Captain Obvious

Nice documentation. Thanks for taking the time to put that together! Couple small corrections... The tube seen on the right side foreground of the flat top in this pic is the ambient pressure vent to the float bowl, not the high pressure side of the suction piston: There are two holes down inside the flat top throat that vent to the bottom of the suction piston, but you cannot see them in any of the pics. We didn't talk about them, but they are above the choke plate. Also, I'm really not sure about the "off idle transient" effectiveness of the power valve. Unless there's some sort of transient second order transient "uber reduction" in manifold vacuum when you change the position of the throttle plate angle, it appears like the power valve does not act like an old school "accelerator pump". It simply has a vacuum threshold above which it starts to supply additional fuel. I know I've heard that belief before and I'm no expert, but my analysis of the power valve operation does not support that concept.

That's a neat setup. And I assume a lot easier to use than a colortune for when you're driving around. What's the response time like? Do you think it would pick up Oiluj's transient states?

I've heard two versions of what is "supposed to happen" when you lift the pistons. First is that it will simply stumble badly (which is what yours does), and the second is that the RPM's will go up for a brief instant and THEN stumble badly. I've also read where people were trying to achieve that "RPM rise before stumble" nirvana and it was elusive. I say that if the colortune looks good at idle, flashes rich off idle idle, engine runs well and doesn't bog when you punch it, and the plugs look good, then you're way ahead of a lot of people messing with their carbs!

Assuming it doesn't take like ten seconds to come back to steady state, flashing rich blipping off idle is a good thing. It means that your piston damper is working. As for anything else except for off idle, I think there's a lot less to be learned under no-load. You're only using a fraction of the needle. The second part about the A/F not changing much with a slow steady change in throttle is an indication that the first few stations on your needle are well sized, but in order to test the rest of the needle, you need a load. Back to your original question, I'm not surprised at all that your car stumbles badly when you lift one of the pistons. You're effectively killing those three cylinders. The only fuel they will get is what happens to head their way through the balance tube. Even with both nuts one extra turn down, I'm not surprised. No number of turns down on the other carb will get fuel to those three dead cylinders. The only thing you can do is make sure the OTHER three cylinders are working as well as possible. Then swap and do the other three. Here's a different way to think about it... When you lift the piston on the rear carb, you aren't testing the rear three cylinders. You are testing the FRONT three. Does that help? Here's where I remind you that I'm no expert though... YMMV!!:classic: Blue? You with me on this?

Which direction does it flash when you change the throttle position? Rich or lean?

Haha! Done!! Let me know what you find with the flow bench stuff. Sounds like a concise way to investigate this stuff. I've spent some time analyzing the SU design, but I'm certainly no carb expert. I am, however, completely capable of having absolutely no idea what I'm talking about! I've got a degree in that.:stupid:

Yes, you're right. The venturi itself by nature has to be a restriction to some degree or it is not a venturi at all. But the amount of restriction the venturi provides pales in comparison to the restriction provided by the throttle plate. The suction piston cannot block all the air. Ever! Even if that little phenolic bumper did not exist at all! The slot cut into the bottom of the piston is still available as an air path even when the piston is slammed all the way down home. At idle, that slot concentrates the majority of the airflow into the center of the piston and directs it right over the nozzle. Concentrated air flow, more velocity, less pressure at the nozzle, and fuel gets sucked up past the needle. Lift the piston, even a little, and you disable that slot. Non-concentrated air flow, less velocity, more pressure at the nozzle, and no fuel gets sucked up past the needle. The cross sectional area of the venturi at idle is about .012 square inches. I propose to you that the distributed area of the crack around the almost completely closed throttle plate is smaller than that and the majority of the pressure drop occurs at the throttle plate, not the venturi.

No, you see... Lifting the suction piston does NOT add more air. The only thing that would add more air is opening the butterfly. At idle, you've got a tiny crack past the two throttle plate butterflies. The engine is pulling as much air as it can past those two throttle plate restrictions. You lift the suction piston, and so what? The same pistons (crankcase pistons) pulling the same air past the same butterfly restrictions. The total amount of air being pulled into the engine will not change. (Again, with the exception of the changes in RPM due to lifting the suction piston.) Yes, you reduce the air velocity at the venturi which (will reduce the venturi vacuum and suck less fuel), but the total amout of air pulled past either butterfly will not change.

I'm no expert, but I believe the above contains a little bit of a misnomer. If you create a "vacuum leak" you introduce additional unmetered unfueled air. Your traditional "vacuum leak" will reduce manifold vacuum because it goes around the throttle butterfly. On the other hand, lifting the piston won't change manifold vacuum (except due to any changes in RPM because of rough running) and it won't introduce any additional unmetered unfueled air. What you do when you lift a piston is decrease the venturi airspeed which renders the venturi ineffective and no fuel will be introduced by that carb. The amount of air flowing through that carb will not change, just the amount of fuel added to that air. So yes, you will see a big change on your wideband because even though the amount of air hasn't changed (again except for any changes due to an RPM change), the amount of fuel has gone down. If you plug a carbs mouth at idle, it will create a huge "static" vacuum in the entire throat of the carb, including the venturi. That huge vacuum will pull raw fuel up and out of the nozzle. That's what "chokes" do.:classic: Or... There's always the possibility that I have recto-cranial inversion. It's happened before!:laugh:

Are there steering wheels from other cars that bolt right onto the Z? I was walking around a boneyard the other day and was thinking about other steering wheels, but the problem is that it's a pain to go through all the effort to start pulling wheels only to find that they don't fit anyway. I've got an OEM rubber steering wheel that needs help and I pulled it off to get a good look at the mounting splines. I found that there are 36 spline points on the steering column and I measured the OD of the splines on the column to be 0.688. Is there a website that lists the spline specs of various vehicles?

Don't get me wrong... I wouldn't get all worried that you're cutting huge chips off the inside of your carbs at any great rate. It's just that I've seen some carbs from other applications that were wallowed out to the point where the butterfly's wouldn't seal anymore and getting them to idle and come off idle without issues was impossible. How long would it take for you to get to that point? Probably years. Another thought is that I've seen some other carbs that had actual seals on the shafts from the factory, and they didn't use O-rings. They used non-round cross section seals with the lip direction such that the vacuum would suck in the direction that would increase the effectiveness of the seal. I'm no seal expert, but it makes me wonder if O-rings are the correct choice for vacuum application. Maybe it's a tradeoff between seal effectiveness and friction? But... Any seal will be more effective than "none". Lastly, I'll cut you some grooves in your shafts if you would like to try putting some O-rings on the shafts to seal inside the bushings in the carb bodies like what Oiluj suggested.:bulb: I've even got a bag of "X" cross section (non) O-rings laying around here somewhere that might be close to size.

Neat ideas Blue. Are you sure that you didn't preload the position of the butterfly plate towards the O-ring? By that, I mean... When the butterfly is completely closed, the side-to-side movement of the throttle shaft is restricted because the butterfly is a tight fit in the carb bore. But when the butterfly is opened, the throttle shaft can move back and forth a bunch before either the butterfly or the linkage bits hit the carb body. If your O-ring is compressed when the throttle is closed, then it will pull the butterfly towards that side of the bore, but when the throttle is opened it may wear the butterfly plate or bore prematurely in that location from the constant scraping every time you open or close the throttle. Not saying it's a big problem. Just a thought.

I'm sure you already checked this, but I just gotta ask... :tapemouth While the idle is in it's abnormally high state, did you manually pull the throttle shaft with your finger in the direction that would lower the idle? Not at any part of the shared linkage... but right where the throttle shaft enters each carb body? Haha!! I know, I know... but I'm Captain Obvious!!

That orifice is a rudimentary fuel pressure regulator. If you enlarge that restriction, it will lower the fuel rail pressure and if you drill it out too much, it will cause a problem. There needs to be enough pressure under all load conditions to force fuel through the fuel lines (which is uphill in spots), filter screens, and push open the float valves. How far can you go (if at all) before it becomes a problem? I have no idea. I slept through that part of class, but I don't think the extra flow will do much for a temperature change in the rail. I can see my return flow because I've currentlly got my return line flowing into a catch can. At idle (light load) there's a surprisingly large amount of fuel being returned to the tank already. Can you take a couple thousandths off without causing a problem? Probably. Would it do anything good for you? Probably not. I'm no expert, but I consider it risky. Haha! Now you've heard two divergent opinions... Good luck with whatever you do, and I hope you kick your VL problems.

Thanks again for the info. There was a time when I could lift that, and I might still be able to, but the risks are too great...:stupid: BTW, I checked out your pic acct. Great stuff there! Thanks for sharing! I was under my Z a little this afternoon. I thought of you and how nice my Z COULD look.

I don't remember where I nabbed this photo, but I've always wondered if it was photoshopped. So, you're saying that if the guys in the rear got the nuts, then this can actually happen?

Rainman, How much weight do you guess you have on the rotisserie? Looks great!

So where did you get the new felt? My felt did the same thing yours did. Was the replacement generic, or specifically made to fit the Z's? BTW - At this point, I've got more than enough "drain" holes in the bottom of my doors. I don't think they can hold water... Anymore. :disappoin

But the 72 round tops have coolant flowing through them as well though. And you don't even have flat tops anymore. I don't know... I'm just trying to figure out why you (with a 73) are seemingly having more problems than the 70, 71, and 72 owners out there. Even the ones in hot climates. Not that MD is "cold", but there are lots hotter. I'm wondering if there's some other 73 remnant that's complicating matters. About the fuel boiling in the filter... I doubt that's where the fuel is boiling. The filter is cool compared to anything that actually touches the engine. Maybe it's boiling in the intake side of the fuel pump and the resultant vapor is backflushing the fuel out of the filter and back into the tank? They don't run any check valves in that line to prevent that, do they? My 74 has an electric pusher pump that probably acts as a check valve, but you don't (currently) have a pump. You could park it hot and then use locking pliers to clamp off the hose on the intake side of the fuel filter and see what happens then.:bulb: If you don't let it go back into the tank, the pressure will go up, and raise the boiling point? BTW - My 74 doesn't have hood vents. I don't think that started until the 280.

So from the factory, you originally had the earlier version of the flat tops, right? I've got a mostly stock 260 with the later version of the flat-tops. Haven't had it long enough to know if I've got a vapor lock issue or not. So why is it that the 73's and 74's are so notorious for VL? It might just be a mistaken impression on my part, but it seems to me you might hear mention about 70-72 occasionally. However, it's not the resounding cry of "known issue" that you get when you mention 73 or 74. The first, most obvious answer that everyone jumps all over is the flat-tops. Well, you've taken that out of the equation already. Next is that the 74 got an electric pusher pump from the factory. But the 70-72 got along OK without one, right? So is there something else about the 73 and 74 that makes them more prone to VL than the previous years? Fuel rail design maybe? You're saying that you parked the car hot and the fuel filter was full when you shut if off, but after sitting for 15 minutes in the sun, the filter was empty? Is that before or after it runs for half a block? That's just sad...

Dang Dude. That sounds like no fun at all. I hope you heal quickly! Since you've got nothing better to do right now than think about the vapor lock problems... There's a pdf document titled "1973 240z 1974 260z Fuel System Modification Plus" over at xenon that talks some about (Datsun's definition of) vapor lock. Probably nothing you haven't seen already, but if you haven't read that one yet, it's an interesting read. Also, what year is your car? I remember that you're running round tops, but I don't remember you mentioning a year. Speedy recovery!!

It really doesn't matter, but I don't get your math... I do this: Assuming batch fired injectors on a four stroke engine... At 1000 rev/min there are 500 injections/min 500 injections/min = 8.3 injections/sec Do I think a high frequency damper might be able to do something to quiet 8.3 pulses per second? Yes. On your VL issues, it sounds to me (the non-expert on such matters) that you're doing everything right except for the removal of the pusher pump.

Haha! Did you look at the list of references for that page and the ones similar? It looks like someone is bucking for a good thesis grade. A lot of the references cited are less than five years old and lots of them are clearly research! This is my favorite... Number 25: Ye, Xiang-Rong; Lin, YH and Wai, CM (2003). "Supercritical fluid fabrication of metal nanowires and nanorods templated by multiwalled carbon nanotubes". Advanced Materials 15 (4): 316–319. doi:10.1002/adma.200390077. Now there's a citation I can trust! As a matter of fact, I was just messing with some metal nanowires and nanorods templated by multiwalled carbon nanotubes yesterday in the shop. Seriously though, good catch. Research continues and concepts are refined. Haha! Back when I was in school, we got by just fine with only three phases. Seems they weren't teaching this at your school either, huh?

So there's nothing up in the engine compartment to mitigate high frequency pulses? You've got the FPR up there for low freq stuff, but nothing for high frequency effects like the injectors opening and closing? I'm wondering if there was a device close to the injectors capable of dealing with high frequency effects and regulating the fuel pressure better than the FPR, it might help with the very hot restart issues we were discussing. The thinking being that it might do a better job of regulating the pressure locally and preventing any vapor from forming in the system, even if that vapor is a recurring transient. Yeah, I went the other way. Engineering first. I would have gotten more out of it if I'd done it the way you did. Heck... Maybe even fluid dynamics and strengths of materials would have been ummm.... dare I say tolerable?

The really unfortunate thing for me is that the main reason I hated thermo so much was that I (at the time) saw absolutely no practical application for it in my future. I barely squeaked through!! Little did I know... If I had known then how much of it was easily applicable to automotive applications, I would have paid attention. There is so much more that I could have gotten out of it if I simply would have given a crap.:stupid: The stupid things you do that don't seem stupid until later.