I mentioned in another post that I had ordered a wideband 02 sensor for my Valiant. I've got it installed and have been working on getting my Carter #9504 AFB tuned. It's been quite the learning experience.

I wanted to pass along some of the things I've learned so that it may help someone else. First, here's my setup:
1969 225 engine, head shaved .090, bored .060 over stock replacement Sealed Power pistons, stock backcut valves with some bowl blending and port matching, Mopar Performance 268S cam. Mopar Performance electronic distributor with orange box. Offenhauser 4 barrel intake. Carter 9504 performance AFB carb. Clifford headers with 2 into 1 pipe with 2.5" full exhaust. TF904 with stall converter, 2400 footbrake, 2800 flash rpm. Timing set at 4 deg. BTDC initial, 34 deg. mech. advance @ 2000 rpm. Vacuum advance still hooked up. This is still mostly a street driven car.

I ordered an AEM 30-0300 wideband O2 sensor kit from Amazon. It has great reviews from what I've found. I welded the sensor bung on the y-pipe side of the front header. It was easiest to get to here and I didn't have to remove anything to weld it on. Also, AEM says the sensor must be mounted 18" to 36" from the manifold flange. It's well over 36" to the 2:1 collector. Technically, I'm only reading the A/F ratio on the front 3 cylinders.

1. Most of the jets and metering rods vary in .003" increments. This makes a bigger change in A/F ratio than i expected. I wish they made the jets and metering rods in .001" increments like Holley does.

2. The metering rods have quite the tolerance range. I've found that the OD can be as much as .002" different between rods with the same numbers stamped in them. You can make minor changes in A/F ratio with the same part # metering rods. I hope the jets have closer tolerances, I have no way to measure accurately though.

3. You need to get the primary side of the carb tuned before changing the secondary jets. I was able to work out a bog when the secondaries open by getting the primary tuned properly. I found out that right before the secondaries opened at about 80% throttle, that the A/F ratio was dropping down to under 10:1. As I changed rods and jets to get the A/F ratio up to about 12.3:1, the bog got less and less. I've found out that the primary jetting has a huge effect on the WOT A/F ratio. If you try to tune your WOT A/F ratio with the secondary jet and your primary jetting is off, you will just be chasing your tail. On my car even if I floor it from a dead stop and the secondaries are open, the secondary air door doesn't start to open until about 3500 rpm. I can hear and feel the secondaries start work at that rpm. It takes a couple of seconds to get from the 2800 flash speed on the converter to 3500 rpm when the air door starts to open(wish I had a converter that would flash to 3500 rpm and still be street driveable, my current converter slips about 500 rpm @ 55 mph while cruising). If you have a stock torque converter this would make an even bigger difference. with a stock converter it will only slip to about 2000 rpm which makes it much more susceptible to bog.

4. This engine runs great cruising down the road at anywhere from 14:1 to about 17:1 A/F ratio(3000 rpm @ 55 mph). It starts surging at cruise rpm at just over 18:1 A/F ratio. At 20:1 A/F ratio it runs like crap while at cruise RPM. Anything under 14:1 while cruising would just be wasting fuel in my opinion.

5. It idles great at 14.5:1 A/F ratio. The emissions sticker says idle A/F ratio should be 14:1 or higher. I also found that turning the idle adjustment screws 1/8 turn makes a huge change in idle A/F ratio. It only takes about 1/16 turn on the pair of screws to change the idle A/F ratio by 0.5:1. After I got the idle mixture set perfectly, the car starts better hot and cold and holds idle rpm much more steady which was always an issue before with the lopey cam. This works much better than using a vacuum gauge which I had done previously.

6. The step up springs only help change the A/F ratio when accelerating from cruise to about 80% throttle. I changed mine to the 8" vacuum rated springs. It was pretty lean while slowly accelerating from cruise to full throttle. Changing from the 5" to 8" spring got the A/F ratio down quicker to eliminate that lean spot. If you put a vacuum gauge on your car and watch the vacuum while rolling on the throttle you should see the A/F ratio change when the vacuum drops below the rating of the spring.

Currently I'm running .089" primary jets, .071"/.061" metering rods and .089" secondary jets. I need to change the secondary jets to .092" to richen up the WOT since the A/F ratio is 13.3:1. My cruise A/F ratio is 14.5:1. The A/F ratio on the primary power is 12.3:1 but drops down to 10:1 if I stay in it for a long time. I've got to do some more testing and changes to see what's causing this. It acts like it needs a metering rod that has a very small change from economy to power step. My current one is only .010" change and they don't come any lower than that. I plan to change to a .086" primary jet and see what it does. Like I mentioned above, the jets only come in .003" size differences.

Hopefully this information is helpful.

Joel Harris

I forgot to mention that while I was changing jets while the engine was still hot that the fuel had boiled out of the fuel line to the carb. When I took the top of the carb off and the floats dropped, a burst of air came out of the needles and seats. There was no fuel left. It was only about 75 degrees outside at the time. Looks like I'll be adding a fuel return line and some of the foil hi-temp insulation to the fuel line to keep the heat out.

Thanks for this detailed info (Joel?). Very useful stuff here for anyone doing tuning with a carb and an AFR gauge.

Lou

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Home of Slant6-powered fun machines since 1988

Yep, I believe that. But as you stated, good luck finding any. Try making your own?

Marvelous. I thought I was the only one crazy enough to try this type of quest. Although I was dealing with a 1-BBl which makes tweaking much simpler.

I'm curious if you discovered the same aspect: the multiple steps on a metering rod are just to get it in the ballpark. If you limit your viewpoint to only one section of rod diameter at a time… let's say the middle section from perhaps 15-8 in Hg… I found the change in A/F ratio is actually inverse of the ideal. During each segment of increase in demand, it absolutely leans out when I wanted it to go richer, and that doesn't get rectified until the next diameter of rod becomes available. My explanation is the fuel being pulled through the jet (around the rod) is apparently proportional to the intake vacuum, even though I expected it to be proportional to the venturi vacuum. (As the foot goes closer to the floor, the intake vacuum doesn't pull as much fuel, hence the A/F goes leaner during that stage.)

Maybe a picture will help. The green line is what I wanted it to do. The purple segments are what it actually did. This graph shows the effects of a 3-step metering rod, and you can see each transition to different rod diameter. Notice that, except for a few key moments, it's always richer than it needs to be. (If you choose leaner than it needs, you tend to blow things up.) Because I didn't need to balance L-R, I could get away with experiments with "rolling my own" metering rod with a continuous taper. (Hobby shop, brass round stock, variable speed drill, sandpaper.) Then a digital caliper. I must have made a couple dozen with various different tapers. Whoever uncovers that box when I die will have not a single clue what all the pretty jewelry is for. And then of course I discovered that the height of mounting on the vacuum plunger, even .002-.003" difference, made a huge difference to final calibration. When you go to this extent of "custom", the main jet size becomes nearly irrelevant because you can make anything function. Note I didn't say you can make it function well. The moment you change any significant variable of weather or altitude, all the homemade perfection goes in the toilet.

I tried to be gentle with the screws, but I wouldn't be surprised if I wore out the threads holding the top of the carburetor together… I shudder to think how much further ahead I would have been if I'd invested all of that time in programming an EFI system. But inquiring minds want to know.

- Erik

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Lots of early Valiants and Barracudas have crossed my path.
Also a handful of other toys for variety now and then.

I didn't think I was the only one going through this. I hadn't thought about modifying the metering rods yet but I'll probably end up there as I'm quickly running out of jet and metering rod options. I changed the primary jets to .086" and the secondary jets to .092 the other day. It made the cruise a/f go to 17.5:1, primary power go to 12.3:1 and wot go to 14:1. I still have the same issue where the longer I stay on the primary power side, the leaner the a/f ratio goes. It will still drop to 10:1 after a while. I can't figure out what's causing that. I may need to play around with the springs. I was surprised the wot a/f went up after putting in a bigger secondary jet. Anyway, more experimenting to come.

Joel

What type of fuel pump are you using?

Leaner? 10:1 is pretty rich.

Let's assume you meant richer, if it's slowly going from 14:1 to 10:1. But the jet diameter is a constant, so for it to add more fuel through the jet, either its effective diameter would need to increase (meaning the metering rod is slowly drifting further and further out?) or the pressure across the orifice needs to change which I don't see how that could happen if your vacuum is at a steady 0" Hg. (Is steady vacuum a reasonable assumption?)

I gotta wonder if fuel is entering through some location other than via the jets.

Have you checked that the float stays floating? I'm just pondering, that if there's enough flow to keep the needle valve off its seat, but then the float didn't come back up, you're going to get more fuel in there than you need/want, and the bowl could overflow down the throat….?

They make little microscopic cameras you could wire up near the carb and send to a screen, that a passenger could admire while you drive… don't know if it would show you anything significant, though. The amount of extra dribbling fuel to raise the combustion from 14:1 to 10:1 is very, very little.

- Erik

_________________
Lots of early Valiants and Barracudas have crossed my path.
Also a handful of other toys for variety now and then.

Erik, I did mean richer on primary power circuit. I'm going to check the check ball in the accelerator pump squirter to make sure its not sticking open. It could be causing that issue. It would only draw through if stuck open at high air flow and would quit when the secondaries open because the signal strength drops in the primary when the secondary open.

Joel

I think this "richer as you stay in it" is probably something to do with the air bleed orifices that control how much air can be pulled in to meter the fuel. These carbs are all built for V8s much bigger in displacement than our engines, so some of this will be inevitable unless you want to start drilling holes or get a carb with screw-in bleeds. This is one reason why EFI makes everything so much easier, once you have it set up.

Also, richening up a jet and seeing higher AFR can be signature of a rich misfire. The AFR gauge reads leaner because the misfire is not allowing all the fuel to be burnt. The misfire may not be heard/felt until it gets extreme. Many, including me, have been fooled by this, but I still recall the first time it happened when I was tuning and found the answer... Dropping the fuel down by ~20% at WOT made it run so much better/faster. As I had richened it, the gauge said I was at 14:1 when I was really below 10:1.

Lou

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Home of Slant6-powered fun machines since 1988

Lou, I wondered how accurate that really was. It's not blowing any black smoke out of the back when it's doing this and my seat of the pants gauge tells me its running well. I'm not sure at what a/f ratio the black smoke start to be noticable. Overall it runs pretty good where it's at. I would still like to check the accelerator pump check ball since I haven't verified it yet.

Joel

I took a little time tonight to check on the accelerator pump circuit on the afb. I took the top of the carb off and thought if I don't see anything unusual I'll put the .089 primary jet back in to see if I get the same a/f ratio as the last time.
When I took the accelerator pump squirter off I noticed that the screws weren't really tight. When I took the last screw out the squirter came off with it. I checked the discharge ball and the brass weight that sits on it and didn't notice anything stuck. It looked nice and clean so I screwed it back down tight. I also pulled the two primary venturies to see if there was anything in them. They were nice and tight and I didn't see anything in those either. I put those back in and changed the primary jets back to .089.
I put it all back together with the same 071/061 metering rods but with the yellow 4" springs. I started it back up and let it warm up, then pulled it out of the shed. I couldn't even get it out of the driveway it ran so bad. The a/f ratio just off idle was going over 22:1.
I limped it back in and tried to figure out what to do next. I had an extra set of metering rod pistons in my jet kit with 065/052 metering rods already on them. I put those in the carb just to see what happened. I couldn't have guessed for a better combination.
On the test drive the cruise a/f ratio was 14.7:1. The primary power a/f was 12.3: and didn't drop anymore no matter how long I stayed in it. It actually went up to 14.7:1 after a while which means it got back above 4" vacuum and the rods popped up. The wot a/f ratio was 13.2:1. So currently my jetting is: .089 primary jet, 65/52 metering rod and .080 secondary jet.

The low rpm power is way up now. It will actually spin those big 275/60/15 tires a little bit on takeoff now. The bog or low power I noticed under 3500 rpm is nearly gone now. I can actually push through the brakes now when doing a brake torque. I don't know if I want to change anything right now. I would have to get a custom size jet or metering rod to fine tune the primary any more. I may change the secondary jet up one size to a 086 to try to drop the wot a/f ratio down to 13:1. I think I'll save that change for the track though to see if it really makes a difference.
Does anyone have any experience with what wot a/f works the best that they want to share? I know it depends on baro, humidity and elevation but if someone can share the range that works good I would appreciate it.

Also, while I was on a test drive last Friday, I was just cruising along about 45 mph and it sounded like someone hit the car with a sledge hammer. I though I had either blown a tire, threw a rod or someone shot the car. I stopped and looked the car over, no blown tire, no thrown rod, and no bullet holes. I drove home with no issues but I noticed a clunking noise coming from the right cowl or front right floorboard. I looked the car over again still didn't see anything wrong. The next day I went out to change the carb jetting again and when I opened the hood, I found out what caused the noise. The passenger hood hinge spring had broken the night before and made the noise. That will be fun to fix since I have to take the front fender off.............

Joel

Sounds great, Joel. I usually end up leaner than the stock jetting on such carbs. I shoot for 12.5-13:1 for best power and don't have any recommendation beyond that.

Lou

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Home of Slant6-powered fun machines since 1988

kesteb, sorry I didn't see your reply until now. I'm running an oe replacement mechanical fuel pump. I should be in the 4-5 psi range but I haven't checked it. I know that you shouldn't run more than 6 psi with an afb. This carb also has the spring loaded needle and seat which I'm not a fan of. If this was on a high HP small block I would swap those with the standard ones.

I still plan on putting a return style fuel filter on it and adding heat resistant insulation to the fuel line. Hopefully this will prevent the vapor issue in the fuel.

Joel

To low of fuel level exposes to many air holes in main well, thus goes lean.