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Many of you know I have an AEM Wideband Failsafe Gauge. I've been playing with it a couple days and will use this thread to document some of my findings, experiences, etc.
Currently the gauge is mounted on Rusty, and after yesterday's experience where the gauge showed Rusty to be running quite lean (18:1) at idle and about right at cruise I rebuilt the carb, finding gelled yuk in it. After rebuilding it the idle AFR came in about 13:1, which is much better and the engine idled MUCH better. So I then took it for a spin, namely up the longer of the two hills I hope to test David's inline 6 on in a few weeks. Then after the crest of the hill I turned around and nailed it. Here's the log from the idling, running it up through 3 gears, and then cruising at 65 MPG, initially downhill but then on flat ground where the AFR goes up. But, a couple of tips in reading it. First, vacuum is shown as manifold pressure, so up is lower vacuum. Second, something is wonky with the RPM. The gauge was set to 2 pulses per revolution, which is obviously giving way too many R's for the M. So I started it up and played with the parameter. It was idling at about 650 RPM and it took 16 pulses/rev to get the gauge to read that. And even then it fluctuated more than it should. So more investigation is needed.
So, what do we learn from that chart? One thing is that the vacuum the Eddy is seeing at high RPM is about 1/2". Another is that the WOT AFR is from 10.7ish at low RPM to 11.4ish at about 5000. Kinda rich from my understanding. And the cruise AFR is also rich, so some tuning is necessary.
Hopefully I can get my head around this meter and be ready to use it when David gets here with his truck so we can not only tune it but determine what it takes to get the excellent MPG he's getting.
Patrick - I agree the tach is way too jumpy, so I'm going to talk to AEM about it tomorrow. Wondering if the long run is an issue or if I need some filtering. As for those tests, but I doubt I have the patience to increase the speed gradually for 3 minutes. Why that long?
CMK/Jim - I agree that it is rich all the way around. But, setting a target of 12.5 at WOT isn't very easy to do if you mean at all points with the throttle fully open. If you look at the three run-ups you can see the AFR go steadily leaner as the airflow increases until the secondaries open, at which point it goes rich again and then starts going leaner as the airflow increases. In fact, that is the nature of a carb - the AFR leans up as the airflow increases unless another circuit kicks in. So, at what point should it be 12.5:1? At the mid-point?
Similarly, the ratio at cruise will change based on the RPM and the throttle setting. I watched today as I climbed the hill at a steady 65 MPH and the AFR went from about 13:1 to over 14:1 as I tipped into the throttle to keep the speed up. (And then promptly went back to 13:1 when the metering rods popped up at 5" of vacuum when I went on down on the throttle.)
Jim - I'm just running the 600 CFM Eddy 1406 on this engine. But, I'm not saying it is too low. I was just surprised how low it got.
Dylan - Make sure you read the part in the other thread about the distance from the ports the O2 sensor should be. Basically that is 18" - 36", but on a six that is going to be difficult to do given the length of the engine.
However, David/1986F150six will have his truck here in a few weeks and we'll be using this one on his so will have experience.
Patrick - The only reason the AFR would change if the timing was changed would be that the RPM changes and therefore the airflow through the carb changes. As said, carbs have a varying AFR with varying airflow if nothing else changes. So changing the ignition timing will change the idle RPM and therefore slightly change the AFR. Or, changing the timing will increase or decrease the power, which will change the amount of throttle the driver gives, and that changes the AFR. Otherwise there is no connection between timing and AFR.
CMK/Jim - I agree that it is rich all the way around. But, setting a target of 12.5 at WOT isn't very easy to do if you mean at all points with the throttle fully open. If you look at the three run-ups you can see the AFR go steadily leaner as the airflow increases until the secondaries open, at which point it goes rich again and then starts going leaner as the airflow increases. In fact, that is the nature of a carb - the AFR leans up as the airflow increases unless another circuit kicks in. So, at what point should it be 12.5:1? At the mid-point?
Similarly, the ratio at cruise will change based on the RPM and the throttle setting. I watched today as I climbed the hill at a steady 65 MPH and the AFR went from about 13:1 to over 14:1 as I tipped into the throttle to keep the speed up. (And then promptly went back to 13:1 when the metering rods popped up at 5" of vacuum when I went on down on the throttle.)
You want 12.5:1 on the top end. Beats the heck out of me how you want make that happen with a carb. When I tuned my Mustang I just changed the fueling table for the RPM and LOAD that the motor was at once I had the MAF dialed in.
You want 12.5:1 on the top end. Beats the heck out of me how you want make that happen with a carb. When I tuned my Mustang I just changed the fueling table for the RPM and LOAD that the motor was at once I had the MAF dialed in.
Can you ease that transition into the secondary?
Ok, let's say the plan is for 12.5 at redline. We are currently running at about 11.3 just before the secondaries open, so we will have to go to a smaller primary jet to lean that such that it approaches 12.5 w/o the secondaries. However, going leaner/smaller with the primary jets means the part throttle AFR will also be leaner if the same metering rod is used - which is what we want as well, but we can easily play with rods after the primary WOT is dialed in.
Ok, let's say primary WOT is hitting 12.5 just before the secondaries open. That will automatically lean the overall AFR somewhat, so a few runs are needed to see where it settles out. Then go a step down in secondary jetting if needed, and I think it will be.
As for softening the secondary opening, no that's not easily done on an AFB. The secondaries open when the airflow hitting their contol valve overcomes a weight on the shaft. The only adjustment is adding or removing weight, but that really just determines when they open, not how fast. For that you need an AVS, Thermoquad, a Q-Jet, or the new Street Demon. However, the current opening doesn't upset anything and I wouldn't want to slow it down any. It provides a bit more oomph and certainly doesn't cause a bog.
Maybe. My sensor is about 10" down the passenger's side exhaust pipe and is 14" - 24" from the ports on that side. IOW, there is an additional 10" from #4 to #1. But on a six I'd bet there is 16" from #6 to #1. So you might be able to be 18" from #6 and just within the 36" on #1.
Gary,
Excuse my ignorance.
Doesn't anyone make progressively wound springs for those carbs?
I only have experience with Holley style secondary tuning
spring on one side of the diaphragm, shafts on the other.
maybe a vacuum restriction would 'temper' the hit.
Unfortunately, this is where the simplicity of the Carter AFB shows up as inability to tune. Fortunately I don't think I need to tune it. But there is no spring nor vacuum, other than that on the bottom of the butterflies, to adjust. All you have is an internal weight on an arm on the end of the shaft. Here's a drawing from Edelbrock's excellent tuning manual:
Imagine an arm on the end of the air valve's shaft with a weight on it. That's what keeps the air valves closed until overcome by the force of the incoming air. But, that arm and weight are inside the carb and getting to them requires quite a bit of disassembly. So it isn't easy and it isn't easily reversible - grind weight off, hope you don't go too far, and weld back if you do. But that problem was fixed in the next iteration of Carter's 4bbl's, the Air Valve Secondary, aka the Edekbrock Thunder series. They have a spring wound around the shaft and a slotted shaft that sets the tension on the spring. Pull the air cleaner, put a small screwdriver on the shaft, loosen the set screw, and give the shaft a small turn to add or remove preload. Very effective but still doesn't address the opening rate, just the point at which it starts to open. So still not as adjustable as the Holley design, although I've never had a problem with them opening too rapidly.
Another is that the WOT AFR is from 10.7ish at low RPM to 11.4ish at about 5000. Kinda rich from my understanding. And the cruise AFR is also rich, so some tuning is necessary.
