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.

I've got a local buddy with one of those pots and he says it's way easier to use with two people. One to blast and one to shake the pot. He and I have discussed mechanized ways to vibrate the pot but haven't implemented anything yet.

Well it might depend on the year, but I found it very easy to remove and reinstall the harness without any major disassembly of other items. The only thing I took out that required any significant effort was my EFI air cleaner. I didn't have to mess with the radiator, hood, or any of that stuff. YMMV

I've seen some intermittent issues with the check valve on the damper stalk where if you reach inside the carb and lift the piston sometimes it's easy to lift, and sometimes it's not. Make sure the oil levels are right and try lifting the pistons a bunch of times on both carbs. Is one carb more consistent than the other? So were you able to find that video?

So it definitely sounds like there were subtle some gauge changes over the years in different spots. My splice points are in the same location as yours, but none of my gauges changed. In other words, the short stubs going to my RHS bulb are the same gauges as the long runs from the firewall. I'm guessing that it was cheaper for them to use the same size gauge there instead of taking the time and effort to use a different one. A tiny bit more costly in terms of copper, but faster to manufacture. And as a matter of trivial interest, the wires on my pigtail that goes with my 77 headlight bucket are also the same gauges. I've heard that may be different on other years as well. There's no reason they couldn't have reduced the size, but for some reason, by the time they got to 77/78, they didn't bother. They just kept the gauges the same. My theory is that when they went to the 77/78 round style headlight connectors, the needed a larger wire to fit comfortably into the larger crimp contact. The choice of gauge may have been driven by a choice of connector used. As for the location of the splices being upstream, I don't think it has anything to do with balancing resistances. I think it's all mechanical. They wanted to spur off the RHS headlight stub, but they wanted it to be mechanical sturdy. So they put the splice some short distance upstream so by the time they got to the pick-off point, they had several wraps of tape to support the wires and not put any mechanical stress on the splice joint. Also gives them the opportunity to insulate the splice point inside the harness without having to deal with insulating it AND branching off the stub all at the same time. I wasn't there, but that's my read.

Yeah, that's my life too. I spent untold hours this past off season putting in headlight relays, a starter relay, and converting over to the internally regulated alternator. My Z's headlights work, it starts when you turn the key, and the alternator charges the battery. Just like before I did all that work. But I know....

Well there's no reason you should have to go four turns down just to get it to run without it being lean, especially with modified N-27's or SM's. If you're running four turns down and you've got the bowl level higher than stock, you probably have fuel pooling on top of your nozzles, right? And if you go from too lean to fouls the plugs in just 1/4 turn, that's unusual as well. About the piston oil, you don't change the amount of oil to adjust how fast the pistons rise. You're always supposed to fill them to the line. If you want to change the rise rate, you mess with the viscosity, not the level. For the link, you can go to youtube.com and search for "XS1100 Carbs Vacuum Slides UNDER LOAD" that's the video I tried to link to above. It sure would be interesting to see a video of what your pistons are doing when you drive your car. So your float level is OK and your fuel pressure is good. Remember though that the bowl level you measured was created while there was very light load on the engine. Maybe you've got a partially clogged needle valve or banjo filter(s). Pass enough fuel at idle to run OK and maintain bowl level, but not enough capacity to keep up under high load. In theory, you could drive the car WOT until it stats to sputter and then push in the clutch and kill the engine in one quick motion and coast to a stop in neutral. Would be interesting to see what your bowl levels look like at that point. Like a "plug chop" for your bowl level. In other words, just because your bowl levels look OK sitting static on the driveway doesn't mean they aren't getting sucked dry at high load.

Yes, I encountered those splices buried inside the harness. I think the wire sizes may have changed from year to year though. My wire sizes (1977) were the same from the passenger compartment all the way out to the bulbs, including beyond those splices. In other words, I got a large R/W coming through the firewall and heads towards the front of the car. Near the RF corner, that large R/W split into TWO large R/W's of the same gauge (on my car). Sounds like yours didn't do that. Anyway, if you've got the flexibility to mount the relay block up near those splices, it could simplify the installation. Hopefully I'll get a chance to draw up a couple sketches today.

Actually, because I'm a loony, I went further than that... I bought a hacked up engine bay harness and stripped out the headlight wires. Then I opened up my original harness starting at the headlights and replaced my original wires with donor wires while pulling my original wires out as I went. I performed this maneuver until I got back to where the voltage regulator branched off. So once I reached the regulator branch area, I had donor wires coming from the direction of the headlights (that were long enough to reach into the passenger compartment) and original wires coming from the passenger compartment (that were long enough to reach up to the headlights). That gave me plenty of extra length centered near the regulator to run up to and back down from where the regulator used to be. That's what I did. I wouldn't wish that on anyone else. I'll make up some sketches of easier ways...

Is that 5 in/hg at WOT under load driving in a gear, or just goosing the throttle sitting still in neutral? Different solution for those two situations.

Actually the height of the piston is more closely described as inversely proportional to the manifold vacuum. Not quite that simple, but basically... The higher the manifold vacuum, the lower the piston position. Here's a video that might help you get a better picture of how the pistons behave: https://www.youtube.com/watch?v=tR63vrfhwr8 It's a motorcycle, but they are also CV carbs with vacuum controlled slides and the concept is identical to the SU's. You can hear the engine to get a sense of RPM and engine loading, and you can also see the throttle linkage moving around in the background. Note the rapid rise of the pistons when he gooses the throttle. Basically, the piston will be highest at full load (WOT and higher RPM's) and lowest at lowest load (coasting). I love that video...

Sorry, but nope. Manifold vacuum never pulls the piston up. The vacuum that pulls the piston up is on the wrong side of the throttle plate to be considered manifold vacuum. The reason the piston drops when you decelerate isn't because the throttle plate keeps the manifold vacuum away from the piston... It's because when you decelerate, there is very little air flowing through the carb. The pistons lift based on airflow. The more air volume that flows, the higher the piston. The less volume that flows, the lower the piston. And when you are decelerating with the throttle plate closed, there is very little air flow volume. I'll dig up some theory of operation documents for the SU's, but I suspect Blue has several at his fingertips. I might just stall for a couple hours and let him post 'em up. I'll let him get into all that Bernoulli stuff. And as for your idea about measuring using a magnified picture... That's a great idea, but someone beat you to it. It's called the "Optical Comparator." You put the part in question on a table and a picture of the magnified part is projected on a screen along with calibrated dimensional lines for measuring. http://en.wikipedia.org/wiki/Optical_comparator I've toyed around with the idea of getting one for my shop, but can't justify the cost or the space it would take up. In the meantime, I'll just have to resort to my calibrated Starrett, my good eye (with magnification) and my steady hands. So you want to send me your N-27's for me to add to my catalog? I'd love to double my sample size!

Yeah, I've done that kind of thing at times, but I really don't like to. Especially for larger wire sizes. I don't mind so much crimping two 20 GA together, but much bigger than that and I don't like it. So about adding completely new wires... On the 4 relay design, I did add a new R/B wire from the relay block out to one of the headlights. The original system daisy chains the filament currents together, but with one relay for each filament, you can't do that. You need another wire run to the front for one of the low beams. The two relay hybrid version shouldn't need to do that however. You'll have both sides HI beams running through the R/W (the largest wire) and both sides LO beams running through the smallest R/B (just like stock).

Yes, the fourth relay is different than the other three. That's why I was saying that three out of four of them are very easy to find (eg they are the same ones used in the Honda relay block I used), but I wasn't sure about the fourth. It might be a very common one as well, but I don't know. I stopped investigation when I went with the Honda block instead.

Hmmm... I'm not sure I'm understanding what you meant, but I didn't have to pull any new wires through the firewall: Original Red from the fuse box for power to right bulb relaysOriginal R/Y from the fuse box for power to left bulb relaysOriginal R/B for low beam actuationOriginal R/W for high beam actuation That's it. The branching for different relays I did out in the engine compartment inside the harness stem to the relays. For example, I needed two R/B wires to control two relays so I spliced a second R/B onto the original one that came through the firewall. Same for the power wires to multiple relays. And since only one of them is carrying any current at any given time I feel completely justified in that. So, yes... I did need extra wires, but I did the splitting inside the harness on the way to the relays. You could, if desired, make all the splitting connections by crimping multiple wires into the relay spade connectors and daisy chaining them using the crimp contacts as your junction points. I didn't do that because I didn't want to complicate the crimps, but that could work as well.

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.

Yeah, I understand. I've found the easiest to harvest is from the 98-00 Civics, but they are on other Honda derivatives as well. Good luck with the hunt!

Here's a snipped from the FSM on which I've made some notes (in red). Might help to determine if it's a wiring issue or a tach issue?

I'm no engine builder, but I'd have a hard time closing that thing up knowing I had pistons that looked like that...

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.

Funny you should ask a question about that... I nabbed these relay/socket assys from Honda/Acuras. I found them to be very compact, orderly, and gangable. They also use a very ubiquitous relay style that you can find pretty much everywhere: . I just posted a bunch of other details and pics in thread about my headlight upgrade experience: http://www.classiczcars.com/topic/51554-a-different-headlight-relay-upgrade-4-relays/

Here's some info on the relays I used for my headlight upgrade. The relays themselves are used in lots of applications and are easy to find, but what really grabbed me about these isn't the relay at all... It's the socket. This relay/socket combo is used in lots of Hondas and Acuras (For example, All 98-00 Civics have at least one assy and two if it has a sunroof). They come in SPST and SPDT applications depending on the original use (for example, horns use SPST and sunroofs use a pair of SPDT). They are easy to harvest and can be found under the left side dash near the driver's left knee. Some models only have one position populated, but have a socket that will accept two relays. Depends on whether it's a manual or auto. And cars with a sunroof will have a second socket (with SPDT relays). Here's the socket and relay combo I used. It's small, comes with a mounting bracket, and accepts two relays: Remove the relays and you're looking at this: What I found really useful about these sockets is if you strip down two of them like this: They gang together!!! So if you need more than two relays and want to mount them all in the same place: Put in four relays and you got this. It's much more compact than the "Bosch style" relays: Since I had just completed an internally regulated alternator as well, I put my headlight relays in the spot vacated by my original external voltage regulator: On the last pic, you can see the retaining feature on the end that makes them gang-able. It really helped simplify the mounting and kept the size as compact as possible.

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?

So they sent you that rusty head claiming that it was a good condition usable piece? Seriously?

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