Since I needed to get a new power washer, I did some driving before I had any opportunity to change the jets. With fewer stoplights on the route and longer runs at constant speed and throttle, I was able to pay more attention to the AFR gauge. At 35 MPH turning 1500 RPMs, the gauge was reading around 14.5. I checked a couple of times at various stops that it was still around 12.2 in neutral like below.

What I learned is I need to start measuring the AFR under load at various driving conditions and make changes based on that information. Time to think about how to capture the RPM, MPH, AFR and fuel flow rate while driving . . . maybe a GoPro camera? I'm sure the kids could use it too.

It was hotter yesterday than when I took the plot below and the idle AFR was up around 15. Plus the engine was warmer which I infer by watching my idle oil pressure drop to about 20 compared to 40 the other night when I took the plot. If I had one of those IR thermometers, I'd be curious to measure the AFR versus carb or manifold temperature. I'm thinking the thermostat opening is not quite the same as the engine being at a stable operating temperature.

Finally, an idle AFR near 15 is too high. When coming to a stop and pulling off the gas, the AFR goes even further lean and the engine RPM drops below where I have set it at idle. It almost seems it might stall. I'll look at this again today, but I need to richen up the idle mixture and the question is whether one should always be a bit rich at idle versus the ideal 14.7.

 

Maybe an option Automobile / Car Racing - Performance Data Acquisition Systems - lap timing & engine monitoring instruments

 

Thanks. That looks workable. Summit also has one I found with similar capabilities.

 

As a proof of concept, I used our old digital camera that has a movie capability to capture a few seconds of the gauges at near steady conditions. I picked off some points from the frames to come up with the data below. Some of the scatter in the data reflects going up and down moderate grades. It also seems to point out that I'm still running rich overall. Changing from the #67 jets to the #65 had no impact on the MPG on average. I think I can make a similar curve plotting MPG versus MPH, although the fuel flow rate data fluctuates considerably.

 

Here's a comparison of MPG versus AFR. Mostly the data points are from driving on level roads, but the points I flagged were reasonable grades up (2) and down (1). While the line is a poor fit, it looks like MPG approximately scales with AFR. Given that most of my driving is between idle and about 1700 RPM, I think I'll drop the jets to #63 to go leaner in this range.

 

Just changed the jets to #63. Here's the slow ramp of AFR versus RPM. I retook the #65 data before the change and also extended out to 3000 RPM. I'm a bit surprised it's flat between 2500 and 3000. Hopefully these jets will make a positive difference on the MPG!

 

Today I made some more AFR measurements with the #63 jets. To make the job easier, I thought I'd try taking some data in other gears. Of course this precludes inferring MPG, but I assumed the AFR would be about the same as a function of RPM. Check out the plot below . . . I'm surprised. At a given RPM, the data shows the carb gets leaner with speed. More air forced into the air cleaner under the hood? That is, a 'ram air' effect? Given my max speed on level streets, I don't think the throttle position was significantly different at a given RPM. After a few more days, I'll report back on the MPG. Now to decide on a data logger which will make this much easier.

 

There's nothing wrong with idle afr of 12.2 if your engine runs smooth there, it likely won' t idle well above 13.0. You need to tune the other circuits on the carb also. Get yourself some tiny drill bits some brass set screws, drill and tap the carb's air bleeds, IFR's and PVCR's. After tuning a holley 770 on my mild 460 it idles at 12.5, cruises at 15.0-16.0, 13ish once power valve opens and WOT around 11.5 to 12.5( still trying to work that out too rich aim for 12.5 to 13.0).it also went from 8 to about 14 mpg doing this too. It's time consuming but well worth the effort, good luck.

Here's some good reading:

Help Holley pros - Page 2 - Chevelle Tech

Tech | Burton Machine

 

Thanks! Great information. I was starting to think about what other changes I could make beyond the jets. As far as mileage goes, I'm seeing a small improvement going to #63 (0.4 MPG), but it doesn't seem all that sensitive to jets. Hence, you're point about looking at the other circuits. Since I have a set of #61s, I'll probably give those a try as well. This should get me in the 15.0 - 16.0 range at cruise.

Can you share anymore of your experience and lessons learned? Of your suggestions, what made the most difference in going from 8 to 14 MPG? Did you see any impact of timing changes?

 

First thing is drill and tap your main body and metering block to accept brass set screws. Sizes are outlined in those attachments. Get the idle circuit tuned first. Increase the idle air bleed until it starts to run out off fuel before the main circuit comes in, then go back down a size. Mine idles at 12.5 and gradually leans out to about 15 as the throttle is opened. Mains don't start until 2500-3000 rpm on my truck anyway. It cruises down the highway on the idle circuit. Before you do any drilling measure the bleeds. PVRC's, and IFR's for your baseline. Use those tiny drill bits to measure.

 

Thanks. In the interest of understanding, would decreasing the IFR accomplish the same goal? And how can you tell when the mains come into play versus the idle circuit? Does the idle circuit 'turn off' at some point or doe the mains become the dominant supplier of fuel with more opening of the throttle?

 

Reducing the IFR's can help too, if you reduce too much it will run out off fuel before the mains come in. Opening the IAB's helps it lean out smoothly into the the mains. Some reduction in IFR may be necessary, play with both you'll get to understand it. The mains come on when the throttle is open enough to pull fuel through the booster. Doing jet changes and take notes will tell when your on the mains. Read those attachment thoroughly, those guys are a hell of a lot smart than me and can explain it better.

 

Thanks - that makes sense. Your suggestions got me to looking more at the AFR and fuel flow rate just off idle to around 1500 RPM. With the throttle barely open, it's running pretty rich around ~12.0. With a little pump of the throttle this easily drops to ~10.0. I also notice that the engine doesn't like to have partial choke. When cold it needs to have more RPM for a few minutes, but the richer AFR causes a real stumble off idle. This seems consistent with the carb being too rich just off idle (which I assume is still using the idle circuit). I'll head down the road of adjusting the IAB and IFR.

Here's the MPG data so far in all my experiments:

 

Hello first off when you started this thread I was sold on a wideband 02, i was thinking about getting one. I too have a 4160 but on a 302 , after a carb and fuel system rebuild I opted to install a percy adjust-a-jet which after getting the truck running tonight I have to say make tuning very easy . Almost takes the fun out of it. Seems as though I can't get it to run at "stoich" at idle do you have the same problem ? Any pointers for a first time carb tuner ? ( I'm us to efi tuning )
Two thumbs up for all your data logging !

 

Thanks. I haven't gotten to 14.7 at idle yet, but it's close. Reading what I've found so far, it doesn't seem to be an issue if it's a bit rich at idle. At this point, I think I've done what I can with changing jets. I could go down to #61, but I think I'd be too lean at the higher end of my typical RPM range and it wouldn't fix being too rich at the lower end.

As whc8100 added below, the next step I'm working pursuing is to 'flatten' my AFR versus RPM curve. From the link below (and others I've looked at), my next mods will be to gain flexibility in adjusting the idle circuit through the idle air bleeds and idle fuel restriction.

Before I do that, I'm going to pull the carb off and make sure the idle transfer slot has the proper exposure and that the secondaries are closed. Plus I'm going to make another pass to confirm no vacuum leaks and the floats are adjusted properly.

Since collecting all the data below was cumbersome (and prone to error), I'm just about finished hooking up a data logger. After doing some research, I got the following which is reasonably priced and seems to work well in my initial testing: SSI-4: 4 Channel Simple Sensor Interface

Here's a picture of it mounted below the ashtray which makes it easy to connect the laptop on the seat. So far I have the AFR hooked up and it agrees with the gauge output. Now I'm working on hooking up the fuel flow meter and then I'll tackle the tach input which may take some interface electronics (thanks to fmc400 for help here). Once this is done, I'll have a much better way to collect data when making changes and save some gas in not having to use multiple driving excursions to get a handle on MPG.