AVS2 tuning

-
mbaird

mbaird

mbaird
Joined
Dec 10, 2005
Messages
9,341
Reaction score
6,555
Location
Boise, Id
I just bought a new AVS2 800 for my 360 and I am confused on the tuning .
My wideband reads 12.5 @ light cruise so I dropped main jets to .101 from .104s but its still rich. So I figure it may be the step up spring because my vacuum at idle is 13. So I replace the stock spring with a lighter one .( or what I think is lighter... this color coding thing is bullshit as they all look alike to me .).. still rich .
I will try some bigger rods but I am already at the edge of the chart .

I ran a 650 version as well that I finally got running clean down low but top end was soft compared to my holley 750vs .

These carbs are said to be easy to tune compared to a Holley but I think thats BS.

Can anyone illustrate a step by step procedure for tuning these?
 
Last edited:
12C211B2-1F9B-4D1D-B480-C9D8FB7E57DB.png
3A6F1C08-943F-4716-B2C2-DC0F5E51E80E.png
 
"Numbers in black are the steps obtainable using Calibration Kit #1480 plus the stock jets and metering rods."

What the hell does that mean ?
The only parts in the Kit are springs /jets and rods !
If they mean for you to change the springs then tell us which one or say "change the springs"
 
Match the numbers on the second page to the numbers in the chart on the first. To the upper right it’s richer and to the lower left is leaner. The ones in BLACK are obtainable using the mentioned kit. Out of the gray area you would have to buy the appropriate rods/jets. There should also be a color code to the springs listed also but be advised the color gets wiped off when exposed to fuel so you’ll have to remember what you installed.
These carbs are easy to tune once you figure it out.
 
I understand how to read the chart and have a wide range of jets/rods. But there are interactions between spring / jets / rod combinations .
What I want to know is if there is a certain order of steps to follow ?
 
The paragraph below is straight from the carb tuning guide. If you think the second step of the metering rod is coming on to soon, use a weaker spring. See below as I believe you mentioned 13” of vac @ idle which means the factory installed spring in your carb is pushing the rod up at idle


• Measure the manifold vacuum at idle. It is critical that the Step-Up spring vacuum rating is always lower that the manifold vacuum at an idle, with vehicle in neutral. One option in choos- ing the appropriate Step-Up spring, is by reading vacuum at an idle, and using a Step-Up spring with a rating of half of what the vacuum reading is at an idle (Example: 14" of vacuum at an idle, would equate to a 7" (Pink) Step-Up spring. Note: Load-factors come into play as well, such as the vehicles weight, gear ratio, etc. The above example is simply a baseline procedure. If it is below 7" Hg, there is a good chance that the Metering Rods are in the up (rich) position. When combined with a high idle air rate this can cause the Nozzles to discharge fuel at idle. Use a weaker Step-Up Spring (see section on Step-Up calibration) to keep the Rods down at idle. With some cams, a stiffer spring (pink or silver) is necessary. Experimentation is the best way to determine which is best for your application.
 
Mopar73340 said:
The paragraph below is straight from the carb tuning guide. If you think the second step of the metering rod is coming on to soon, use a weaker spring. See below as I believe you mentioned 13” of vac @ idle which means the factory installed spring in your carb is pushing the rod up at idle


• Measure the manifold vacuum at idle. It is critical that the Step-Up spring vacuum rating is always lower that the manifold vacuum at an idle, with vehicle in neutral. One option in choos- ing the appropriate Step-Up spring, is by reading vacuum at an idle, and using a Step-Up spring with a rating of half of what the vacuum reading is at an idle (Example: 14" of vacuum at an idle, would equate to a 7" (Pink) Step-Up spring. Note: Load-factors come into play as well, such as the vehicles weight, gear ratio, etc. The above example is simply a baseline procedure. If it is below 7" Hg, there is a good chance that the Metering Rods are in the up (rich) position. When combined with a high idle air rate this can cause the Nozzles to discharge fuel at idle. Use a weaker Step-Up Spring (see section on Step-Up calibration) to keep the Rods down at idle. With some cams, a stiffer spring (pink or silver) is necessary. Experimentation is the best way to determine which is best for your application.
Click to expand...
One thing to do is to loosen the screw on the metering rod covers and twist them slightly and snug up the screw. That way you can check what’s happening with the rod. Also changing the rods without changing the jets will change the air fuel ratio.
 
I have done all of this . My idle mixture is good @ 12.8-13.4 and the engine idles nicely down around 700-750 rpm . Vacuum is up to 14" now .

I am at .101 main jet and .068/.057 rod but my afr is 13.5 @ 2200 rpm cruise... Seems like a lot of jet change
for a small jump in afr ?
I may try a .098 jet and the stock .065/.055 rods.

Just seems like a big change in jetting for a carb that everyone touts as being spot-on out of the box. And of course the hassle of changing jet sizes is a pain . Luckily I have lots of those Micro-Clips ... lol
 
mbaird said:
I just bought a new AVS2 800 for my 360 and I am confused on the tuning .
My wideband reads 12.5 @ light cruise so I dropped main jets to .101 from .104s but its still rich. So I figure it may be the step up spring because my vacuum at idle is 13. So I replace the stock spring with a lighter one .( or what I think is lighter... this color coding thing is bullshit as they all look alike to me .).. still rich .
I will try some bigger rods but I am already at the edge of the chart .

I ran a 650 version as well that I finally got running clean down low but top end was soft compared to my holley 750vs .

These carbs are said to be easy to tune compared to a Holley but I think thats BS.

Can anyone illustrate a step by step procedure for tuning these?
Click to expand...


I you use a smaller main jet, you lean out cruise and WOT whether you want to or not. If all you want is to lean out your cruise, grab a pair of calipers and measure the metering rod you have now on the fat part. Whatever that is, you want a metering rod with a .004 bigger diameter there, and the same diameter on the small.

If you can’t get that, you have to do the math and find a main jet that gets you .004 on cruise and has the same WOT size you have now. Which means the WOT part of the metering rod needs to be .004 bigger (or whatever the number is that you are trying to hit) so you only lean cruise and not both cruise and WOT.

That chart is a PITA. I measure every metering rod. I’ve found some that aren’t what they claim to be and that can jack you up.

You are dealing with the springs, the main jets, and two different diameters on the metering rod. You have to juggle all that crap until you get what you want.
 
Maybe my engine is just really efficient ? But my mpg doesnt reflect that . Lol
 
So your vacuum is 14” @ idle and as soon as you touch the gas the vacuum level drops, the spring pushes the rod up and your running on the second step of the rod with the 7” spring. Have you installed a weaker 6” or 5” spring?

The jets are no harder to change than a Holley carb except you don’t spill fuel everywhere.
 
yellow rose said:
I you use a smaller main jet, you lean out cruise and WOT whether you want to or not. If all you want is to lean out your cruise, grab a pair of calipers and measure the metering rod you have now on the fat part. Whatever that is, you want a metering rod with a .004 bigger diameter there, and the same diameter on the small.

If you can’t get that, you have to do the math and find a main jet that gets you .004 on cruise and has the same WOT size you have now. Which means the WOT part of the metering rod needs to be .004 bigger (or whatever the number is that you are trying to hit) so you only lean cruise and not both cruise and WOT.

That chart is a PITA. I measure every metering rod. I’ve found some that aren’t what they claim to be and that can jack you up.

You are dealing with the springs, the main jets, and two different diameters on the metering rod. You have to juggle all that crap until you get what you want.
Click to expand...

Which is why I say
"Easy to tune my ***!"

Holleys have nice compartmentalized circuits . Much easier to make a change without effecting everything else .
I guess I should do the math on total area between rod/jet combo but Im lazy .
 
Mopar73340 said:
So your vacuum is 14” @ idle and as soon as you touch the gas the vacuum level drops, the spring pushes the rod up and your running on the second step of the rod with the 7” spring. Have you installed a weaker 6” or 5” spring?
Click to expand...

I have tried all the springs. The ones that are too strong make it idle rough . I assume the rods are bouncing at idle.
 
mbaird said:
I have tried all the springs. The ones that are too strong make it idle rough . I assume the rods are bouncing at idle.
Click to expand...

You are correct the stronger springs are pushing the rod up at idle. I would use the weakest spring and tune from there. Sounds like your running on the power portion of the rod mostly now.
 
If my idle vacuum is 14" and my cruise vacuum is 20 (For example) then the rod should be fully planted in the jet and I should be cruising much leaner than idle ? Correct ?
 
Mopar73340 said:
You are correct the stronger springs are pushing the rod up at idle. I would use the weakest spring and tune from there. Sounds like your running on the power portion of the rod mostly now.
Click to expand...

I forgot to mention my idle screws are about 1.25 turns out. So my springs should be correct .
 
That’s correct. If you have a vacuum gauge you can see while driving to verify the 20” for example but it most likely doesn’t take much throttle to be below 14” and the rod is up which effects fuel mileage.
 
Oh yeah .. 1 more ***** .
The jet size imprinting sucks ! None of them are readable even with a magnifying glass ! Certainly they can afford to lazer etch them . I resorted to measuring them with drills .
 
If it makes you feel any better I’ve never had one close out of the box. I have the equivalent of 4 or 5 of those strip kits. I believe people who say that have no clue how close it is just that it runs ok.
 
Mopar73340 said:
If it makes you feel any better I’ve never had one close out of the box. I have the equivalent of 4 or 5 of those strip kits. I believe people who say that have no clue how close it is just that it runs ok.
Click to expand...

I was thinking that exact thing .
 
My wife keeps saying "it runs just fine"
And "its shifting better now"
I say no its not .. can you feel that ? It sounds wrong ... she thinks I am an idiot ..lol
 
Its possible I have it set to tight .

I dropped to .098 mains and the stock .065 x .052 rods and it jumped to high 13s... stopped and swapped for .068 x .0555 rods and now it is in the high 14s - 15.5 on a cruise at 2200 . But now it stumbles on light tip in of throttle. I will try stronger springs . The accel pump on this 800 came from factory on the biggest shot . (Hole nearest the body

)
sccachallenger said:
Any chance your vacuum advance isn't working?

And appreciate the discussion/explanation of how this carb works!
Click to expand...
 
The clues are there....
IF the transfer slot position is correct, then idle quality should not change whether the rods are up or down. That is because there is insufficient airflow to activate the main system.
I suspect your pri t/blades are open too far at idle { big[ger] cam? }. Depending on a number of factors, low speed cruise can be off the transfer slots, NOT the main jets...which would explain why changing to smaller jets made no difference.
Another indicator that the blades are open too far is if you screw the mixture screws all the way in & the engine keeps running.
The first step in carb tuning is actually....ign. Get the correct amount of timing at idle that the engine WANTS; it will often be more than the factory amount because the factories had to be conservative. Once dialled in, check t/slot position. Also check that PCV is working. If it vibrates, vacuum is too low for it.
 
You must log in or register to reply here.
-

Back
Top