Captain Obvious

I'd be hard pressed to believe that a backfire could cause fuel to shoot out of the banjo connection. I don't understand the path of pressure that could possibly make that possible. If you've got atmosphere venting nipples on the fuel bowl lids, it's impossible to build any significant pressure in the bowls. And even if you did, how would that pressure get backwards through the needle valve to the banjo connection? How far is this fuel shooting, and are you sure it's got anything to do with the backfiring?

Awwww crap. I thought you were taking the lead on arranging escorts for the show or something. I completely support the concept of meeting and being able to get in touch, but I don't have a smart phone. Will this system work for me on my Neolithic flip phone? I was thinking we could just exchange phone numbers or set up arrangements to meet at one of the local brew pubs. I've been known to make an effort to visit all of the local breweries in any new town I visit. I sure hope there aren't twenty of them...

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.

I haven't tried that Eastwood paint, but I have to tell you that you make that outfit look good! Party on, my friend!

The temperature threshold for the cold start valve actuation is pretty low (below about 70F). If the engine bay is hot, the cold start wouldn't be expected to actuate anyway. In other words, once the engine is warm, there's no difference between having it electrically connected or completely disconnected.

Well I'm not expecting that you'll find anything wrong with the cold start injector, but it is "possible". Prudent to check, but I'm not expecting a revelation. So what else.. Injector that sticks open some when cold? Again, it's possible, but I wouldn't put any money on it. Have you checked the validity of the signal coming from the incoming air temp sensor? Been a while since I went through the theory, but IIRC, the air temp sensor has less of an impact than the water temp sensor, but could still be causing some kind of issue if it's not accurate. To check it, keep in mind that the temp-resistance curves for both the water and air temp sensors are the same, so when the engine is cold (and the coolant is the same temp as the surrounding air) you should get about the same resistances from both of them. Maybe you've got something wrong with the fuel pressure regulator that gets better when the engine warms up. For example, a leak in the vacuum control hose to the regulator could cause fuel pressure to be too high. And if that leak somehow minimizes as the engine warms up... Like I said, just tossing stuff out there for discussion.

I'm assuming you had little footies on the bottom to keep the sand out of your toesies? Hahaha!! You had to know you were going to regret that!

I got nothing that wouldn't still be an issue after it warms up. CTS and AFM issues wouldn't go away once warm. Cold start valve could be leaking even though it's electrically disconnected, but that wouldn't go away once warm either. AAR valve never does anything to affect mixture, it's idle speed only. Maybe the richness curve vs temp is just steeper than you're used to on newer vehicles? I mean, it certainly isn't as well controlled was it can be with todays vehicles. Maybe it's operating as intended and you're just expecting different? Maybe it IS still running rich after it warms up, but you don't notice it as much because it does a better job of completely burning the charge? Just tossing out ideas...

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:

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.