Idle mixture settings on a dual carb set-up

I am trying to tune the dual Carter W-1 carb set-up on my Chevy Inline 261. I have looked at the factory settings that list all the information for the stock single carb set-up. The single carb set-up calls for the idle mixture screw to be set at 1 1/2 turns out. Am I correct in assuming that on a dual carb set-up I should have each idle mixture screw set at 3/4 of a turn out, therefore equaling a total of 1 and 1/2 turns out total. I currently have it set at 1 1/2 on each and it seems to be running rich.

Thanks.

 

No.

Start at your 1.5 at normal idle rpm, then go in until you see some rpm drop. Then come out a bit. Do that on both carbs, but you really should have a UniSyn to see if both carbs are equal air flow at idle. Or, if you have really good hearing, you can get them sort of close by comparing the air noise from each.

The reason to get them close is that if one carb is barely flowing at idle, you will not get a good feel on the idle drop when turning the screw inwards on that carb.

 

Total agreement with F&J. UniSyn is the only way to go. It will turn you rough runner into a dream machine.

 

I would start with all the screws at 1 and 1/2 turn off the seat. Do not tighten the screw, just barely snug. Then out. As far as adjustments, make sure you adjust all the screws the same amount, in or out. This is as you set the idle with the throttle linkage disconnected and the idle screws so either one will raise the idle.

 

I just got my Uni-Sync in the mail today. I have a couple hours tomorrow to play with the carbs. Spending tonight trying to do some research to have a game-plan for tomorrow. Thanks.

 

With that being said, can someone give me a play-by-play on how to tune with the unisync. The instructions only mention making adjustments to the idle speed screw. There is no mention to the mixture screw. The Carter W-1 only has two adjustment screws. One for the idle speed and the other for the fuel mixture. Do I set the mixture screw at 1 and 1/2 turns out and then only tune with the unisync by adjusting the idle speed screw?

 

Did I miss something here. At 12:57 you ask for help and at 1:10 it is suggested you get a Uni-syc tool to equalize the carbie running. Then at 1:14, that's only 4 minutes later you say you got your Uni_syc tool in the mail. I want to know who will ship and get it to you in only 4 minutes.

 

The UniSyn is just a tool to measure or in better terms, Compare the air flow with the other carb or carbs.

Get your flow exactly the same at idle with carb linkage disconnected. Then make sure each idle stop screw is working against the stop.

Now with the flow equal, start to turn one idle mixture screw inwards to get a slight drop in rpm, then bring it back out just enough to get that rpm you had. (Most carbs setting info says to leave it at a 25 RPM drop to have it cleaner at idle for emmisions.)

When you get both carbs idle mixture correct, then you can hook up the linkage and check the high speed flow by re-adjusting the UniSyn valve to allow for more flow. Read the direcections..it's easy to do. If the high speed is way off, you need to adjust your linkage or remove any slop that is causing one carb to flow more.

One last thing; The Idle mixture screws are idle only, they do not affect mid to high RPM mixture.

 

start out at 1 1/2 turns out tighten in until it starts to miss then back off 1/2 turn and you are set. use the unisyn to synchronize the carbs to the same idle air flow then lock down the linkage..........

 

Gee, thanks Dickster for the play-by-play and time stamps. You didn't miss a thing. I am trying to dial a carb set-up in and had a question about a specific setting on the carbs. The Unisync instructions do not mention the idle mixture screw settings, so I thought I would ask here about it. My apologies...

Anyway, thanks F&J, that clears up alot of questions I had. I appreciate it.

 

Set your idle mixture screws to get the smoothest idle or most manifold vacum.

 

On your UniSyn tool; It most likely does not have adapters to fit various carbs. Just use a short length of radiator hose, or plastic plumbing fitting or even a piece of cardboard tube, if it fits good on the carb top.

The UniSyn will have a flat wide foam rubber gasket to seal to whatever adapter you make.

 

i WOULD LIKE TO say that i think the idel circuit does change the mixture through all speeds

when setting HC and CO on a 2 gas analyzer and realizing lowest readings on the idle air screws you will improve mileage, we used to do this time in and again and it always made a difference in mileage

when I was a 19 year old i run 2 Wa1 s on a 235 and they work really nice but you cant buy a manifold for $20 anymore!!

I dont want to rain on anyones parade but that is what i was taught somewhere proabbly at Bill Sauers Auto Tech which was over 20 years ago wow where did the time go, did a lot of tune up after that and had a lot of fun learned about the big gap plugs there and running lean mixtures

oh when i was a kid my 235 got over 20 MPG but who had money for gas then!!!

 

This may (or may not) help:

http://www.thecarburetorshop.com/Dual1barrelcarbs.htm

Jon.

 

Ever try it with a vacuum gauge and a T fitting??? I never messed with a UniSyn before... What do those things run for brand new??? ($$)???

 

I paid about $30.00 shipped from jegs for it.

 

Ditto on the Uni-Syn/Uni-Sync tool.

As to syncing by ear, a length of vacuum/aquarium/washer hose with one end in your ear and the other at the same point inside each carb works surprisingly well. With care you can nearly equal the precision of a sync tool. If the carb size/design, and the hose being used, allow the hose to be inserted this far, the best spot for the hose is where the butterfly meets the throttle bore. But caution, if you have the hose well into the carb venturi, don't rev the engine with the hose in your ear!

First, the carbs need to be individually synced with the linkage disconnected or at least not holding one way or the other. Then, the the linkage should be set so both carbs start to open at exactly the same time. Getting this step perfect doesn't matter on a race car, but a street engine will run nicer with the linkage carefully adjusted.

 

Cool! I think I can scrape that up... Thanks! You really should check with a vacuum gauge after you fiddled around with the UniSyn just to see how close they are to each other in idle mixture settings..

 

You also need to have heat under your intake.
See information at the Stovebolt site.

 

fantastic tip, i also am learning to work with the UniSync tool and Vacuum gauge, any way i was trying to figure out why my front carb would not get the bobber to come up to the middle like the rear carb untill i was down looking at the idle screw as i was adjusting it and i saw that i could see day light thru the carb top and the UniSync.
pressing down on the UniSync and the bobber came up and let me adjust the idle speed, but very aqward trying to adjust the air/fuel mix and holding down the UniSync.
i am going to take an extra carb today and go to Winchester Auto to find the hose that will fit and make my adapter.

thanks again F&J great advice.
rich

 

bought a 1940 master deluxe the end of last year been working on getting things fixed, has 2 W-1 carters and trying to get them set up. Been reading this post which is very helpful , first thing I did was turn in idle screws to start, both were out 2 1/4 turns came out 1 1/2 on both, went in and idle dropped came out to 2 seems good but driving down the road it surges below what I would say is 2000 rpm, have a unisyn coming this week. My questions are should I stay at 1 1/2 turns out on idle screw to sync it and if carbs are out of balance how to I even them out,with the idle speed screw or ??? thanks in advance, really looking forward to getting it on the road.

 

Remove your linkages from the carburetors.
Set both carbs at 1-1/2 out.
Start engine.
Run each idle screw in all the way.
Not at the same time though.
(You should experience a variance in idle)
If you don’t then you have a vacuum leak)
If all is good turn them back to 1-1/2.
Install unisys….Adjust your carb (THROTTLE) Not idle screws.. (without attached linkages) where both carbs are reading the same. Reattach your gas pedal linkage by adjusting their lengths. Do not move the carb linkages, it will change your carb unisys setting.

 

I've been running dual W-1s on my 250 for quite a while. Here is some of what I have found when tuning them. I have a wideband Air/Fuel ratio gauge mounted in the truck.

On a 250, the UniSyn tool doesn't work all that well in as delivered condition due to the very low air flow at idle. I pulled the tool apart and cleaned up all the ports to get maximum flow through them. When it was assembled, they left a bunch of casting flash around the holes. After cleaning it up, it works much better, be sure to grease the moving parts to help it seal.

Both of the W-1's must be the same carburetor number, metering the same, and adjusted to same. You basically have a two barrel split apart feeded three cylinders each. I used a dial caliper and measured the length of the linkage, just to be sure they were equal.

W-1s use the throttle lever to raise and lower the metering rod, the hight of the rod is critical, as its a three step rod. The lift at full throttle is also a critical measurement. Here I set up a dial indicator to be sure both were set equal. Just .015" difference in height can change the air fuel ratio a point or more.

To adjust the air flow, unhook one carb from the linkage. Have the UniSyn tool wide open and set on the first carb. Hold it down tight to the carb and slowly close down the tool thumb screw till the red float ball is mid site glass. Without touching the thumb screw, move the tool to the second carb. Using the idle speed screw, adjust till the red ball is the in the same spot as it was on the first carb. Keep moving from one to the other till both have the red ball in the same spot ( only adjust one idle screw ). Once both are equal, you can slowly adjust both to the desired rpm. Recheck using the UniSyn tool.

Moving on to the mixture screws. Start at factory setting, you should only have to make a slight adjustment rich or lean off the original setting. Mine reacts to the smallest turns, the air fuel ratio changes quickly from really lean to really rich, so thinking the screw is a clock face, turn only a few minutes at a time. Having a dwell meter with low rpm tachometer will help, the rpm will change with each adjustment. I tried a vacuum gauge, but it wasn't as accurate as the low rpm scale tachometer.

When you are happy with the idle speed and mixture screws, throw the UniSyn back on and double check the airflow. When happy with that, attach the rear carb linkage and double check the air flow. Adjust the linkage to equalize the airflow, NOT THE CARBURETOR!

 

My experience with a pair of WA1s on a 235 with a Tattersfield intake, the air intake screws are adjusted while listening to my dual exhaust. The front carb has more effect on the left pipe and the rear carb will have more to the right side pipe. They are adjusted to get a smooth and rhythmic put put out of both pipes, with no skip in idle. Once that is done, I don't bother with a Uniysn as they don't work as well as my ear. Listen the air flow into the to carbs and adjust the idle screw to get the best idle and match the pitch of the sound made with the air rushing into the two carbs. You will hear the difference between the two carbs when not in sync. It will be easy to adjust them to be and sound in sync. Try it and see how it goes. My two carbs have been in tune for years and have never given me or the engine any trouble.

 

These guys have got you covered. I will mention that once you have synced the 2 carb's throttle openings
and reconnected the linkage, blip the throttle a couple times and recheck. The better you have them matched, the better it will run at light throttle. Just because it was adjusted once isn't finished. You want it to repeat. Same with mixture. Do set it for best running and even between carbs, but blip it a few times and make sure it returns to a happy place.
Having the return spring on one or the other carb can mess this up. Best to have it attached to linkage. Just recheck to make sure.
As far as mix, I prefer using lean drop unless it's got a big cam. Stock engines pull ~18 inches of vacuum. If you are sure of the health and it's got a big cam that only pulls less than 15 inches, adjust to best RPM.
For Lean Drop, get everything adjusted as well as possible, then using a tach, adjust the mix to the point where the mixture screw is adjusted out just enough to hit highest idle speed. Another 1/16 or eighth turn will be a plateau of still a good mix, then even more will start to 'roll' rich. Find that just peaking point on the lean side. Then turn each screw in slightly, trying to get a 50 RPM drop total (half for 2 carbs). Too much in will be too lean, and the engine will 'shake' lean while the RPM drops. It takes a few tries and to develop a feel. Just keep your eyes on the tach and feel how the engine responds. Final adjustment should be at that peak lean point or just slightly in (50 RPM). It will rev crisper and smoother, start better and have less sooty plugs. Having it too rich at idle will tend to foul plugs, be harder to start and will tend toward sluggish acceleration. It also tends to load up at very light throttle, like cruising thru the parking lot.

 

Trying to adjust idle/air mixture on one or multiple carbs without using a vacuum gauge is fruitless in my opinion. You're looking for the highest vacuum reading you can get as you adjust each screw, but also need to try to get all the adjustment screws to be close to the same number of turns out once you're done.
This means you're going to be doing a lot of tweaking in very small increments, and going over and over until you've reached peak vacuum, and close adjustment at each corner. You may need to adjust the throttle setting rpm's also as you go if it gets too low or too high as you get the mixtures set.

 

I got a unisyn and disconnected linkage and have both idle mixtures at 1 1/2 turns out adjusted unisyn so ball is in the middle on first carb and when I put on 2nd carb the idle goes up when I remove it the idle goes back down kinda hard to adjust when the idle does that and idea why that happens and what I can do to correct?

 

Sounds like the idle mixture is to lean on the second carburetor. When using the unisyn I put it over the carburetor as fast as I can get my reading and remove it quickly.

 

Putting the unisyn over a carb creates a stronger vacuum signal in the carb. This can pull more fuel. If the rear carb is lean, this is just like putting the choke on part way.

I use the unisyn to set throttle opening (idle speed screws) on both carbs the same.

Using it for mixture isn't really the best due to the vacuum change I mentioned. I prefer a tach and/or a vacuum gauge to set mixture.

I'd suggest first setting all carb idle mixtures the same. Pick something from 1/2 turn to 2 turns. Set them all the same. After synchronizing both carbs, then adjust mixture. First, turn one all the way in until lightly seated. RPM and vacuum should drop. If it doesn't you can adjust until the tank runs dry and it won't be right. Do this for each mixture screw to make sure each affects mixture and idle speed/quality. Now go around and adjust each one say a half turn. You will see what happens to the RPM or vacuum. Keep adjusting all of them equally until you get that best RPM or vacuum. Finally, go back around and one at a time, note position and turn it in and out a bit for best number. Take at least a minute per screw and let it 'settle' in it's new mix. You may do this a couple times. Keep trying to get that top vacuum or RPM number. By now, you should have a 'feel' for when it's too rich (rolls) and when it's too lean (rocks).

 

@Joe H touched on an important point. Sync both carbs to the same air flow. THEN connect linkage. Recheck. At this point everything should be synced.
I tend to back off one idle speed screw, noting which is still holding itself, the linkage and the other open. Which ever is easier is probably best. Then set mixture and any needed idle speed is adjusted only at the one holding it all open. After all adjustments are made, check sync again.