LS 80Vette

George...... I forgot to mention, on the jet/rod chart, 12% lean on cruise and 6% lean on power, lies between 25 and 26, but closer to 26 ( 12% cruise and 4% Power )so that's what I will go with, ( 26 ).....

gkull

My take was he put together a 383 project and did not want to incur additional expenses of getting another carb.

For the record I only have one carb for my Vette. A modified Demon full on double pumper flowing @ 825 on the flow bench. My present little 383 also has a single plane and only about 10 inches of vacuum at 900 idle with my big solid roller and @227 cc big intake ports with big valves.

It runs fine. But not with an out of the box carb or distributor. It also get 20+ mpg avg on 1000 mile road trips.

The carb is way too big........... But when you floor it my little motor goes with mid 500 HP

gkull

It should help, but jets are cheaper than rods. You might just go down to 110 jet and keep the existing rod. First you have to get it going without wet carboned up plugs. Then go for fine tuning.

Page 16

http://www.edelbrock.com/automotive_...412_manual.pdf



The Edl manual and charts just suck. That is where in my first posts I referred to Math and area of a circle. Do you understand the relationship between the stepped diameter of the rod and the jet diameter as to restricting flow to get a A/F ratio?

Thanks V2racing for having a little faith in me!

keithinspace

God bless. I'm sure you're doing something right!

Cesar Rodriguez

I have a Road Demon 725 on mine.. It works very nice... but she is surely thirsty.

scottyp99

If you don't understand what gkull means by "the relationship between the stepped diameter of the rod and the jet diameter as to restricting flow to get a A/F ratio" here is an explanation, taken from Lars Grimsrud's paper on tuning Q-jets. The rods and jets in an Edelbrock carb work the same way:

"Metering rods, suspended from a power piston, “plug off” part of the area of the main
jets by being inserted into the jets. These rods have a “fat” diameter and a “skinny” diameter: The number stamped into the side of
every metering rod is the “fat” diameter indicated in thousands of an inch. This part of the rod is pulled into the main jet at cruise, at
idle and at other high-vacuum operating conditions (light throttle). It produces a lean operating condition for good fuel economy and
good throttle response. When engine vacuum is lost, indicating a high-power condition, the rods are pushed out of the jets by spring
pressure, and only their “skinny” tips, or power tips, remain in the jets. This richens the fuel mixture for peak power."

I hope this helps,

Scott

gkull

Scottyp99 - you have it right. So if you do the flow area of the jet minus the diameter of the rod area is the total flow area.

So the EDL charts only give these big jumps in % ranges. So if you do some math you can fine tune total flow outside of EDL's limited choices

Lars understands the Q-jet. The tapered rod has a linear transition. where the stepped rod is an abrupt A/F change. It is not a perfect solution and I'm not sure why Edl didn't follow it.....

The 750 Edl that I used for 10 years had all kinds of mods. Polished pistons, tapered home made rods, springs, drilled primary blades, Off road needle and seat, chokeless, squirter changes, and wood 1/2 thermal manifold spacer. It is still running today on a friends 428 Mustang

scottyp99

I would think that the difficulty in modifying rods would be in keeping them both exactly the same. Doable, though, with some patience.

Scott

gkull

Area of a circle:

http://www.calculatorsoup.com/calcul...ane/circle.php

Model #1412 & #1413
Rod/Jet Reference Chart
REF # MAIN JET METERING ROD CHANGE FROM BASE
1 1433 (.113) 1453 (071 x 047) none - stock calibration

Pi X R2

1/2 of .113 = .0565

Area = .0100287


1/2 of .071 = .0355

A= .00395919

I'm gooing to start rounding the numbers.

.0100287 - .00395919 = @ .006


The proposed #26 on the chart rod and jet change

26 1432 (.110) 1456 (073 x 047)

1/2 of .110 = .055 A= .0095
1/2 of .073 = .0365 A= .0042

A= .0053 Which is @ 12% leaner

I proposed not following the chart and just changing the jet to a .110

1/2 of .110 = .055 A= .0095
1/2 of .071 = .0355 A= .00395919

A= @ .0055 Which is @ 9 % leaner on cruise

gkull

Scott, time consuming. I used lead solder to increase the diameter of the power, small diameter of the stepped rod. Then I used 600 sand paper to taper the small end so it would transistion into full rich.

Anything is better than a step and I used a rear wheel dyno and oxy sensor to dial in the best A/F ratio. Today you can just buy the A/F ratio gauge for your dash and really dial in your carb

scottyp99

OK, so the OP has decided to work with what he has and try to get it tuned in for the engine's needs? Sounds good to me. Although, he sounds like he doesn't know that much about how carbs work, and just wants someone to tell him what he needs to do. I can understand that attitude, but a good working knowledge of the basic principles of how a carb works will be worth expending the effort to gain. It just makes it so much easier to get a carb dialed in if you know a little bit about what the changes you are making are actually doing. It also makes it easier to understand some of the answers to questions you may have. There is plenty of material out there on the internet, if you can take some time and look for it, it will serve you well. Here's a decent article to get you started:

http://www.popularhotrodding.com/tec...k/viewall.html

Good luck with your carb, I think you're in good hands with gkull.

Scott

LS 80Vette

George, I more understand the concept with the % now and I can figure other % if I need to....It's just hard to understand that such a little difference in area (a few .000 ) would make a difference. But if you scale everything higher, keeping the same ratio of increase between everything, then it's easier to see how it can make a difference.

The challenge is going to be when I adjust the mixture screws, either by vacuum or idle, because the idle is lopey, and the gauge needle does a lot of jumping...

gkull

You turn up the hot motors idle up to 1000 or even 1100 after setting the max timing. You attach the vacuum gauge and then adjust the idle screws. As you get the screws close to perfect the idle will increase along with vacuum. so you have to keep turning the idle back down.


the idle has to be high enough to have a fairly steady vac needle.

You might also try turning up the idle timing just for setting the carb. You will here it smooth out around 16-20 initial.

I sat down with a hand calculator and wrote down all the calculated diameters of both of the rod steps and jets in the ranges that I thought I would need. So I could just look at the chart and pick out of my carb box any jet and rod combination i needed for minor adjustment as the air density alttitude changes.

I never had anybody help me, it was before the internet. I just got to thinking one day about Edl's 4% or even greater choices are not what I wanted and I thought about how to figure out what they were actually doing. I also took alot of math in high school and college. So i knew most formulas by heart

scottyp99

In a conventional 4 stroke gasoline engine, the fuel mixture being used to run the engine is around 14:1. That's 14 parts air to 1 part fuel. That's a lot of air and a small amount of fuel, relatively speaking. That's why such a small difference in the area of the orifice that's metering fuel can make such a big difference. Also, when you increase the diameter of a circle, it's area increases quite a bit more that you might think. It's because the area of a circle equals the square of the radius of the circle, multiplied by Pi (3.14). If that just sounds like gibberish to you, just remember that a small increase in diameter gives the circle a lot more area. Like, a circle with twice the diameter has a lot more that twice the area. Not sure if you're having trouble with the area of a circle or not, but a lot of people do, so I just thought I'd throw it in there. Also, high school was a long time ago, so if any of this is incorrect, feel free to bust my stones about it. Just for yuck-yucks, here's a Pi joke:

Math teacher: Pi r^2

Bubba: Pie are not squared, pie are round. Cake are squared!

Oooooo........I can hear the groaning, even over the internet! Sorry.......

Scott

Shark Racer

Agree with most of the above except for the joke and I'd correct you and say stoich is closer to 15:1. (14.7:1) (and more like 14.68...) (assuming pure gas)

Luckily the typical lambda(O2) controller reads 14.7 at stoich regardless of fuel makeup.

scottyp99

Well, I did say around 14:1, just because the a/f ratio changes according to a bunch of variables, like load, rpm, etc. And, it's safer to err on the side of being a skosh rich.

Actually, this may be a good chance to get your view on something I just read recently. It said that the actual stoich for E10 is almost exactly 14:1, and I guess that makes sense to me, because of the alky. If this is true, shouldn't we all be shooting for a/f ratios around .7 lower than we are used to with straight gasoline, when using E10? What are your views on this, gkull? And, is the OP using an a/f meter? I recently picked up an LM-2, and it works great. Makes carb tuning much more precise. Will the LM-2 read 14.7 stoich no matter what fuel I'm using? Hmmmm, I guess it's time to re-read the instructions.

Scott