A little clarification on adjusting idle air mixture on eddy carb

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71DodgeDemon340

71DodgeDemon340

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hey guys, did my 2 year plug change the other day and noticed the plugs are black, definitely running rich. Im running a edelbrock thunder series avs on my 340. I know the idle air mixture hasnt been adjusted in about 6 or so years. Edelbrock says to adjust them one at a time, here is a simple step by step listing of their procedure.

1.Fully warm engine and ensure choke is fully open.
2. Air cleaner in place.
3. Set desired speed with the idle speed screw.
4. Adjust the IMS on ONE side to get the maximum possible RPM. Do not go rich beyond the maximum speed point.
5. If the above changed the idle speed more than 40 RPM, then readjust the speed.
6. Adjust the side OPPOSITE of that in Step 4 to get maximum RPM.
7. Reset the speed.
8. Carefully turn each IMS to again get the maximum idle RPM.
9. Go leaner just enough to get a 20 RPM drop in speed.
10.Reset the speed to the desired RPM.
11.This is a Lean-Best Idle Set. Setting richer than this will not improve idle quality or performance, but could tend to foul plugs.

Is it better to do it this way or just use a vacuum gauge?

If i use a vacuum gauge do i set the idle speed then hook up the vacuum gauge and turn one of the mixture screws at a time or do they both have to be turned back and forth simultaneously to find a happy medium?

Any help or guidance is appreciated
 
Either way but black plugs can be from more than idle mixture. Simply short trips or short running time without getting things warmed can cause that, or "big cam" (Not all that big) can cause other circuits in the carb to run rich.

Once you find the peak "lean" towards the lean side, just at the point where vacuum or RPM "just not quite starts" to fall off, in other words, "lean towards lean" rather than towards rich side of the peak

It is IMPORTANT that engine be fully warmed. If it is cool in your area, some cardboard partially blocking the radiator, temporarily might do it
 
Both ways work and probably get you about the same adjustment.
 
67Dart273 said:
Either way but black plugs can be from more than idle mixture. Simply short trips or short running time without getting things warmed can cause that, or "big cam" (Not all that big) can cause other circuits in the carb to run rich.

Once you find the peak "lean" towards the lean side, just at the point where vacuum or RPM "just not quite starts" to fall off, in other words, "lean towards lean" rather than towards rich side of the peak

It is IMPORTANT that engine be fully warmed. If it is cool in your area, some cardboard partially blocking the radiator, temporarily might do it
Click to expand...
Too cold of heat range plug can screw with you too.
 
Thanks for the tips guys, i may try to do it edelbrocks way first and see how it goes. I do also take it on short trips, 10-20 miles but it always is up to operating temp rather quickly even when its cold out. Its not a huge cam but its decent.
 
I guess my main concern with using a vacuum gauge is you cant do one mixture screw at a time right? You have to do them both back and forth until peak vacuum is achieved?
 
It doesn't matter much if you use rpm or vac for adjust for strongest pull.
Vacuum is more sensitve and will better inform you of efficiency if you compared it with another tune at the same rpm.

When you're setting the idle mix in neutral, the only load on the engine is the internal friction. This why it is often better to turn the idle mixture screws a bit richer (1/16 to 1/4 of a turn) after establishing the best idle in neutral.

I'd experiment with the part throttle and cruising mixtures too (if there is a safe place to test).
The Carter design gnerally has two idle restrictions. So off idle/ very low throttle is not user friendly adjustable.
But the higher speed cruising and part throttle mixtures (50 -70 mph) can be leaned by changing the rods. Do this in small steps. When you go too lean the engine will surge or trailer hitch. You'll have to slow down then to complete your drive back to the shop. Then change back to a little richer. Use the chart in the instruction book to make sure you haven't altered the wide open throttle AFR. If you have, then change the jets to get that back where it was using the chart for guidance. Obviously don't play games by going WOT while testing for lean in cruise. Test for WOT at the track and to be safe experiment by going richer first.
 
Thanks a lot!

So it may seem like a dumb question but the idle air mixture only affects af ratio at idle correct?
 
71DodgeDemon340 said:
I guess my main concern with using a vacuum gauge is you cant do one mixture screw at a time right? You have to do them both back and forth until peak vacuum is achieved?
Click to expand...
'
You have to go back and forth a few times regardless of RPM or vacuum. This in part is because most intake manifolds have some sort of balance / feed through between the two sides. Of course an open plenum/ single plane manifold is moreso
 
Thanks so much you guys, for the guidance and quick responses, i started out at about 1.5 turns out. Ended up with the driverside screw a half turn out and the pass a full turn out, rpm at 800 engine up to temp, verified timing and its at 16 degrees btc. The jets and rods shouldnt need to be changed i wouldnt think so anyway, they are still the ones that the engine was dynoed with at the engine builders. I may try to see if i can swing by his place one weekend and let him double check the idle mixture.
 
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71DodgeDemon340 said:
Thanks a lot!
So it may seem like a dumb question but the idle air mixture only affects af ratio at idle correct?
Click to expand...
Well it depends on your Transfer port sync, and whether or not the idle ports have to augment them.
If your Transfer slots are not supplying enough fuel at idle, then you have to increase the mixture screws to make it idle. Then when you tip in the throttles, maybe you get a a lil tip-in sag until the transfers wake up.
If your Transfers are supplying too much at idle, then you will need to lean out the mixture screws,for a nice idle . And again you get tip-in issues.
In N/P, the mixture screws can work even at 2000 rpm; just put the carb up on the fast idle cam, and find out
 
AJ/FormS said:
Well it depends on your Transfer port sync, and whether or not the idle ports have to augment them.
If your Transfer slots are not supplying enough fuel at idle, then you have to increase the mixture screws to make it idle. Then when you tip in the throttles, maybe you get a a lil tip-in sag until the transfers wake up.
If your Transfers are supplying too much at idle, then you will need to lean out the mixture screws,for a nice idle . And again you get tip-in issues.
In N/P, the mixture screws can work even at 2000 rpm; just put the carb up on the fast idle cam, and find out
Click to expand...
I need to study up on your Transfer slot tuning. Heck, I have never even changed jets or metering rods in a Carter/Edelbrock. They run pretty good right out of the box. I don't have anything with a radical cam though. Thanks for your technology AJ!
 
71DodgeDemon340 said:
Thanks so much you guys, for the guidance and quick responses, i started out at about 1.5 turns out. Ended up with the driverside screw a half turn out and the pass a full turn out, rpm at 800 engine up to temp, verified timing and its at 16 degrees btc. The jets and rods shouldnt need to be changed i wouldnt think so anyway, they are still the ones that the engine was dynoed with at the engine builders. I may try to see if i can swing by his place one weekend and let him double check the idle mixture.
Click to expand...
A rolling dyno only represents a WOT run - similar to a drag strip but without the wind resistance. This is great. So you have a solid baseline for wide open throttle AFR.
Hopefully they provided you a file or at least a print out. (Although sadly this seems to be an exception rather than the rule)
A dyno with eddy current can be used to tune part throttle and cruise but that takes a very knowledgable dyno guy. Very very few guys are at that level.
 
Remember A/J, an Eddy should have the extra metering restrictions for off idle.
 
Mattax said:
A rolling dyno only represents a WOT run - similar to a drag strip but without the wind resistance. This is great. So you have a solid baseline for wide open throttle AFR.
Hopefully they provided you a file or at least a print out. (Although sadly this seems to be an exception rather than the rule)
A dyno with eddy current can be used to tune part throttle and cruise but that takes a very knowledgable dyno guy. Very very few guys are at that level.
Click to expand...

It was while the engine was on the engine dyno. I have yet to have it on a chassis dyno
 
Trying to adjust a carburetor without a vacuum gauge is the way to get it "close" but it will never be dead on. Attach the vacuum gauge to a manifold vacuum port. With the engine fully warm, and in drive with the parking on and wheels blocked, neutral for a manual, adjust idle speed until it is correct. Then adjust each air screw until you reach the highest reading on the vacuum gauge, then out 1/2 a turn leaner. Then readjust the idle speed as needed.
 
I had a vacuum gauge from work on it but it was one of the cheap ones, seems like it was aaround 13 in/hg at the best i could get. May be time to invest in a quality gauge? Ive also heard the bigger the dial the more accurate it is. Are there digital ones available? Are they any good. I know ideal af is 14.7 in/hg. Doesnt seem like im far off but some of that could be the cam

Stubbled on this article
Air-Fuel Ratio Meter Tuning- Car Craft Magazine

So about 13 in/hg is ideal?

Also not sure if it was my gauge but it didnt seem like it varried to much when turning the idle mixture screws. Ended up setting them up by listening to the rpms
 
RustyRatRod said:
Trying to adjust a carburetor without a vacuum gauge is the way to get it "close" but it will never be dead on. Attach the vacuum gauge to a manifold vacuum port. With the engine fully warm, and in drive with the parking on and wheels blocked, neutral for a manual, adjust idle speed until it is correct. Then adjust each air screw until you reach the highest reading on the vacuum gauge, then out 1/2 a turn leaner. Then readjust the idle speed as needed.
Click to expand...

You mean leaner or richer? I thought once the best vacuum was obtained you richen it up just a hair?
 
71DodgeDemon340 said:
You mean leaner or richer? I thought once the best vacuum was obtained you richen it up just a hair?
Click to expand...
If you're working in Drive, I'd go for best vacuum and leave it at that. It showing strongest pull with load. can't ask for better than that
71DodgeDemon340 said:
So about 13 in/hg is ideal?
Click to expand...
Depends on the combination. Don't shoot for a number, adjust for the best you can get.
 
RustyRatRod said:
Trying to adjust a carburetor without a vacuum gauge is the way to get it "close" but it will never be dead on. Attach the vacuum gauge to a manifold vacuum port. With the engine fully warm, and in drive with the parking on and wheels blocked, neutral for a manual, adjust idle speed until it is correct. Then adjust each air screw until you reach the highest reading on the vacuum gauge, then out 1/2 a turn leaner. Then readjust the idle speed as needed.
Click to expand...
Got any ideas on how to calibrate or adjust two Eddie carburetors on a tunnel ram with no vacuum port and a 4-speed?
 
With a manual trans;
Depending on your overall starter gear,and your cylinder pressure;I highly recommend not to try and run the Vcan full time off the manifold. Your low speed will get too jumpy as the Rs fall, and it will not pull itself very well, at low mphs. This is because the pressure pulses will be too strong. Instead, as the Rs fall to under 650rpm in gear, you want to retard the timing, down to perhaps 5* at 550. This will weaken the pulses. The engine will lose power,and smooth out. She might want a slightly higher idle speed screw setting in compensation.
Of course, with a mechanical D this is not possible to do. But if yours gets jumpy, now you know why.
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This is why I only run 12 to 14 idle-timing. Then I run a lot of advance in the Vcan to compensate.I also run a dash-mounted, dial-back, timing device (with a full range of 15*) so I can retard it as far as I need to up to ~8 degrees. In this way I can get the idle down to 550@4mph, a brisk walking speed. On flat,level,hard ground it has just enough power to pull itself.
These things gain importance as the cylinder pressure rises.
When the engine is cold she wants a lot of advance. If it doesn't have a choke, you gotta baby sit the throttle for a couple of minutes. Normally the choke also increases the idle speed, sometimes to the point that the Vcan gets activated for several degrees. Here is another place that I use the Dial-back timing device; this time I dial in 7* for a total of 20ish degrees. And set the low-speed circuits a tad rich. In this way, I don't need a choke, and it is GONE.
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Here's how I set the mixture screws on a manual trans; fully warmed up of course. I put the carb up on the fast-idle cam at 1800 rpm or so. I put the air cleaner back on. Then with the engine at ~1800 I set the mixture screws for the highest rpm. Then I add 1/8th turn rich for a Holley and 1/4T for anything else,kick the fast idle down, and shut the car off. The extra 1/8 T is for cold-idle compensation, during the warm-up. Then I screw the mixtures screws in counting the turns as I go. Then I add the turns together and divide by two, then reset the screws to that average number.
If the car won't idle after this, I might bet a dollar the throttle valves are not far enough up the transfer slots. Rarely are they too high. So crank up the curb idle until the engine idles sweetly but too fast. Then retard the stinking timing to whatever idle speed you want. Then fine tune the idle quality one last time with the idle SPEED screw, now adjusting idle fuel with transfer slot exposure.
In this way the mixture will be pretty close @1800 in low gear, for most of the time. At 1800 the Vcan should be on line as well.And the D will have cranked in about 6*. If you had the initial finalized at say 12*, now you have 18* plus Vcan . Mine has 22* by this time, so my total is 40*. Because of this, I can set my carb pretty lean at 1800rpm on a warmed up engine. But this does not help the engine one bit when she's cold, and neither does setting the mixture screws any other conventional way. And nothing irks me more than a neighbor babysitting his carb for 5 minutes on a cool morning, so I don't want to be the guy blipping the throttle with twin 3" cannons pointed his way,lol.
BTW, I use 1800, because with my combo, 1800rpm gets me the following cruising speeds; 13,21,29,41,52.I have it geared this way because the pipes are singing a relaxing melody at 1800. Some of these speeds correspond very closely to popular posted cruising speeds, making pulling fuel economy as good as it gets.. If you have 3.55s and 27" tires, you'll be similar. If you have 3.23s , then your rpm of interest might be 1650 to achieve those speeds; but your Vcan may not be on line yet, which if it's not,is gonna suck for economy. To prove it is or is not, you will have to Tee into the spark-port and run a vacuum gauge up to the windshield, and take her for a ride, paying particular attention to the rpm/vacuum,at the common cruising speeds.
Of course if you don't care about fuel-economy, then I guess none of this is relevant.
In any case; Happy HotRodding.
 
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