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I'll probe the air bleeds to check. I'm only measuring the drivers side at the moment as I'm told the dual plane manifold makes each side of the carb feed both sides of the engine i.e. left barrels feed 1, 7, 4 & 6 and right barrels 2, 8, 3 & 5. Seems to me AFR should be the same both sides.
I really need to physically check the primary slots and secondary throttle balance. The carb is supposed to be a 4160, but has dual feeds and a secondary metering block without idle screws. See old photo. I understand there is a secondary power valve but haven't had it apart yet. All my work on IFR has been on the primary side.
Are you using the tailpipe probe for your LM-1?
It seems there have been reports of tailpipe readings running too lean on some cars at idle, but OK on other cars. A C3 without a crossover pipe has very strong exhaust pulses. There is very low flow at idle, you could be drawing air back in.
Porsche guy's dyno experience: Compared tailpipe probe to a welded in bung, t/p probe went lean at low rpms:
Corvette guy's experience and solution (Jim Lockwood): >I use the tail pipe clamp from Innovate but, as designed, it permits too much extraneous gas to enter and this gives false lean indications at low speeds/low flows. To solve this I made an extension that sampled exhaust gas farther into the tail pipe:This completely solved the false lean indications. I get accurate AFR indications at all engine speeds from idle up to wherever I chicken out. At low exhaust flow, there is a slight lag between throttle input and AFR indication. An upstream bung might get rid of this, but I don't find it to be much of a limitation so I just live with it.<
This is very interesting and important! I really have to be sure that there is no measurement error or I'm wasting my time.
I am using the tailpipe sniffer on the LM-1. I have a 2 1/2" system on my 69 and a crossover - H - pipe. I do have a bung, I think on one side only and it is upstream of the H pipe, but I'll check and switch to that when I get back to testing tomorrow. That way I can see if there are measurement errors.
The measurements of AFR at idle taken at the bung are wildly different to the tailpipe!!! Like 21-28 at the tailpipe and 12 at the bung!!!
See attached measurements.
For now I've left it at 1 turn out on the idle screws. Let me know if you think I should choose the leaner settings at 3/4 of a turn. (Finally the AFR is VERY sensitive to the idle mixture screws!)
You'll see I've increased the timing to 15deg initial. I have not yet removed the carb to double check the transition slot at 0.020 and balance but will probably do that soon. Quick question - are there IFR in the secondary metering block that affect the idle mixture, it does not have idle screws? I've only played with the primary metering block.
I'll start measuring fuel economy and put a clean set of plugs in it.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Think it depends on the age and cfm of the carb. The newer bigger carbs have 4 corner iar on top that are replaceable so I would guess you could tune the secondary ifr as well but other than setting the Idle transition slot I don't think you would need to unless you are big dollar racing. As for your carb I would try just setting the slot to see if it helps when the secondaries open.
I deal mainly with tripowers which are a 6 barrel with detached secondaries. The stock setup for that has no secondary tuning ability at all other than cracking the rear throttle blade to get a stable idle. I modified my center carb significantly that I can control my idle through it
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
NOw with your idle at 13.5 I would look at your tune for your driving experience...you don't drive at idle but you will be in the transition a lot and cruise at higher speeds. Te transition slotis your main fuel up to about 2000 rpm. I would start tuning those for fuel mileage or performance. You need to tune one at a time leaving the previous alone so it is a set value in your calculation. They all add together as you accelerate so you are in idle, then idle+transition, then idle+transition+cruise which includes how your accelerator pump comes in if you stomp it plus the power valve if you lug the engine. Your power valve could be set high enough it comes in too early and you are running too rich while you are cruising and not know it. My idle vacuum is 13 hg and I still have a 9.5 hg power valve in their from a previous tune and it should be a 6.5 . I can see it open when I'm climbing a hill at 70 on the highway as the vacuum drops my afr will go from 15 to 10.
I know mine is wrong I just haven't put a lower valve in it yet
Having an afr gauge and vacuum gauge you can see while you drive it is a next step in detailed tuning unless you have a chassis dyno.
Now it's time for some driving tests.
You can do it with or without gauges.
Just see how she runs and make notes.
Try all the circuits: 1500/2000/3200 cruise, easy accel, med, & hard.
Low rpm, med , & high.
If you get a bog or a rattle make a note of how and when, rpm, etc.
A vacuum gauge and the AFR gauge make this infinitely easier and more accurate.
If you do not want to permanently install these things, go for something like this, but you will probably need a co-driver to read it and take the notes! LOL This is an NCRS car getting a fuel psi check because of a bog. Turned out the fuel pump was getting too weak for a 454. You could do similar with a vacuum gauge, and the LMI AFR should reach inside. I know it data logs, but you need the vacuum and rpm and AFR all at the same moment.
OK so I've taken few steps back ....
I removed the carb to see that the primary throttle transition was sitting at about 0.080 and the secondary closed. I adjusted both to 0.020. I also found that the secondary side had the 0.055 IFR so I added 0.014 wire there to match the primary. The engine ran immediately at a much faster idle 1800+ so i needed to adjust both throttles back evenly about 1 & 1/8 turns. You'll see from my notes attached, that the transition has leaned out. I think I need to adjust IFR wire size to get back to where I was.
A lot more work to do and I'll probably pull the carb off again and recheck everything before I do some real road testing with a passenger to record vacuum and AFR under various conditions.
I'll keep you posted on progress.
Joe
Ok you are making good progress!
It's good you checked the "slot" size. That would cause issues.
Now take GOOD notes,
and please only change ONE thing at a time.
(You changed 2 things at once up above, the slots and the IFR)
That way you can be sure what each change did.
It can be a time consuming process, but you can get it purring / growling by the end of it.
Pinging is usually from timing.
I would address that first, if it runs OK.
That 15.6 is mighty lean. But if it runs OK you can leave it.
It will surge, misfire, etc, when it is too lean. Then richen it back up a little. Mine was so lean it surged when it was cold out, but not in the summer.
Agreed, I think I would put the secondary IFR back where it was.
Last edited by leigh1322; Jul 5, 2024 at 01:59 PM.
Agreed, I think I would put the secondary IFR back where it was.
Hi Leigh,
Do you think I should simply remove the secondary IFR wires I added or change all four from 0.014 to 0.010 to get the same overall reduction as 0.014 in only two, but with all four IFR the same?
In any case, I'll bring the timing back to 11deg and see what that does to the pinging first. (agree one thing at a time, rookie error )
Joe
It's all experimentation now.
But I would tend to leave the secondary stock, and do all the changes on the primary, IFR restriction wise.
Some recommend keeping both slots exactly the same.
And others recommend keeping the front at exactly .020" and doing any needed idle adjustments with the hard to access secondary screw.
That method only really works if you put a hex head screw in the upside down secondary screw.
I have never done it that way, and it does seem a little extreme, but as long as you do not need to change the front from .020" very much, it shouldn't matter. But you do want to keep them pretty close to that, so you may need to tweak the back once or twice to get the idle speed correct.
I have always heard it is best to keep the front slot, between .020" and .040". A method I have done is to measure how many idle screw turns (from "0") it takes to get to the .020" slot position, and then how many more to get to .040", and write them down in your notebook. That is the range that you must stay in on the front slots. And those notes will enable you to check that easily in the future without pulling the carb.
Got rid of the pinging / rattling by bringing initial mechanical timing back to 10 degrees advance.
I have removed the 0.014 IFR restriction in the secondary and back to just in the primary.
Pulled the carb again to see all throttles are almost closed with slots barely visible. Recorded the turns from closed to 0.020 and 0.040 as Leigh suggested.
That said, the 700 rpm idle speed is stable and achieved with only a 1/4 turn on both throttles and well short of the additional 1 or 1/2 a turn needed to get to 0.020". I've kept them both the same at 1/4 turn for now.
See vacuum and AFR results at no load today. I've left idle mix at 1&1/4 turns and probably don't want to go richer.
Hi All,
I probably should mention that I've checked for vacuum leaks. Can't find any but I do run a PCV into the carburetor base and that could be introducing sufficient air at idle to require the throttles to be closed down.
So I end up with transition slots practically closed.
Joe
Hi All,
I probably should mention that I've checked for vacuum leaks. Can't find any but I do run a PCV into the carburetor base and that could be introducing sufficient air at idle to require the throttles to be closed down.
So I end up with transition slots practically closed.
Joe
Hi Mel,
Yes, the PCV came with the ZZ502/502 Deluxe Kit. I mentioned it in case you guys thought there was any merit in going to a breather / catch can instead of PCV.
Joe
Hi Mel,
Yes, the PCV came with the ZZ502/502 Deluxe Kit. I mentioned it in case you guys thought there was any merit in going to a breather / catch can instead of PCV.
Joe
Each of those setups would require a different tune to the primary idle/cruise circuit.
PCV is a benefit. Leave it in.
Race cars don't use them, but then they do not run at part throttle either, so a PCV would be basically useless there. Street cars need them.
I like your afrs now, all 12s & 13s.
And you got rid of the 16 which could cause a lean stumble.
Looks like you are making great progress.
So how does it run now?
The stumbling you noted at 1000 could have easily been from too much timing. Is it gone now?
Remember one change at a time!
Since the afrs look good it should be pretty good with slow throttle.
If it is ok slow, but stumbles on quick throttle, that is accelerator pump tuning
Last edited by leigh1322; Jul 13, 2024 at 06:45 PM.
Got rid of the pinging / rattling by bringing initial mechanical timing back to 10 degrees advance.
I have removed the 0.014 IFR restriction in the secondary and back to just in the primary.
Pulled the carb again to see all throttles are almost closed with slots barely visible. Recorded the turns from closed to 0.020 and 0.040 as Leigh suggested.
That said, the 700 rpm idle speed is stable and achieved with only a 1/4 turn on both throttles and well short of the additional 1 or 1/2 a turn needed to get to 0.020". I've kept them both the same at 1/4 turn for now.
See vacuum and AFR results at no load today. I've left idle mix at 1&1/4 turns and probably don't want to go richer.
On to road testing next!
Joe
Hi All,
Finally got some road testing done. Time and weather stopped me from getting this done earlier. See results below. (enough data to get a picture but not every data point)
I'd like your opinions, however my take on it is that I'm still a bit rich at idle, cruise and light acceleration and about right for medium, strong acceleration and WOT.
Since idle, cruise and light acceleration are on idle and transition circuit I'll probably try adjusting idle screws in by an 1/8 turn to lean out that circuit ... as Rescue pointed out, I spend a lot of time driving around on that circuit.
)
