Jetting for reduced choke size?

[webbslinger]

What a great start for 2020! Today we got a break in the weather and took our S4 out for a final test with it's new 33mm chokes. Ever since it's been back on the road, 18,000 miles ago, it's had 34mm chokes and while it had a lot of power when wound up, low end torque was lacking. That is now fixed.
With the holidays, 2 family birthdays, bad weather, and having to do a radiator repair, the job took about 6 weeks.
Actual tuning would have gone much quicker though if I hadn't had the wrong expectations for what jets the smaller chokes would like. That has led me to wonder what others with more experience might have expected.
My car has 151's Webers and with the 34mm chokes it was using 130 mains with 160 air cors on F16 emulsion tubes. The idle jets were 50F9 and the accelerators were 45s. An immensely helpful A/F guage is hidden in the glove box.
The engine is bored to about 1615 cc and it has Dave Bean's stage 3 cams which have about .413 lift. It has the stock crank and rods and it made just shy of 130 hp at the wheels with the above setup.
So I'm curious: before I tell you what jetting I've arrived at, does anyone care to throw out what ballpark jetting changes you would have expected? I was suprised.

[nmauduit]

something like idle 45 air 200 and main 130 or less (if you've kept the F16)?

[types26/36]

At a guess,
M/J....120
A/C ...155
EM/T ...F11
IDLE ....45F8 or 50F8

[webbslinger]

Changing from 34mm to 33mm chokes has (so far) resulted in the following jetting changes:

Choke 34mm 33mm
M/J. 130 135
A/C. 160 140
EN/T. F16 F16
IDLE. 50F9 50F9
ACCL. 45 35

Usually when trouble shooting I try to change one thing at a time, but I figured that was out the window with a choke swap. So I abandoned the jetting I had and started with book Sprint jetting.
Sprint jetting was so lean I could barely coax the car out of the driveway at first.
After several jet swaps I bagged the F11 EM/Ts and went back to the F16s and the other jetting it had with the 34s to reboot from there.
This engine seems to require F 16s, but with 130 M/Js it was still wheezy and the a/f meter was reading way lean. That was so surprising I bought a new o2 sensor but it read the same as the old one, and the plugs confirmed lean as well.
So I put in 135 M/Js and immediately it felt good although it leaned out too much at higher rpms with 160 A/Cs.
Now the A/F stays in the 12.5 area under WOT from 3500 to 6500 and it idles in about the same area and the plugs look good. It runs smooth from 1000 to 6500.
I don't get it. My first thought was that the M/Js were marked wrong but they aren't and regardless, the same 130s used with the 34mm chokes were lean with the 33mm chokes. (I measured the chokes.)
The car used to get over 30mpg cruising and a bit less when exercised. I'm holding off saying it's tuned until the milage is confirmed OK as before.
Any speculation as to how these results are possible, or are they not possible and there's something mysterious/stupid going on in my garage?

[pauljones]

I cant comment on the jetting. What I can say is when I went EFI and had it setup on the rollers,I was 105bhp at wheels, great A/F Ratios everywhere. Cruising around and 40mpg plus was a genuine figure on millage.

[nmauduit]

Any speculation as to how these results are possible, or are they not possible and there's something mysterious/stupid going on in my garage?

I would think the actual cam profile has a lot to do with the requirements when fine tuning the jetting, that is why it always pays to spend some time on the rolling road for a given modified engine one wants to make the most of.

ps: I bet you can obtain a bit more if you're willing to let AFR go above 12.5 at high revs - I consider 12.3-13.3 a rather safe power band for a mildly improved street LTC, and I would think your rear bumper is likely to get spotted with black sooth if you never go above 12.5

[webbslinger]

Thanks for your advise and comments. The engine builder just said to keep the afr "under 13" so it's helpful to know that isn't a hard limit. I'll look closer at the 45 A/Cs.
I'd like to dyno it again sometime in the future, but first I'll run it awhile to get more accustomed to it's new behavior and see if there are any weak spots.
But my initial impression is that the increased driveability with 33 mm chokes is a remarkable improvement over the way it was with 34s. Not just noticeable, remarkable. It is going to be even more fun to drive than it has been working on it.

[webbslinger]

I meant 145 air cors, not 45s.