Since the very rich AFR at idle I am experiencing with the N27's is not acceptable, I decided to order some SM needles and give them a shot. A comparison:

While the differences at each used station are "small", they are evidently quite large when tuning SU carburetors, as I found out watching How SU carburetors work parts 1 through 4 here: https://www.youtube.com/@ACDodd/videos

Anyway, they were only $40 delivered, so not an expensive thing.

I swapped those in last weekend and made a few more runs to collect AFR. Doing so, I saw some changes and made some new observations.

First the plots:

Idle:

I am quite happy with that. 14.7 to 1 is stoichiometric. For idle speed, I think that would be ideal. But I think this is great.

cruise:

For the recording of cruising above, I was mostly on flat road holding the speed at an indicated 50 mph in third. My RPMs are now being recorded, but honestly, I don't know which to believe, my tach, or the LM2, as there is a notable difference off of idle. Generally, the LM2 is reading higher by as much as ~500 rpm. I plan to notate some speeds in gears and tach and LM2 rpm readings and compare with calculated info based on transmission and differential ratios and tire diameter to see which is more accurate.

Again, for cruising, I like the AFRs I am seeing here. I do not notice any surging or anything out of usual when slightly lean by this much while cruising. It should be good for fuel economy.

acceleration:

Now, for acceleration, I am less pleased. The first steep rise is first gear. I think I pressed record just after the light turned green and applied full throttle in first, then shifted to second and applied full throttle, then shifted to third and applied full throttle - very briefly. My focus points are seconds 2-3 and 6 to 8 and a half. For those, I am in a steady state - full throttle - flat road. In first, the AFR is a bit under 12. In second, it looks like about 12 to 12.2. So, rich. What got really interesting is that when I shifted to third and again applied full throttle, the engine essential "stopped" as the AFR spiked up to north of 20. I suspect the float bowls went dry.

I was able to repeat this a second time. So, I will need to investigate this further.

Now that I have watched the video series mentioned above, I will plan out my next steps differently. I won't be messing with needles for now. I need to check the springs in the carburetor to see if they are original and in good condition. And I need to investigate my dampers. They may have too much wear. One of the indications that I have an issue with either the springs or the dampers is that (and this is with both the N27 and the SM needles), the AFR is going extreme lean on throttle off deceleration. It is worse with the SM needles - going north of 20. But it is bad with the N27s as well, going north of 18. Lots of popping out of the exhaust...

Based on the info in the videos, that is either spring or damper.

Edited 17 hours ago17 hr by inline6

Other than that anomaly in third gear, I think the numbers look excellent. My understanding is that you WANT to run a little rich at WOT, and from watching the engine experts on "Engine Masters", I believe that 12 would be great.

In general, the it seems the desired conditions are:

A little rich at idle (because engines never want to run at idle, so you reward their good behavior with a little more fuel).
A little lean at cruise (because you can, and gasoline is expensive).
A little more than a little rich at WOT with 12 being just about ideal (because the extra fuel helps keep cylinder temps down which helps prevent pre-ignition).

I'm no expert, but that's what I've gleaned from listening to people who claim to be.

Now that anomaly in third gear... That's concerning. I remember asking a bunch about fuel delivery earlier.

And just to be annoying, I remember asking about dampers and springs earlier as well. Haha!! 😄