I installed an AFR gauge and a vacuum gauge in my '26T coupe with an SBC 355. It has a Quick Fuel SL 600 VS carb on an Edelbrock rpm dual plane manifold. I have been making changes and then driving it for a while. Then make another change. I started out with AFR in the 11-13 range in all modes, I have played with every circuit in the carb. My AFRs are now in the 14-16.5 maybe 17 at times at part throttle cruise, depending on the weather. My WOT AFR is about 12-12.5. The engine operates beautifully with this tune. No hesitations, bogs, or lean backfires. I wouldn't want any circuit to lean out any further. Except idle. My problem is I cannot get my idle AFR any better than 12.4:1 and still make the transition off idle good. I can actually get the idle AFR to a little better than 14:1 by leaning out the idle mix screws, and it idles well, but the transition circuit goes lean and starts giving an off idle hesitation. The next size up squirter might fix that hesitation, but I haven't tried that yet. It seems counter-intuitive to economy. I wonder if I have worked myself into a corner. Should I start the whole tuning process over, starting with the idle set at 14:1 with the idle mix screws and start richening up the other circuits? It has been about a 5 month process to get where I am. I am really happy with the performance other than the idle AFR. What's my next move?
I commend your tuning dedication, not many folks use a dedicated AFR gauge with a carb. If it's healthy at your current idle and transitions well. I'd call it done. Are you just chasing the number? It's a modified car, ideal idle for a stock vehicle may not apply. WOT and Cruise AFR would be most important to me, multiple cruise mph ( 30-40) and ( 70-80) to cover around town and highway. If a squirter change and better idle AFR are the goal, give it a try.
Next size squirter or the accelerator pump cam. There is a kit you can buy and experiment with. I think my Holley 950 HP uses the pink one.
https://www.motortrend.com/how-to/zero-to-hero-this-guy-reveals-dual-quad-tuning-secrets/ You may find some help in here.
Don't mess with jetting or cruise circuit or anything else or start over. What do the plugs look like? A rich idle circuit will load them up and foul pretty quick. 12.5 sounds pretty rich for idle. Kinda want to give an engine what it wants versus "chasing numbers", but yeah. I'm sure you've seen this page? If not... https://www.tccoa.com/d2/members/392bird/tuning.htm
I have heard of guys changing air bleeds on carbs that have them to fine tune. Sounds like a little more air vs fuel is needed on the primary side.
Yeah it runs great. Any body that drove it would be happy. My background is circle track racing. My carb knowledge consisted of curb idle, idle mix screws and main jets. I didn't care much about the carb as long as it would drive through the pits and run good at 4500-7500 rpm. Now being more into street cars, I wanted to learn more about carbs. So I decided to take this carb on for economy. I've read all the books, Vizard, etc., and now have a pretty good understanding of the different circuits. I have gone from about 12 mpg to almost 16 mpg. That's in town driving. This car seldom sees the highway. I just feel that it could be better yet. I'm not really looking for a "number" other than the bottom line MPG number. And I believe the last improvement is to be found by leaning the idle AFR. I know it will idle well at 14:1. I just need to get it transition well. That could however result in less mpg. I think I will give that a try. Aerodynamic wasn't in Henry's engineers' vocabulary when they built this car, so that's not helping anything either. You are correct, but I don't think I am at that point yet. It takes about 20 minutes to make a tuning change on the carb, then drive it for a couple of weeks, doesn't take much time out of my busy (LOL) schedule. Besides I enjoy solving these kinds of problems.
Have you tried tuning the vacuum advance, preferably using manifold for vacuum source. Adding more or less "in the can" timing can also alter the a/f ratio at idle. You seem to be real close to perfection. My q-jet responded well to tuning with actual numbers.
There's something to be said about your methodology. But remember that the engine doesn't spend much time idling under most circumstances. To make much of a difference in fuel mileage I'd think it would have to be idling pig-rich, rolling black smoke out the tailpipe. If you're working with something hotter than a stock cam, you may already have the throttle plates open slightly past the transition port to calm down the idle. Since the transition ports provide only a small amount of enrichment for a short period of time, it may not be worth a lot of extra effort. And increasing the accelerator pump volume will probably cost you more in mileage, even though it would likely improve the off-idle feel.
On a Chevy V8 always tune your AFR off # 6 (leanest) next is #3 Second leanest cylinder Its a balance game to play, Most will not go this far in tunning, I do not want to burn a piston in 6 or 3. -1,5,7 & 2,4,8 run close to same AFRs ,, Intake no matter the brand plays a big part of air fuel ratio delivery anything over a stock intake manifold.
I haven't messed with the air bleeds much. I did try up 2 and down 2 sizes from stock on the idle air bleeds, but didn't like either one. But that was with a different combination of everything else. So maybe an idle air bleed increase might help. I'm thinking that might carry through the transition phase, though.
How does the cam profile affect what AFR is needed at idle? I know in the old days, I'd always have to run a rather fat idle on an engine with enough cam to give it a rough idle. This was back when I used a 2 gas analyzer, looking at CO and HC. It was pretty common to go up to 5% CO to get the transition acceptable, and also keep HC numbers down.
I think while in town driving the engine is idling or just off idle more than most people realize. This engine is 9.96:1 CR. It has .043 quench. ProMaxx 183cc aluminum heads w/64cc chambers, 2.02/1.60. Cam is hyd. roller 215/224@ .050, 284/296 adv, .470/.490 lift on 112 LSA. Probably makes 325-350. Smooth idle, no lope. Trans slot is set at about .020 exposure. Idle speed screw can be turned about + or- 1/4 turn and stay in good relationship to throttle blades. Any gross idle adjustment is done on the secondary throttle stop. I replaced that stupid short set screw with a longer screw so a 1/4" wrench makes it easy. Yeah increasing the squirter could very well end up with less mpg. That's why I have resisted doing it. But I'll never know for sure unless I give it a try. I can always change it back. It's not like changing a cam. I would like those plugs to clean up. IIRC there are 12 or 13 points of adjustment on this carb. The number of combinations is probably close to infinite. I can play this game the rest of my life.
Yeah, this cam at 215/224 and 286/294 has a lot of overlap. The way I understand it is cams with a lot of overlap usually need a rich idle to overcome reversion, but this is a fairly mild cam that produces no lope and I know it will idle well at 14:1.
Cam profile dose affect AFR balance in intake & runner on naturally aspirated more so then forced induction, from idle ,cruise, wot.
A couple of thoughts; like others have said, idle mixture doesn't have a large effect on fuel economy unless you idle the engine a lot. One size larger squirters won't have much of an effect unless you are constantly on and off the throttle, which is a fuel economy killer anyway. The last thing is that this thing is getting close to 16 mpg in town, that is pretty amazing.
Is your fuel straight gasoline, or is it E10? The Stoichiometrically-correct ratio for straight gas is 14.7:1 The Stoichiometrically-correct ratio for E10 is 14.2:1. If you can get it to 14:1, you might be rubbing the edge of the accuracy of the gauge and Oxygen sensor.
The low engine speed operation with your cam, induction , compression ratio , piston & combustion chamber & exhaust system design MAY make it impossible to do any better than you have !? I'll add this , 55 years ago , in college , one of my classmates rolled in in a new cobra after spring break , complete with one of those 8- stack pornographic Weber setups on it ( c'mon you'd a been drooling too) and from that point until the end of the school year , his " mechanic" showed up every couple weeks for " tuning" .....from 300 miles away ( daddy flew him in) As well as that car ran ( & it screamed) it would not vary the tune on its own , no carb ,or cab expert, can .
When I started this project, I bought a range of main jets, IFRs, Air Bleeds, power valves and PVCRs, squirters and pump cams so I would have what I needed. I have three different QFT carbs, a 600VS, a750VS, and an 850DP on different engines. These parts fit them all. I only have the AFR gauge on one of them. I figure after I get to be an "expert" and get the Model T coupe all tuned in I can swap the AFR onto the T roadster and start on it.
Fuel is straight gas. So 14.7 is stoich. I don't understand why my gauge would be "rubbing the edge of accuracy"? I can get the idle AFR to 14 easily. I haven't tried, but I could probably get it to 15 or higher. Trans is 700r4. 3.89 rear gear w/28" tire. 1900 rpm at 55 mph. I almost never get on the Interstates with it. Ignition is Pertronix Flamethrower III. 18 initial +16 mechanical = 34 +14 vacuum @7", all in @2500. NGK 5671A-7 plugs @.035". Taylor wires. Zero detonation. Idle speed is about 800-850 in park. 700-750 in gear. Torque converter is one step above stock stall in the 1800-1900 range.
Somewhere you have to make a compromise between economy and driveability. I would make it by running the idle a bit fat, but playing with the squirter might be a better way to go. You can adjust both the size and the duration, so maybe play with a big bigger squirter, maybe not so big a cam? anyways, let us know what you end up with, so we can learn...
Well, I got the plugs out, but can't get a pic that would show anything. The ceramic is dark brown and the surface is sooty black and are pretty consistent through all eight. The pics just look like a dark spot. They all show rich as hell. I hadn't looked at the plugs in a while. They certainly verify the 12.4 idle AFR.
I admire your hard work in learning this science and testing each solution. Quite a journey. Education is always a worthwhile pursuit. I'm not sure how you will know when you've reached your goal. From a practical point of view, how many 'around town' miles does this rod do a year? On my rod, I drive about 6k miles per year. About , 80 per cent highway. I stopped working on mpg improvements when I averaged just over 20 on a trip of two thousand miles, with a mild/stock 350 SBC. Keep us informed of your trials and progress on this project.
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