No gauge is 100% accurate. 99% accuracy is 1% off. 1% off on the scale from 11-18:1 is a big deal. The scale is 8 whole numbers, and most gauges only displays whole and tenths. Add or subtract 1% (0.08:1) from the number that you see, assuming that it was already rounded up or down for the display, and your number will be off. For example, if you were at 14.57, and you added a 1% error rate (0.08) it will round to 14.7 (14.57 + 0.08 = 14.65, rounds to 14.7). Take that reading and go the other way and the gauge could read 14.4. So, a 14.57 reading could display anywhere from 14.5 to 14.7 (14.57 - 0.08 = 14.49, rounds to 14.5). See if you can get idle to 14.5, and leave it there. I would not set lean-cruise past 16. Detonation begins well before you can hear it. If you can hear it, it is doing damage.
I would keep stepping up the idle air bleeds by two sizes, until you can get 14.5 at idle, by turning the screws no more than 2-turns out. The bleeds are cheaper in kits that have a range of sizes. Is your carburetor equipped with intermediate air bleeds? Have you adjusted the accelerator pump arm adjusted to zero gap?
I believe the original idle air bleeds on that 600 are 0.039". For perspective, they go all the way to 0.095", mostly in 0.001" steps, so there is plenty of room to tune.
Here's the way that it was laid out to me over 40-years ago. At idle you are on the idle circuit. When you are on the primaries, you are on the primaries, and the idle circuit. When you are on the secondaries, you are on the secondaries, the primaries, and the idle circuit. The idle circuit never goes away. It is always contributing fuel and air. If what it is contributing is far too rich, and the primaries are good, they are both not properly calibrated. Lean-down the idle. Richen the primaries. Take the extra fuel from where it should not be, and put it where it belongs, then adjust your pump shot to get you up on to the primaries.
That is a last-ditch patch for poor tuning. The carburetor in question is tunable 96-ways from Sunday. You add more air at idle by putting idle air bleeds with larger holes.
I've read that the "idle" circuit is actually in play up to around 35 or 45 mph, it doesn't add a lot of fuel but it does add some. It is kind of misnamed in that it is important to be adjusted correctly for just off idle acceleration and above where a lot of driving is done. Very little air flows through the venturi at idle, carburetors get around this problem by utilizing manifold engine vacuum through the idle circuit to get good velocity for very fine fuel atomisation. That's one reason why correct ignition timing is so important, as it has a direct effect on the manifold vacuum. The carb depends on a strong crisp signal for metering. At the moment of acceleration, there is a "lag" where extra fuel needs to be added until the air velocity picks up again a split second later. That's where the squirters or pump shot comes in. During WWII and gasoline rationing it was common for people to disconnect the carburetor accelerator pump to save fuel, every little bit helps I guess. If the spark plugs are fouling then there is room for improvement. It might be that leaning out the idle some and fattening up the squirter or whatever will cancel each other out. One thing I noticed is a lot of people fixate on 14.7 or "stoich", and spend a lot of time trying to achieve that. It's too lean for acceleration and too rich for flat ground cruise and it may not idle well there either, so near as I can tell it really doesn't have much utility for tuning purposes.
When I first got my Falcon the plastic arm on the accelerator pump was broken. I drove it like that, in the city, and freeway, for two years. I have been told this feat is impossible.
On my current Holley 950 HP, since it didn't come with threaded IFR's I did the wire trick. Insert small gauge wire into the IFR hole to limit fuel at idle. Experiment with different gauge wires to see what works. It made my mixture screws less sensitive and have them around of a 7/8 turn out on them I also went more extreme and drilled out my metering block and baseplate for ported vacuum. The carb now is very streetable. I have 14-14.5 at idle and light cruise I can get it around 15-16 and still run low 11's on a drag strip.
For anybody that's interested in the more technical aspects of setting idle mixture and idle speed . . . This may be more than you want to know about the subject. https://www.holley.com/blog/post/he..._setting_an_optimized_idle_mixture_and_speed/ About a quarter of the way down the page there is a specific mention of having to possibly run the idle mixture just a "skosh" richer than a "lean best idle" setting to at least partly compensate for a lean off-idle hesitation.
Im curious to know the Ones that are using AFR gauge where are you taken a reading from / location . this is on "Naturally aspirated Engine" 1 O2 ? Or One on each cylinder If only using 1 O2 @ collector , on bank side of 2,4,6,8 and @ idle , 14. Ish or leaner ( higher the #) # 6 cylinder is much leaner then gauge says 16 ish & cylinder #3 ,15ish . Idle @ Cruise , If 1 O2 on bank 1,3,5,7 collector , # 3 is leaner then 1,5,7 & 6 is leaner then #3. @ WOT Gauge says 12.5 ish cylinders are tight / close 1,5,7,2,4,8 # 6 & 3 are closer to 14 ish or leaner . Egts monitoring is also information These are not exact numbers, A Chevy V8 ,( not Ls ) 6 & 3 aways leaner unless its been addressed , & engines slightly modified overstock to radical HP. This applies to a Single carb or multiple carbs, the only way to have close to equal AFR on ever cylinder is to Extensive work to intake & runners & some time on heads . Or One single carb with its own separate runner to that cylinder , ( more then one make adjustment after running temp reached because of expansion) For as many cylinder you have. This is why Mechanical Injection was popular because separate jet on each cylinder for all forms racing, With experience and testing and time can be successfully used in street driven.
You are correct in that the O2 sensor on a single bank is just an average of 4 cylinders. The fuel curve with many carburetors as installed or in practice is often pretty far off, so this is more than accurate enough to make a big improvement.
Fine thread and info. My only suggestion is to maybe take a look at the early/ mid 80 emissions carbs. There was some “setting” used to adjust with a dwell meter. I’ve no idea what or how it worked, but seemed to make the V8 Chevys pass emission tests. I understand you’ll not be going to a computer ect, but maybe knowing what it controlled (I don’t ) and how can assist in the efforts to your goal.
The information I try to contribute is to help others ,I don't know everything but I do a lot experiment , & around people searching for HP /Dyno Over the years . Before I was able to get my hands on some informational tools used, I have used a go pro to fim thing going on in driven conditions , Information, Information!! I know most I am doing / trying does not apply to 90% of ones here , But there are one here can help the week in cruiser & H-Der
over the last 40 year a lot of develop and research went into manufacturing producing engines, then the Hot Rodder Racer improves ,but once you get away from certain aspects of the original design it's give or take to figure out how to hop up & keep this particular topic AFRs balanced . Then its lead to Sequential EFI . Control of each cylinder.
You are aware all of this will change this Winter? The best you can do is tune for 'average' conditions. I tuned on the fat side this Summer after a new Holley install. It will lean out a bit when air density rises in a couple months. Joe
I could not agree more. Was just thinking as the OP is working with a carburetor,he might benefit from what GM did to keep them emissions friendly. Might be or may not be something to learn. I wasn’t trying to bring in PCM/ECMs, but more thinking along the lines when I’m trying to fix “2xxx” vehicle, I revert to old tech. Just wondering if understanding what “new” tech can do to assist. Heck, he has an O2 sensor
I think on the early GM computerized carbs, there may have been a dwell setting or specification for the mixture control solenoid. And that's just about all that I know on the subject . . .
The idle air bleeds are 70 front and 39 rear on the stock carb, however I have removed the stock secondary metering plate and replaced it with the QFT 34-9 metering block which gives me 4 corner idle. At the same time I replaced the 39 rear idle air bleed with a 70 air bleed. This was done, after conferring with QFT tech, because every 4 corner idle carb I have seen runs the IABs with all 4 corners the same. This was all done before I started this tuning process. I have a range of air bleeds. They are the easiest thing to change on the carb. Certainly worth a try. The Quick Fuel carbs use an idle feed restrictor. It is a small jet at the top of the idle feed channel in the metering block. Stock size IFR for this 600 carb on the primary is a 31. This channel feeds the idle port and the transition slot. This restrictor greatly affects idle and the amount of turns out on the idle mix screws and thus the AFR on rpms up to about 2500 rpm. (Holley says 2000 rpm.) I know this to be true from my experimenting. I am down to 28 IFRs and my idle mix screws are out 1/2 turn from light seat with idle at 12.4:1 AFR. I can squeeze it down to about 3/8 turn and get 14 AFR, maybe a little better, but the transition suffers. If I go to larger than 28 IFRs I start to loose control at the idle mix screws. Too large and the IMS become non functional. This 600 comes with a pretty rich tune right out of the box. Initially, with this carb on a stock Goodwrench 350 I had no control at the idle mix screws. In an effort to gain control, I modified it with the metering block for 4 corner idle and to be able to lean out the IFRs front and rear. It worked great on the stock Goodwrench and gave me back control with the IMS out 1/2-5/8 turns out. Plug color was good. A little rich. Tan but not black. Then I rebuilt and modified the engine, (threw everything away except the block, crank, and carb) used the same carb, and installed the AFR and vac gauge. Everything new. Now I have black plugs. I just did a leak-down test and a compression test to make sure the engine is healthy. It just now has 2000 miles on it. #1 cylinder leaks at 3%. The rest are at 1-1/2% -2%. 180 PSI + or - 3PSI. The engine is pretty healthy. Okay. I think Plan A will be to change the IAB's from 70 to 74 at all 4 corners. Plan B: If that leans out the secondary, I may go to the 74s on the primary only. Plan C: Go back to 70 IAB, squeeze the IMS 's down to about 3/8 turn and put in a bigger shooter. I think what this thing really needs is a restrictor in the idle port to separate the trans slot from the idle port . I haven't read anything anywhere of anybody doing this though. I can testify to #6 being the leanest. I have 2 #6 melted pistons hanging on the wall of shame in my shop. One my fault. One my engine builders fault.
I'm slightly more familiar with Carter and Rochester carbs than I am with the Holleys. But it looks like they still use their simpler, external float adjustment system. If you have the time and the desire I'd be interested in knowing just how much (or how little) the AFR is affected by small changes in float level. Also, does a float level change have a distinct effect on the idle and transfer circuits, or mostly the main circuit?
I know on my twin Carter W-1s ( 1948 Carter one barrels ) I run on my 250 inline six, float level makes just as much difference on the AF as a metering rod or jet change. Even .060" hight adjustment of the metering rods can change the AF 3/4s of a point.
With all the discussion about IFR 's & IAB's & their cumulative affects , one thing I don't see mentioned is throttle flap position / venturi air introduction . I would think that has a far greater effect on the amount of emulsion & combustion air the engine " sees" for combustion @ any / all rpm.
I was going to mention this, it's actually the fuel height that is the issue. The float height spec is just a bench setting to get it close. Since there are tolerances in fuel pump output pressure and flow variance and whatnot the fuel height needs to be checked at idle after installation on the engine. Holley style carburetors make this easy. The fuel height level in the bowl exerts a column pressure, all of the fuel passages and orifices and the rest of it are calibrated at the factory for this. So the short answer is yes, a float height or fuel height change of just a 32nd in. or so is equal to a jetting change. It's easier to "pull" more fuel when the level is higher. A defective pump, incorrect fuel height, or maybe both could bugger up the idle circuit, sure. The float & needle + seat arrangement sort of reminds one of a toilet bowl but unlike a toilet bowl the idea is to maintain a constant average full bowl at all times from idle all the way up to cruise and full throttle.
Quick Fuel is even easier than Holley to set fuel level. It has a sight glass instead of a plug. It even has ridges in the bowl casting where the fuel level should be. I always make sure the fuel level is at the prescribed level. I see no reason to introduce another variable into an equation already loaded with variables.
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