Hot Rods Fuel Idler Screws

I am running 2 - 500 cfm Edelbrocks on my 1st Gen 392 Hemi in my HotRod
How to adjust fuel mixture idler screws. Have seen on here how to abjust on 1 Edelbrock, but not 2? My question, is it same steps on each carb for a 2 carb set up.
Jack

 

I set them both the same for starters.

 

I adjust all 4 screws at the same time. First get them all gently seated then out 1.5 turns. Then start the engine and get it warmed up and then go through the process of getting it to it's best idle, starting by backing them all out 1/4 turn and see what happens, etc. Make sure all of them get the same number of turns.

You also have to get the idle speed screws set the same, so the carbs are both open the same amount, and then when you change idle speed, adjust both screws the same amount.

But it's not the end of the world if they're not all perfectly identical, you just need it to idle OK, and idle doesn't win races.

 

You're running dual starters with idle screws?

 

I should say to start with. As Jim said is the approach I use.

 

I knew that. It's just sometimes my mind wanders and I'm never quite sure when it's coming back.

 

Without one of these it’s just a guess. The Unisyn is attached to a carb cover. Both are bought from Summit. If you’re idling on both progressive linkages or not. I check mine maybe once a year. Just adjust until the red pills are the same.

 

gEEz, what the Hell? Whatever it is, don't think it's something I need. Thanks anyway.
Going with Squirrels method and a va***n gauge.

 

Use a vacuum gauge, adjust one carb to highest vacuum reading, move vacuum gauge to other carb and repeat, don't forget to cap your vacuum port when finished.
You would want to repeat this process at minimum of 2 times since the first carb is playing off the non adjusted carb on the first go around, most of the time I do it 3 times.
Like was mentioned set both carbs identical with idle adjustment screw and all to start with.

 

I use a Wideband to fine tune mixture settings. Seems to work well and very handy if changing jets or other fuel mixture related parts.

 

A vacuum gauge will have the same reading no matter how you adjust both carbs if you are reading manifold vacuum and the manifold is open to both. Treat both carbs as one big one with four idle screws. If you turn one, turn all four, same with idle speed, don't turn one without the other. Once you get the mixture where you like it, you can back off the idle speed, then slowly bring them back up to idle speed. Have a tachometer were you can see it, when both are really close to the same, you will see the rpm change when you back one off, or speed one up. When set correctly, both should change the rpm the same amount if you turn the screws the same amount. The Unysinc works pretty good, but with a good ear and tach, you can achieve the same results. A wideband A/F gauge is the way to go for tuning.

 

Well ! It would certainly seem that everyone has a method, or variance thereof ! Confusing ,isn't it . Sure glad I never felt the need to ask ..

 

I set my dual carbs the same way I set my single 4 barrel Holley with 4 corner idle mixture screws. Once all 4 screws are adjusted they should be the same number of turns out at each corner. No difference between two 4 brl. carbs with primary only screws than one 4 brl. carb with 4 corner screws.

 

A number of guys have said how to set the carbs up right, but not really how to go about it.

I will offer a sort of tutorial of how I do it.
There may be better ways to do it, but I get good results my way.

I will say, I have never used a Uni-Sync to set up multiple carbs or a vacuum gauge for carb tuning or a Manometer, used to tune Rochester FI that I am quite proficient in.

I find that I can listen and hear and feel the engine tone change, long before I see the vacuum gauge reading change while adjusting.
I will use a vacuum gauge though, for troubleshooting and to determine how much vacuum the engine will produce at idle, how worn out the engine is, how lumpy the cam is etc.

With that said, here is how I adjust multiple carbs with idle circuits.
Holley 4 corner idle carbs similar.

1: Remove throttle linkage from both carbs.
Make sure choke or chokes, if equipped are open fully and that fast idle cam is not affecting the throttle opening when setting the throttle plate openings.

Screw both idle speed set screws out until the screw is not touching the throttle lever, now the throttle plates are fully closed.
Double check fast idle cams are not interfering.
Then turn the set screws back in, just until the throttle plates start to move.
Do this on both carbs.

Mark the screws with a reference line, in the same direction on both.
Now turn each screw in an equal amount, say 2 turns to start with, lining up your reference marks the same on both carbs.
At this point, both carbs throttle plate openings are equal.
Now reattach throttle linkage, making sure both carbs throttle levers are fully seated on set screws, adjust linkage if necessary.

2: Turn all fuel mixture screws in until lightly seated.
Turn all screws out an equal amount. Lets say 1-1 1/2 turns.
If you choose, put reference marks on the screws, the same direction, so you don't get them messed up later in the tuning.

3: Now you are ready to start the engine.

You will probably have to adjust idle speed somewhat and also fuel mixture screws equally, to get idling reasonably.

Warm engine up, then start adjusting all fuel mixture screws as equal as possible, A little at a time, to obtain the best idle possible.

You will have to adjust both idle set screws as your going, to keep at a reasonable idle speed the engine is happy with.

Once that is done, shut off the engine.

4: Now turn each fuel mixture screw in counting the turns until seated.
As hard as you may have tried to turn all equally, you may find some way different.
Write each number down.
Example, you may find one is at 3/4 turns, another at 1 turn, another at 1/4 turn etc.

Add the numbers together and divide by the amount of fuel mixture screws there are, in your case 4.

Then turn all mixture screws back out that number and fine tune again turning all an equal amount a little at a time.
Adjust final happy idle speed.

You may find the engine may idle smoother with some fuel mixture screws slightly different, but will all be pretty close to each other.

This may seem complicated and time consuming, but its really not.
I can do this reasonably quick, as I do a lot of them and they are pretty messed up by the time the car comes to me.

This is all ***uming the ignition is set up properly, initial timing, total timing, advance rate etc.
Most cars I get in the shop for poor running issues are due to a combination of poor ignition set up and a really messed up carb adjustment in an attempt to make run better.

Good luck

Bill

 

If they wind up the same number of turns out ,there really wasn't much point in there being more than one , or even to be adjustable ...

 

I have 2x4 setups on all my old cars, the mixture screws aren't the same amount of turns on each carb. I'm **** about tuning my carbs too, one corner might be 3/4 turn while another might be 7/8's a turn, it all depends on the intake design and the engine itself.
That's why I mentioned earlier it's a repeated process when setting them.

 

Not so if you do it right. Every carb manufacturer recommends that you end up with all idle air screws at the same number of turns. And if you're patient enough it's easily done. Simply turning them all to one point without trying to get maximum vacuum reading wont result in a good running engine. But once you've adjusted each you can take some more time to adjust the settings and easily get them all equal, and the engine running well.

 

The exhaust sniff test works okay if the mixture screws are close. Lean or rich are pretty much unbearable to stand behind. A 14/1 AF ratio isn’t all that bad.