Measurements for stock SU jet needles?

[Stanley]

After a couple hours on google search, I'm guessing there's no such thing as official measurements for the stations on the stock needles (N-27's, etc.). British SU's sure, but not Hitachi's. Maybe it's a secret. The same question was asked here in 2008 and no good answer.

 

So has anyone (with a good eye, steady hand, and a good micrometer) measured and charted the stock needles? I've got some stock N-27's, modded N-27's, and some well-worn N-58's (I think), and well as the current SM's (data on those since they're British) and need to do some more polishing, but as I'm a novice at this, I would like to know where they started at, in case I want to buy or make some different ones.

 

I spent (wasted) a couple more hours today measuring and tabulating (top to bottom, and bottom to top, as a check) stations on the stock and modified N-27's, but my ten dollar Harbor Freight digital micrometer and maybe my measuring technique wasn't up to it. Won't post the results in case someone think it's correct. Typically off by about 2/100th's of an inch, maybe more. Had to zero it after almost every measurement. Looks like the measurements need to be within about 1/100th to correlate with the printed specs (for  British SU's) and document any modifications. According to the SU book, 5 seconds of finger pressure on the needle (in a drill press) with 200 grit takes off about .01", and the two needles should match within .01" at any station, so ...

 

Other option is to wing it, grind away and not worry about mistakes.

Edited June 1, 2015 by Stanley

[Captain Obvious]

There's no way a digital HF caliper is going to cut it. The resolution is only a half thousandth, and the accuracy (and that's advertised accuracy) is one thousandth. It's good enough to toss in your tool bag to take to the junkyard and tell the difference between a .68 wide rotor and a .72 wide rotor. Not nearly good enough to take measurements off the needle stations however.

 

A couple years ago, I measured some (what I believe are) N-27 needles. They came out of a pair of round top carbs I bought. I didn't own them from new and I have no idea how many other hands had been into them messing around with things. I may have been the first person in there, or the tenth. The needles may be OEM stock, or may have been replaced and changed many times. And unfortunately the numbers on the side of the needles is no longer readable.

 

What I can tell you is that the carbs appeared to be unmolested and the needles are N-somethings printed in black ink (not stamped into the brass). I can also tell you that the car I had the carbs installed on ran great with those needles. Not too rich, not too lean. The carbs worked great. Bottom line is that I believe, but cannot guarantee, that they are N-27 needles.

 

Lastly, I have a good eye, a steady hand, and made these measurements with a quality micrometer. I'm not infallible however, so would I would claim accuracy only to maybe plus/minus maybe two tenths.

 

So with multiple qualifiers and caveats, here are my numbers:

 

 

 

[Captain Obvious]

So were your numbers anywhere close to mine? Or are you in the market for some better measurement tools? 

Edited June 2, 2015 by Captain Obvious

[Stanley]

Here's the average of the two measurements I recorded. The two I recorded were the same at stations 1,6,7, and 14. All the rest differed by 10 to 20.

 

Here's the difference between my measurements and yours. +10 means I recorded .010" higher, etc.

 

STA     1       2       3       4       5       6       7       8       9      10      11      12      13      14

          +10   +5     0    +15     +15   -20    -20    +15   -15     -5       0        0      -15      -15

 

So no and yes.

[Captain Obvious]

Yeah, there's some significant differences there. Could be measurement tools, technique, or both. If you're sure what you have are stock N-27's I'd be more than happy to check them for you. Send 'em over, I'll record them and send 'em back.

 

And BTW, I'm pretty sure it was just a typo, but when you said .010", I'm thinking you actually meant .0010", right?

[Stanley]

Right, .0010 not .01. 

 

I bought two sets of N-27's from MSA last year. The needles weren't marked in ink or stamped, so I'm not sure. I got the pair of SM's for ZTherapy a few months ago, they had SM printed on the base. My '73 has 3-screws installed by previous owner; the needles in them were stamped N-58. '72 Nissan engine manual lists N-58 as emission spec for USA (and Canada?), N-54 as non-emission, standard elsewhere. N-27 stock for all earlier. MSA told me I needed early nozzles to go with the N-27's, something I don't understand.

 

My measurement technique is pretty low-tech. The first run I held an architect scale against the needle, put my thumbnail at 1/8" points, and mic'd against the thumbnail. Second run (point to base) I stuck the needle through a strip of thick paper, moved the paper to 1/8" points on the scale, and mic'd against the paper. All without my glasses.

 

I'm thinking about biting the bullet and getting a Mitutoyo.  The HF is handy to have but I can't get the same reading twice with it, and need to zero it constantly. I may have a job coming up where I have to reverse-engineer a bunch of damn used pallet racks; last job like that I used the HF and it was a hassle to never get the same readings. So I guess I can justify the Mitutoyo for that. I'll be using a $120 instrument in a dusty, filthy environment, but I guess they're built to take it. Wondering if I should go dial or digital.

 

Interesting that the SM's are way fatter (leaner) at the base than the N-27's. Maybe that's why it's popping or burbling if I stomp on it without downshifting.

 

Information on the various Hitachi SU needles has to be available somewhere. Probably only in Japanese, but there's google and other translators.  I read a post on an Australian Datsun site that someone took their nozzles to a machine shop and had them drilled to British spec just because they couldn't find that information. ZTherapy sells oversize nozzles, but that would make it way too rich at top end with the SM's, with stock displacement. I'm flowing more air though, so I need a slightly richer needle. So polishing seems like the best option.
 

[240260280z]

Jet nozzle size (diameter of the drilled hole) is a tuning parameter. You can buy nozzles with different sizes.

 

The fat SMmay work better in a larger nozzle than  in a 0.100" standard nozzle .

[Captain Obvious]

I'm not sure I understand MSA's comment either. The original nozzles should fit fine with any 0.100 needle.

 

I've heard the comment about the SM's being way leaner at idle, and I disagree. It might seem like that at first glance when you look at the numbers, but don't forget... You never ever use station 1.

 

The nozzle adjustment nuts are M10 x 1.0 thread, so at your standard 2.5 turns down, you're already at least .098 down from station 1. Then you consider the fact that even at idle, the suction piston is pulled up a small amount and you're very close to station 2 if not past it. At station 2, you're a little leaner than stock, but by the time you get to station 3, you're already richer than stock. The transition point is somewhere between 2 and 3.

 

In other words, I would expect the SM's to be pretty much identical at idle and get richer from there. I'm no carb expert, but that's my read.

 

As for the needle measurement method, I don't think this is a job for a caliper. Too springy... I think a good micrometer with a ratcheting knob (for repeatability) is a better tool. I used a Starrett on which I had just checked the accuracy, and I did it all under magnification. Even so, It takes some technique to get repeatable results. But as for your HF caliper, I can't imagine it's so bad that it gave you trouble reverse engineering pallet racks. I gotta believe that's far enough from rocket science that you could probably get away with just a scale! I would have expected the HF to be at least accurate to +/- .004"... That has to be good enough for a pallet rack, right?

 

I looked a bunch for the specs on the N-nn needles and came up blank. I suspect the reason is that they were custom made by Hitachi for Nissan. In fact, I'm thinking that might even be what the "N" stands for.

[240260280z]

^{N is a sideways Z}

[Stanley]

I think the problem with the SM's, at least in my case, when you hit the gas and the RPM's aren't high enough, the piston drops lower than it rides at idle. I'm seeing about 16 in/hg or more at idle, but the vac can drop below 5 in/hg at WOT from say 1500 RPM. I have to downshift when I floor it, or ease off the gas when the vacuum drops to 5. When going from cruise to WOT from about 2500 RPM it's similar, no popping but burbling starts to happen when the vacuum gets to about 10 which is still below vac at idle. Not sure about this, but I expect piston height is linear with vacuum regardless of load. If so, the pistons get close to the bottom. My modified N-27's worked better than the SM's in there now, only burbling a little going WOT from freeway speed cruise. Stock N-27's were not so good.

 

You're right, the HF caliper is accurate enough for pallet racks, it's just irritating that they won't give the same measurement twice on the same metal, and you have to keep zeroing them. It would be OK if I was paid by the hour. Outside calipers are no good for them since you have to take a lot of inside measurements. Owners buy narrow ones with thin, non-seismic baseplates cheap and load them to the roof. All that measuring, testing and semi-rigid, non-linear frame analysis is total waste of time since the baseplates (tear-out at the anchor bolt holes) determine how much junk they can pile up there, but the building dept. wants to see it anyway.

 

Besides, the Mitutoyo looks pretty cool.

[Captain Obvious]

At the risk of telling you stuff you already know, I'm not sure you understand the suction piston operation... It's not pulled up by manifold vacuum. It's pulled up by the vacuum difference between the mouth of the carb and the area right behind the suction piston. In other words, not the area on the engine side of the throttle plate, but the area just BEFORE the throttle plate.

 

The suction pistons don't operate by manifold vacuum, they operate by airflow speed through the carb.

The more airflow through the carb, the higher the vacuum just on the downstream side of the piston and the higher piston is pulled.

The intention is to keep the air speed through the carb venture the same regardless of how much air is being pulled through.

That's the "Constant Velocity" part of the "CV" operation.

 

So with that in mind, when you goose the throttle, the piston won't fall but will be pulled up. Quickly. Too quickly in fact! And if you pull the piston up too quickly, you'll run lean during that transition and sputter when you increase the throttle position (Kinda exactly what you're describing). That's why there exists the whole damper stalk and oil in the pistons. To slow the rise of the piston when you nail the throttle.

 

So I agree that it sounds like you're running lean on transition, but I would start by looking into your damper oil level and viscosity or the operation of your jiggly bits on the end of your damper stalk. Those jiggly bits are a check valve intended to slow the piston rise but allow fast decent. I've seen more then one occasion where those little jiggly bits didn't work properly resulting in a lean transition situation.

 

And as for your caliper, I'm not poopooing the idea of you buying a good Mitutoyo. I think it's a good tool choice. Go for it! Just don't try to use it for needle measurements. 

[Stanley]

Plenty of  stuff I don't know in SU hot rod school. Anyway, I'm starting to understand why I was wrong about piston position varying linear with manifold vacuum. It was hard to understand until I thought about deceleration. In that case manifold vacuum is at it's max, but throttle is closed and the piston drops because the closed throttle keeps manifold vacuum from pulling it up. Right so far?

 

Seems like needle dimensions could be accurately measured with a photo. Take a hi-def photo of the needle from an angle that minimizes distortion, import the JPEG into paint or similar, make a large bitmap image and copy into a CAD program. Blow it up as big as possible without blurring. Measure the needle length with micrometer, drawn a line equal to that length (or say 100 times the length if that helps resolution) on the image, and pull the image in or out until the needle length matches the line. Trace the needle image with the CAD. Draw 1/8" stations on the traced image, set dimension accuracy to 0.001 inch and dimension them. Move the image out of the way and there's an accurate (enough) diagram.

Edited June 3, 2015 by Stanley