Today I was able to take some data with the logger. One interesting surprise was my ability to measure RPM without a direct connection to the unit. I had put a few components (resistor, zener diode, cap) together on a board to help condition the input and had it sitting on the transmission tunnel. Happy that it was working, I then noticed that the wire to the logger was hanging in the wind! With none of the ignition system wiring shielded, I'm not too surprised my foot of wire became a nice antennae. I called the company and they weren't too surprised either especially given the high input impedance. It tracks my Autometer tach so I'll leave it for now. Here's a screenshot of the software gauges working:

With my son driving the laptop, here's a screenshot of part of one of the logs we took this afternoon. This shows the AFR (black), RPM (magenta), and Fuel Flow sensor (red) as a function of time. This was starting at a stop light and accelerating up to cruise in 4th. The fuel flow meter puts out pulses and has a DC offset due to the meter I showed a few posts ago. I need to see if the software can count the pulses, or add a circuit myself to do the job.

Finally, the LogWorks software that comes with the data logger has some useful functions and plots such as this scatter of AFR versus RPM. While it looks like a shotgun blast, it's pretty clear from the density of data that I'm running rich overall (the dashed horizontal line is 14.7) and especially between idle and 1500 RPM where I'm at most of the time cruising around town. Now I can start tweaking the carb again.

 

Nice chart work ! I have a question , dont want to hijack your thread but I have a lean at idle problem I'm consistently 11-10 afr at idle and no matter what I do to the idle mixture screw I can't get it to change. Is this going to cause long term problems and any known fix for it ? Looks like you are 13-14 at idle I'm stumped ?

 

With running 10 - 11 at idle and the mixture screws have no impact, it sounds like you're getting fuel in from somewhere else. Assuming you have a holley, I'd start with making sure the float level is correct, then I'd look to make sure the transfer slot exposure is 0.020". Check that the secondaries are closed as well. If the carb is adjusted correctly, turning the idle mixture screws all the way in should kill the engine. With that AFR I'd think your plugs would look pretty black. I suggest searching the forums or starting a new thread that has the details of your current setup. Good luck!

 

I took the carb off and measured the exposure of the transfer slots - 0.020" on one side and 0.025" on the other. Given I'm idling about 750, I can probably close the throttle a tad to reduce these a few mils.

While hunting for some numbered drill bits, I thought I'd experiment with putting a small diameter wire in the IFRs to hopefully affect my rich condition just off idle. From what Holley told me, the IFRs in my 1850 are 0.029" (although I've seen 0.026" a few places on the internet). Using a 34 gauge strand of wire reduces the IFR cross-sectional area by 5 - 6%. Incidently, before I started peeling apart wiring to find the one above, I tried the smallest wire leads I found on a capacitor. This dropped the area by 30% which was way too much - nothing I could do would get the engine to idle.

The 34 gauge wire definitely made a difference in a few ways. First, I was able to adjust the idle AFR to 14.7 which I wasn't able to do previously. I could even make it lean which was what I saw when I first started the engine. Backing out the idle mixture screws ~1/8 turn dropped it to 14.7.

Below is another scatter plot from a similar drive to the other one I posted. Clearly one can see that the average is much closer to 14.7. Cruising around at my typical 1500 RPM it's even running on the lean side some now.



But the most encouraging benefit was an appreciable improvement in the MPG!



I'm still rich right off idle, but given that I'm in the 15s at cruise, I don't think I want to try a larger wire and further reduce the IFRs. Right now, I need to do some research on whether it might be better to take the wire out and open up the IABs. The question on my mind is if this will help flatten the curve so I'm closer to 14.7 across the RPM range I typically use.

 

I'm just starting a similar project on my 390, just in a bare bones kinda way.

Definitely interested in following this one. Even have the same carb, though there is probably a 5000ft elevation difference.

Some of the reading material that was getting me started...

http://members.tccoa.com/392bird/carbtech.htm

 

That's a good link. Like they show in the article, the Quick Fuel carbs look really attractive from a tuning standpoint. I saw one at the local speed shop today and will try one of those if I ever need another carb.

 

I'm really on the fence. I may go ahead and get the metering block set. I have had problems with the secondaries ever since I built this carb back in 98-99. I think the problem was a cheap knock off gasket set, but I'm not sure.

The carb was given to me, so it might not have worked right before I got it. Once I closed off the needle and seat for the secondaries it ran fine, ran it for years before I put my 4100 on my Comet.

The QF secondary metering set takes jets where the factory piece didn't.

 

I've been working with the software that came with the datalogger to see what different ways I can look at the data. One useful means of looking at the AFR versus RPM is a table that bins all the data points from a run. In the example below, I chose to show the number of points in a bin. This way one can get a sense for the average AFR for a given RPM band. It varies quite a bit as this includes accelerations and cruise.



My motivation here is to figure out what change to make next. From driving around and looking at the AFR gauge, I can see I'm still running rich off-idle, but in a reasonable AFR range during cruise. I also wanted to get a sense for when the main circuit comes into play. If I turn the table into the plot below, it looks like the mains start about 2200 RPM.



So from the above, I think my next change will be to open the IABs which should help being too rich off-idle. Down the road the plot also suggests a drop of a couple of jet sizes as well, but I'll wait until I get the off-idle range improved first.

 

Well, my plan to open up the IABs didn't work out as expected. When I called Holley last week, they told me they measured 0.075". So a 5/64" drill bit is conveniently 0.078" which would be an area increase of 4%. Unfortunately the drill bit went right through the bleed! Since a 12 gauge wire wouldn't fit (0.081"), it must be somewhere around 0.079" - 0.080" (somewhere on the 'net I recall seeing 0.079"). So tomorrow I'll get some larger numbered drill bits and leave it for now.

With the carb already torn apart for the work above, I thought I'd go ahead and try a smaller wire in the IFRs. The next larger wire I could find was a #30 (0.010") which should reduce the IFR cross-sectional area between 11% and 14% depending on the original diameter. Going down this road was also sparked by checking how far the idle mixture screws were turned out from being seated. I was somewhat surprised by seeing only 1 full turn. So less fuel from smaller IFRs should take me in the direction of 1.5 turns which is the usual starting place. As I was hoping, I backed out the screws 1/4 turn with the #30 wire to get a 14.7 idle AFR. I was also able to close the throttle blades a bit and drop to just under 700 RPM. I plan on driving the truck to work a few times this week so we'll see what impact this has - hopefully some further improvement in the MPG!

 

It likely won't idle well at 14.7. It will likely be a lot smoother at 12.5 to 13.0.

 

So far it seems to idle well around 14.7 although from driving around yesterday, the idle AFR varies with engine temp and dropped to the high 13s. I'll put the vacuum gauge on tonight and see where that leads me with the idle mixture. As you say, it might run better richer. It certainly didn't like the low 15s when I first started it with larger IFR wire.

Speaking of which, the #30 wire might be too much. On the longer runs between stoplights yesterday, the AFR is 15.5 - 16.5 cruising about 1600 RPM. To accelerate I now have to be more conscious about tapping the pedal to pump some fuel in the carb and richen the mixture.

From a qualitative standpoint, these recent changes to the IFR have the engine running smoother in general and the exhaust doesn't smell near as bad as when I started this project.

 

After reading this thread I'm starting to like my mileage. It's a 77 that just rolled over to 30k (130k I'm assuming) that got 12.35mpg the week after I got it and 12.85mpg after a tune up. It's a 2wd, C6, 351m, 2bbl, lwb, single exhaust truck. Only mods are a flowmaster, k&n filter and keeping everything maintained. I've got a wide band A/F meter and have been wondering about putting it on to help tune but most people are getting worse than I am so I might just leave it well enough alone.

 

That's great - almost 13 MPG is impressive. Out of curiosity, is this city or highway driving? All the data I'm showing here is from my daily commute to work which I average about 25 MPH. Once I improve how I collect data from the fuel flow meter, I want to look more at MPG versus MPH.

 

It's a little of both, I live in Killen AL by the TN state line and drive 14+ miles to work one way in Florence. Today I'll be filling up and checking it again. Also I've been pinching pennies to do a transmission upgrade, a mechanic told me the valve body from a E4od might fit in a c6 and with the correct torque converter it would give me 4th gear

Anyone heard of this or done it?

 

Just confirmed it will not work.