I have been driving my Z a bit with a wide band 02 installed. Generally speaking, the car is rich at idle and lean everywhere else. If I am not mistaken, the needles I have are the stock N-27. I cannot confirm because they have not markings.

After reading the following post, I got to thinking... If I have the needle station measurements, and I can record my AFRs for each of the general categories of start, idling, pick up, cruising, and top speed, then might I be able to calculate the station measurements of the needle that my engine wants?

Not that I know of. It would give you an idea of where the stations need to change and whether a little or a lot but I dont think you can calculate exact station size required from wide band afr numbers. You would also need a visual reference for piston height (station being metered) at a given AFR. There are quite a few SU needles available with station sizes but like tuning webers there is some educated guesswork

I assume that with enough equipment and calculations, you might be able to predict what would work "right out of the box", but that's way above my pay grade. I would do the same as Patcon and use the AFR numbers as guidance as to which general part of the needle needs to change.

Here's a great video of how the pistons in CV carbs operate. This is a motorcycle, but the concept is the same. You can tell by the sound how much he's into the throttle, and you can see what the pistons do in response.

I love this video:
https://www.youtube.com/watch?v=tR63vrfhwr8

I don't have time to really work on any details right now, and probably won't for a couple days. But, just for starters... Is there any chance that you're running into a fuel compliance issue? By that, I mean, maybe your fuel system is able to provide adequate fuel at idle, but once you start loading the engine, maybe you're sucking the bowls down?

And if you let it idle for a minute before shutting it off, the bowls refill before you get the chance to check them? I consider it a long shot, but just maybe? Something like the banjo filters are partially clogged perhaps? Long shot, but I have to ask.

Oh, and the reason I posted that video is that it can help give you a good idea of where your pistons are under different conditions. Making it potentially easier to figure out where along the stations you need to make adjustments.

Like for example... Just because you are at WOT doesn't (necessarily) mean that you're running at a station way down at the tip of the needle. It depends on the amount of air flowing through the carb, not directly on pedal position.

If you have a small camera, you could rig it up to take a similar video of the carb slide on one of your carbs. Might be informative.

And about the AFR numbers you're getting... I'm no carb expert, but I've generally found that the SU's run rich at WOT, not lean. Maybe your rebuilt engine flows a whole lot better than stock? Maybe it's something else.

It doesn't sound like a vacuum leak to me. That would be more pronounced at idle. A problem with the slides? Like dampers not working right or weak springs? Fuel supply compliance?

What happens if you pull back on the choke lever while you're driving? Does everything clear up and look better?

Just thinking out loud. ☺️

Edited Friday at 12:21 PM4 days by Captain Obvious

21 hours ago, Patcon said:

Not that I know of. It would give you an idea of where the stations need to change and whether a little or a lot but I dont think you can calculate exact station size required from wide band afr numbers. You would also need a visual reference for piston eight (station being metered) at a given AFR. There are quite a few SU needles available with station sizes but like tuning webers there is some educated guesswork

You are probably right, Charles. I'll tell you what I find myself thinking about though. The open height of the pistons in the domes at cruise, for example, is likely very close if not identically the same with needles that are not radically different from each other (and with all other factors (oil, springs, nozzle heights, etc. the same). If cruise is stations 3, 4, 5, and 6, and I am getting an AFR of 16.0 at "cruise", then wouldn't I be able to run a calculation to determine what percentage increase in fuel flow I would need to achieve an AFR of 14.7 instead, for example? And, subsequently, could I not calculate the change needed to the annulus area, and thus back into the width of the needle that I would need at "cruise"?



If the issue is which of the four stations is "cruise"... then that might be determinable, perhaps by recording operation of the pistons in a video (with markings or indicators for readability). Also, the needles do continuously taper. So, if I am seeing a consistently lean AFR through the entire range of "pickup" and "cruising" (e.g. 16 AFR for tip in, pick up and cruise) then I may need the same percentage of fuel increase across all those stations.

20 hours ago, Captain Obvious said:

I assume that with enough equipment and calculations, you might be able to predict what would work "right out of the box", but that's way above my pay grade. I would do the same as Patcon and use the AFR numbers as guidance as to which general part of the needle needs to change.

Here's a great video of how the pistons in CV carbs operate. This is a motorcycle, but the concept is the same. You can tell by the sound how much he's into the throttle, and you can see what the pistons do in response.

I love this video:
https://www.youtube.com/watch?v=tR63vrfhwr8

I don't have time to really work on any details right now, and probably won't for a couple days. But, just for starters... Is there any chance that you're running into a fuel compliance issue? By that, I mean, maybe your fuel system is able to provide adequate fuel at idle, but once you start loading the engine, maybe you're sucking the bowls down?

And if you let it idle for a minute before shutting it off, the bowls refill before you get the chance to check them? I consider it a long shot, but just maybe? Something like the banjo filters are partially clogged perhaps? Long shot, but I have to ask.

Yah - nice video!

I do plan on at least using the AFR info as guidance as to which general portion of the needle needs to change. I have spent some time reviewing specs for needles. Here is a small selection of the few I have found to not be obviously bad choices:

From here, I plan to set the mixture screws at 2.2 turns out (per the factory manual spec). Then I will make some recordings with the LM-2 wideband. My RPM signal is not being capture accurately at the moment, so I will have to address that first. I think fixing that will assist with some of the repeatability that will be needed for testing.

When I do that, AFRs will change from what I have seen thus far, as right now, the mixture screws are about 2.7 turns out. While I am seeing around 11 at idle, and 16s pretty much everywhere else (other than throttle lift off, which will cause AFR to hit 19-20), I may find that idle moves to 14 instead, and everything else jumps past 18. Who knows. But, I will attempt to get some AFRs with 2.2 out on the mixture screws and use those as my N27 "reference" AFRs.

Once I have these, I will review the above needles again, comparing with the N27, and consider the change in "general direction" that will be necessary at each station.

Regarding sucking the bowls down, I believe I would observe lean mixtures getting leaner as RPM's increase. In other words, during heavy throttle applications (I haven't gone full yet, but I have gone close) as the revs build, I would expect AFRs to go leaner. I don't have the data to show that is not happening (yet), but I will when I get the RPM signal working properly. From my driving experiences thus far, accelerating near full throttle through several gears in succession, can be repeated, with no notable increase in AFR at any point in time during the series of "pulls". Simply put, the AFRs are lean, but very consistently so.

10 hours ago, Captain Obvious said:

Oh, and the reason I posted that video is that it can help give you a good idea of where your pistons are under different conditions. Making it potentially easier to figure out where along the stations you need to make adjustments.

Like for example... Just because you are at WOT doesn't (necessarily) mean that you're running at a station way down at the tip of the needle. It depends on the amount of air flowing through the carb, not directly on pedal position.

If you have a small camera, you could rig it up to take a similar video of the carb slide on one of your carbs. Might be informative.

And about the AFR numbers you're getting... I'm no carb expert, but I've generally found that the SU's run rich at WOT, not lean. Maybe your rebuilt engine flows a whole lot better than stock? Maybe it's something else.

It doesn't sound like a vacuum leak to me. That would be more pronounced at idle. A problem with the slides? Like dampers not working right or weak springs? Fuel supply compliance?

What happens if you pull back on the choke lever while you're driving? Does everything clear up and look better?

Just thinking out loud. ☺️

I will do a little run tomorrow, weather permitting, and see what pulling the choke lever up does to the AFR. I have done that a couple of times when the engine is a touch cold and the slight hesitation (from being lean) goes away.

Edited Saturday at 12:05 AM4 days by inline6

I do not know if the afr number to the fuel supply is directly relatable

I dont believe a super lean afr is 0 fuel and pig rich is an Afr of 1. Its a ratio so an afr of 1, is 1 part air to 1 part fuel. How is each part measured? What is the unit?

Stoic is 14.7:1

So if you're at 16:1 your fuel needs to go up by 8.8% I believe, maybe, possibly...

So does it really hurt anything to run a little lean at cruise? I would presume better airflow for cooling so overheating shouldn't be a problem and I would guess you would also be seeing better cruising mileage if you're a little lean. Probably something way wrong with this line of thinking.

Modern cars tend to run a little lean at cruise. You just have to be careful. Too lean will burn valves and damage pistons

On 3/14/2025 at 11:04 PM, Patcon said:

Stoic is 14.7:1

So if you're at 16:1 your fuel needs to go up by 8.8% I believe, maybe, possibly...

Yes, that is the general concept I was thinking, if I know the percentage more fuel that I need, then I just need to do the calculation properly to back that into the increase in annular area needed to achieve that.

https://en.wikipedia.org/wiki/Annulus_(mathematics)

On 3/14/2025 at 11:13 PM, w3wilkes said:

So does it really hurt anything to run a little lean at cruise? I would presume better airflow for cooling so overheating shouldn't be a problem and I would guess you would also be seeing better cruising mileage if you're a little lean. Probably something way wrong with this line of thinking.

"a little lean" at cruise is desirable. That is, lean enough to reduce gas consumption, but not so lean as to cause any noticeable surging. 14.7 at cruise would be a good choice.

Yesterday, I noticed that my exhaust joints from the header to the pipes are not sealing well - they leak. So, I need to resolve that before I can take new baseline AFR readings. I have some new exhaust clamps coming today that should solve the issue.

Edited Sunday at 03:48 PM2 days by inline6

I went through a few sets of needles after I installed an AFR gauge in my Z and got a better idea of what was needed. I didn't get too scientific with it, other than making general observations at idle and different throttle positions like you have then looking for a needle profile that was bigger or smaller where adjustment was needed. I'm running a mildly built L28 with SUs. I started with SM needles, which I had to run super rich at idle to get decent AFR everywhere else. I tried SB needles also, but settled on RH needles as the best range for my engine. This is a great website to compare needle sizes https://www.classicminidiy.com/technical/needles. I never found a good source for needles in the states and ordered mine from https://www.sumidel.com/ out of Australia. The conversion rate was decent a few years ago when I ordered, so the price wasn't as bad as their site appears. Good luck!

11 hours ago, Ninjaneer11 said:

I went through a few sets of needles after I installed an AFR gauge in my Z and got a better idea of what was needed. I didn't get too scientific with it, other than making general observations at idle and different throttle positions like you have then looking for a needle profile that was bigger or smaller where adjustment was needed. I'm running a mildly built L28 with SUs. I started with SM needles, which I had to run super rich at idle to get decent AFR everywhere else. I tried SB needles also, but settled on RH needles as the best range for my engine. This is a great website to compare needle sizes https://www.classicminidiy.com/technical/needles. I never found a good source for needles in the states and ordered mine from https://www.sumidel.com/ out of Australia. The conversion rate was decent a few years ago when I ordered, so the price wasn't as bad as their site appears. Good luck!

Thanks for the reply. I overlooked the RH, but it looks much like the OA6 - leaner than the N27 for the first two stations and richer at every other station. I'll add it to the rather narrow consideration set.