fall for the idea that the secondaries need to open ASAP; that's just not true with that carb. The power delivery needs to be seamless. If you loose a half second on the transition, you end up slower in the speed contest.

You gotta remember that the engine does not suck air in.
The atmosphere pushes air in, trying to fill the low-pressure area in the cylinders that the falling pistons and ring-seal is creating.( see note 1)
As the rpm rises, and with only the primaries open, the air flow thru the throttle bores picks up ever increasing speed. Let's say at 3000rpm, the engine is processing 300cfm, thru the primaries. If you then slam the secondaries open at 3000rpm, the engine is still only gonna process 300 cfm, which is all it can do at 3000 rpm. Now all 4 throttle-bores are sharing the same 300 cfm.Subsequently the airspeed thru the primary throttle bores HAS to drop.
When that happens, fueling in the primaries, drops in proportion, back to 150 cfm level. While in the secondaries, fueling is just starting up.

So your job is to make the transition between primary flow only, to shared flow, as smooth as possible;
The easiest way to do that, is to prevent the secondaries from slamming open.
The second, is to replace the Vsec, with a double-pumper.
The third, is to get a higher-stall TC, so that the sag happens with no load on the engine; try a 3500,lol. or for street; maybe a 3000.
Or you can do a combination; like a 2800TC and 3.73gears, and a 650DP.
Or you can just slow the daymn secondaries down........... lol

Note1
Talking about ring-seal. Some guys , are under the impression that leakdown is not "bad" until around 10%. Well; that may be acceptable for some ..... on the compression side of what the rings do.
But the rings also have to create the low-pressure in the cylinders, to cause atmosphere to move in that direction. So the poorer the ring-seal, the less air will find it's way into the cylinder, and the faster the engine spins the bigger the problem will be, and the more intake duration will need to overcome it.
Don't be misled, 10% ischit. 4% is a good and reasonable target. Less is better.

Course if you have a 440, I guess you could afford 10%,lol.
Another thing to keep in mind is if you have a 440, the total volume in that chamber, swept plus chamber,etc; could be as high as say 1030cc, with the piston parked at the bottom.
Whereas the 273 might have say 620 total. But after you stick a dome into the 273,and after the late Ica on the big-cammed 273 finally closes, maybe 420cc .................. Now; if the ringseal is down 10% ..............................
This is what makes the modest-cammed 360 so attractive to me; 830cc with flat-tops, and maybe 650/700 after the intake closes. That is able to make a lotta torque right where a streeter needs it.
In comparison;
the 340 could be 770cc/660 after the intake closes.and
the 318 could be 730/580.

Engines are airpumps.
To make power it takes rpm to process sufficient air.
But to make street-torque, you gotta be able to process air at lower rpms. 660 is a suitable target, allowing you to run less gear, say 3.23s.
Whereas 580, being 12% less than 660, might require 12% more gear, mathematically 3.68s, which round to 3.73s.
 
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I used to have 2 1/2 turns on the secondary spring in my '69 340 Dart. That starts from the close of the secondary flap . So, release the spring, turn till the flap is closed, then 2 1/2 turns. It's a great street carb. Did not like it at the drag strip.
 
Stock 3.21 gear. I dont recall what I set the timing at. I think its 16 initial and something like 36 all in. Vac advance is connected to timed port. No tach so not sure when the timing is all in. Its a msd 6C box with a 90's vintage mopar performance distributor so it should be coming in earlier than a stock distributor.
 
The hardest thing to except is the smooth transition between the primaries and the secondarys. when it bogs, and it try to put you face in the windshield. Then it comes back on you get slammed into the seat. Feels GRATE!!!

When it is working right you don't feel the secondaries and thing they are not working.
Your butt odometer(how it feels to you ) will lie to you every day and twice on Sunday.
If you feel the secondaries come in it was falling off and loosen you up in the seat before the eng was able to overcome the extra air of the secondaries and push you back in to the seat.

That was the hardest thing to accept and overcome..... "Smooth is Fast". feeling the secondarys coming on is slow. Your butt will lie to you every time and tell you that it is not so.
In fact i never got over it. Keep opening the air door sooner until i felt the secondarys coming on and my et would slow.

As i just couldn't get my mind to accept that......... i when to a double pumper and never looked back.
 
Cudafever said:
As i just couldn't get my mind to accept that......... i when to a double pumper and never looked back.
Click to expand...
Same here.
swapped a 780 Vsec off, for a 750DP. More tirespin is more funner.
But I swapped the 780 off, also because throttle in the corners was just too tricky to get right. So now we just slide on thru every corner I dare to, with the throttle acting like a big variable-speed electric motor.
 
I have been playing with a 800 avs for a while and am really pushing it beyond its comfort zone. Not sure what info will be helpful, most of this won’t help you, but some interesting things I have noticed. The base casting on the AVS is the same as the base casting on the AFB/performer. Meaning the holes for the counter weight shafts are there even though it has an AVS top. I epoxy the holes closed to keep air from bye passing the secondary boosters. I’m running .116 jet in front with a 6842 rod. and .107 in the rear. Now I’m running it on a 410 CI stroker with some serious effort heads. I did a test to determine when and how fast the avs door was opening. I just had no idea. It could have taken milliseconds or 2 seconds. So here is what I found. Hammer it in third gear from idle and the door starts to open at 2500 rpm and is fully open at 3000 rpm taking less than .300 of a second. It was REALLY snapping the door open. Any other vacuum secondary carb uses springs to open the throttle plates, With the spring the rate doesn’t change if you pull more air through it. I can never feel it and I have never noticed it go lean. I am on the converter up to 2500ish at launch. So I didn’t worry about it. Last weekend I helped tune a serious effort 499 B block that has to run an 800 AVS. It was snapping the door open so fast it would go LEAN. It would recover and really pull hard, it was just something very interesting to see. It probably has to do with how the dyno applies the load and I sure we are pulling more air through them than you really should. To date I haven’t really play with the door, but believe I will need to revisit that. Also I have found that at that level they REALLY like to be fat in the primaries. I have run three stages rich in front three stages lean in the secondaries. So you figure bring them back to stock and you are fine. Nope! Really doesn’t like that! I have seen the same thing on a serious effort hemi running AFB’s. Needs to be fat in front apparently to help cover the lean spot right after the AVS or counterweights snap open. I do know you can’t really crank on the door spring. It can break pretty easy. I was surprised that the 800 AVS could support 600 hp being it is really NOT a race carburetor.
 
furrystump said:
I have been playing with a 800 avs for a while and am really pushing it beyond its comfort zone. Not sure what info will be helpful, most of this won’t help you, but some interesting things I have noticed. I was surprised that the 800 AVS could support 600 hp being it is really NOT a race carburetor.
Click to expand...

I had one on my 360 and it would go lean at the hit on the start line so I did a bit of investigative work to see why. With the floats set to their correct heights the fuel level was just too low in my view to fill and keep the pump shot well filled. So I cut up an AFB top at the front and adjusted the float settings to see where it actually should sit with various heights on level ground. They didn't match up so I ran the 800 AVS as high as possible to get it where I needed it but if you go through the Carter literature you'll find that they recommend lower float levels with a bigger needle and seats so that would have been the better way to go. When you consider the original Carters and how the feed to the pump shot well has a one way check valve at the bottom of the bowl feeding into the well you understand how it works. Later carbs do not have this feature so it needs to be higher to get into the slot. Add to that the inertia of forward movement and the fuel is being pushed to the rear of the carb it makes it harder to get there as well. So check the "actual" fuel height. Just something else to look for when tuning your carb.
 
MoparMike1974 said:
I have owned a dozen carter /edelbrock carbs over the years and they have always worked great out of the box. This one (1906 650cfm) has a bad stumble when you mash the gas. Pulls hard once it recovers. Basically you have to ease the pedal into the power mode transistion or the engine falls flat.
Its basically a stock 73 340 that has been freshened with new bearings, rings, valve job, etc. Got about 1500 miles on the rebuild. Upgrades include air gap intake, TTI exhaust, and thats about it. Cam is a comp with specs comparable to a stock early 340.
Trying to use the calibration chart that edelbrock provides but im a little confused with it. I did try swapping around the step up springs first but that didnt seem to help any. Right now im one stage rich on the chart and honestly it seemed to run stronger with the factory calibration. I did not change the secondary jets and now im thinking I was supposed to.
I was thinking about going back to the stock needle and main jet with the orange spring, and stepping up a size or two on the secondary jet. Will this help my transition stumble from the primary to secondary?
I have moved the accelerator pump to the heaviest setting. I have also tried adjusting the secondary air door all different ways. I have not found any instruction on how to properly set the tension on that air door.
On another note, is this 650 big enough for my 340? I do have an new 800 cfm AVS sitting on the shelf I could try but I really think the 650 should be sufficient for this engine.
Click to expand...


Trust me on this I had the same issue with the same carb, upgrade the squirt nozzle to the largest size I believe it is 43. I did that and god damn the bog is gone and now I go sideways! Thank me later!
 
Don’t mess with metering rods or anything just upgrade the squirt nozzle I wasted a lot of time with springs rods and jets then I finally went back to stock and changed the squirt nozzle. Bog was gone then I fine tuned The carb leaning it out and putting a spring that matches my engine vacuum.
 
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