Measurements for stock SU jet needles?

[Captain Obvious]

I did some investigation into the springs a while ago and as part of that, I determined the spring constant of the stock springs through experimental measurement:

and also verified the validity of my measured results using theoretical calculations:

In the end, I come up with approx 0.110 pounds/inch (or about 1.76 oz/inch).

 

As for which spring would make for a suitable replacement, I don't know. The spring force rating system you described above that lists a force (like 2.5 oz. for example) is based on a compressed length and without knowing that compressed length is, there's no way to compare it to the stock spring.

 

So while I can tell you that k = 0.110 lb/in for the stock springs, I don't know what the spring constants are for the aftermarket replacement.

[Stanley]

Found this for springs: Yellow (AUC1167)8 oz.@ 2.750", Red (AUC4387)4.5 oz.@ 2.635", Blue (AUC4587) 2.5 oz. @ 2.625". From official SU handbook that I couldn't print from.

Also found this PDF, with full needle profiles, kind of old but probably haven't changed or added new ones:http://mgaguru.com/mgtech/books/pdf/Tuning_SU_Carburetors.pdf

There's the SU "Needle Profile Chart" on SU carbs site. Couldn't find a PDF anywhere. Not expensive but looks like you have to order it from England.

Compared to your calcs for stock springs:

K yellow = 8 oz. / 2.75 in. = 0.18 lb/in = 2.91 oz/in

K red = 4.5 oz / 2.635 in = 0.11 lb/in = 1.71 oz/in

K blue = 2.5 oz / 2.625 in = 0.06 lb/in = 0.95 oz/in

Looks like red springs are about stock, blue for performance (with richer needles only, otherwise worse), and yellow for super gas mileage and poor performance.

Edited June 6, 2015 by Stanley

[Captain Obvious]

Unfortunately there's no guarantee that you can take the 4.5 oz @ 2.635 inches and simply divide it to a result of 1.71 oz/inch. However, if I needed to buy springs and that's all the info I had, I would do the same thing and start where you suggested with the "red" coded ones and hope for the best.

 

AUC 4387 - Red Piston Spring. Load at length 4.5 oz @ 2.635 in. Suitable for Normal Suction Chambers.

In order to really compare the two, you would need to:

a) compress a stock spring to the length they listed and measure the force to do so, or
2) apply the same force they did to a stock spring and measure the length, or
c) determine the spring constant of the aftermarket spring by measuring the change in force required for a change in deflection, or

d) calculate and predict k. But in order to do that you have to know a lot more about the springs in question:

 

 

[Stanley]

Right, need a red spring and a new #23 side by side for comparison.

 

That PDF on link I posted in #26, is from 1968, and does not have a number of needles that were added after that. For example, the BCA, BCE, and BBZ needles that SU recommends for 240z (according to source quoted on zparts.com) are not on that list. Some interesting tuning ideas, though, and nice picture of mechanic listening to carb with rubber hose.

 

Wonder if the #23 Hitachi SU springs are NLA. I'd guess the SU's for roadsters and 510's used different springs since they used different needles, which might confuse the issue of part numbers.

 

It might also help to measure an N-54, stock non-emission needle for the '72 three-screw carbs, for comparison.

[Captain Obvious]

And here's another complication to the whole spring conundrum... Say you have a couple (of what you assume are) stock springs, but none of them are the same length. The only reasonable assumption is that a previous owner has tweaked some or all of them in some misguided attempt to increase power or something?

 

But here's the thing... The beauty of measuring and or calculating the spring constant in the proper way is that (within reason) the tweaking doesn't matter. Unless they really really screwed up the spring, you should still have a range somewhere in the middle of travel where the spring constant is still relatively linear, and it should still be pretty much the same as it was before the spring was tweaked. So even though the springs I have may have been stretched when compared to their original length, I stand by my number of 1.76 oz per inch.

 

And that's the big problem with the way the catalog lists the color coded springs "4.5 oz @ 2.635". It doesn't have portable meaning. In other words, even if the stock springs I have USED to provide 4.5 oz of force when compressed to a length of 2.635 inches, they won't do that anymore after being stretched by a previous owner.

 

On the other hand, "k" is portable, so assuming a new spring you would buy is the correct length when purchased, you could easily pick the appropriate spring if they were listed by spring constant.
 

I don't remember where I dug up a copy of that old pdf, but I have that one, and I've never looked into purchasing stock spring replacements from Nissan. I have no idea if they are NLA.
 

About other needle alternatives... I dug through the tables a while ago and turned up what I thought would be the closest replacement for the stock N-27 needle. I took a quick look through my notes and didn't turn up the number, but when I get a chance, I'll look some more. IIRC, it was a two letter designation, not three. I'll try to see if I can find that info.

[Stanley]

Meant to get back to this sooner. Bought a good calipers and worked out a measurement technique but haven't got around to it yet. Still using SM's, not satisfied though.

 

Found this today, seems really cool, don't know if it's new or I just didn't see it before.

http://www.mintylamb.co.uk/suneedle/

 

Also, made a PDF (from much longer source document) of .100 in needles:

Needle charts pp 47 to 49.pdf

[Stanley]

Ordered a set of red springs (and yellow set) from APT last week, and installed them today. Had them side by side with the ones in the carbs and took some measurements (with a cheapy micrometer I keep in the car) and a 3 oz. fishing sinker. I'm calling the original ones #23's, but I don't really know. They look identical to each other, and seem to be in good shape.

----Unloaded L,----Coil O.D.-- No. coils-- wire dia.,--L w/ 3 oz. wt.

#23's---3.95 in.,-- .95 in.,--- 30,-------0.33 in.,------- 2"

red's-- 6 in.,---- .95 in.,---- 35,-------0.035 in.,------ 3.875 in.

The difference in wire dia. probably due to crappy micrometer. Length with the 3 oz. sinker on it is only good to about 1/8".

So took it out for a short test ride. They worked. No other changes. Now I can stomp on the gas without it leaning out and popping through the carb. Surprisingly (well, maybe not, if you think about it) my idle rpm dropped a couple hundred revs. So I set it back where it was, without adjusting the mix.

Probably still need to change or modify the SM's, but seems like a big step in the right direction.

Edited November 22, 2015 by Stanley

[Captain Obvious]

Cool. Sounds like progress.

 

I'm trying to run the calculations to compare the springs you have to eachother and to what I've got like, but the results are only as good as the data input. A thousandth here and there can really make a difference.

 

Also, when you count the number of coils, that's supposed to be the number of "effective" coils and doesn't count the partial coils at the ends. For example, my (what I think are stock) springs have probably 30 coils total, but if I don't count about a turn-n-a-half at each end of semi-formed coils, I get about 27 effective coils. When I get a chance, I'll take a pic of one of mine to show you what I mean.

 

And the measurements of wire diameter and OD of the spring need to be as accurate as possible. What's this "crappy micrometer" look like?

[Stanley]

I counted each of the 1.5 coils at top and bottom as one coil. Micrometer is harbor freight digital. It's ok, but last digit is iffy and it needs to be zeroed after each measurement. I'll check O.D. and wire dia. of  #23's with good Japanese mic in next few days. It's .033 in. not  .33 in wire dia.  I think wire dia. for the reds is listed in SU literature.