Captain Obvious

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.

So is the damper intended to reduce changes in pressure in the positive, or negative direction, or both? In other words, suppose your fuel rail is supposed to be at 30 psi... Is the damper supposed to provide some temporary volume to swamp out pulses above 30 psi, or supply a transient supply of fuel in the event that the fuel pressure drops below 30 psi? Or both? I've never thought about it that intently before. You got any idea? I mean, who knows what's happening at speed. You got the injectors are opening and closing. They open and fuel goes out, so physics dictates that the pressure must drop. And then they snap shut, probably momentum hammering the fuel rail and sending a very high spike back through the line. And then you have the fact that the pump output is probably not be a perfectly steady pressure either. So what's your take on the damper?

I'm guilty as charged, and I apologize. I would like to be the first to admit that I've offered nothing to help with your original question. I found the topic interesting and got sucked in with the intent of defending the laws of physics. And just when I thought I would be able to resist, something like this happens... Uhhh... No. There exists a temperature (shown on the phase diagram I linked to earlier as Tcr) called the "critical temperature". The critical temperature of a substance is the temperature at and above which vapor of that substance cannot be liquefied, no matter how much pressure is applied. In other words, above Tcr, there can be vapor only. No liquid, and certainly no solid. (Haha! Ignoring superheating, of course.) I'm really trying to give up the physics. Really!! As a matter of fact, thermo was my worst class ever. I absolutely hated it.

Please forgive me for not knowing the intricate details of the Z's FI system, but... You made mention earlier of the batch type injection concept where that all the injectors open at the same time for the same duration. Is that what the 280Z uses?

Nice description of the gradient path, and I'm with you. Assuming the temperature is high enough, somewhere between fuel pump and the intake valve end of the pintle the gasoline will cross the phase change line from liquid to gaseous. Still not sure exactly where, but it seems from yours and others experiences that it's in a place that affects performance for the first few minutes of operation. I'm buying it. My only point of contention is that I'm still not seeing the "superheated" part and we are really running the risk of wandering off into the weeds with this part of the discussion so I will try once and then let it go. I believe the use of the term on the geyser page is a misnomer as well. I know you had to pass thermo, right? Just because you have a hot liquid under pressure does not "superheated" make. That just means you have a hot liquid under pressure. With all imperfections in the rock surfaces and turbulence in the water, I can tell you that there's no superheating in a geyser. It's too unstable for that. You can change phase and flip back and forth across the liquid/gas phase line all day by varying temp or pressure as your theory suggests without ever becoming "superheated". In order for a liquid to be superheated, it has to incorrectly exist in liquid form when conditions place it in the gaseous area of the phase diagram. In other words, your substance is at a temperature and pressure that SHOULD result in a gas, but you are incorrectly a liquid instead. Ask yourself the question "What is keeping it in liquid phase?" If the answer is "Pressure.", then you're not superheated. If the answer is "Uhhhhh... I don't know. It really SHOULD be a gas but it's not!!", then you're superheated. Does that make sense?

You think you're boiling fuel even with a 30 psi rail? So where exactly is it that you think the fuel boils? And do you think it's boiling a little at a time on each injector pulse and blowing only vapor into the manifold, or do you think it's sustained and bubbling back into the rail as FastWoman described? One thing that's completely clear from all of this discussion is that with my carbureted Z, I'm simply screwed... And not that it really matters, but I'm still unclear on why you're bringing the concept of superheating into this... You really don't even need it to support the beliefs under discussion. But in any event, it doesn't detract, it's just probably unnecessary.

STP has nothing to do with this. It's all about vapor pressure at elevated temps. You're nowhere near STP. Also, your use of the term "superheated" has become is a little confusing to me, so it might be prudent to make sure we're talking the same thing... What's your understanding of "superheated"?

Yeah, You're right. I probably should have said "the upper limit on vapor pressure is supposed to be tightly controlled" Research indicates that the intention of the gov't is to reduce the amount of fuel evaporation into the atmosphere as much as possible while still allowing the fuel to work well in application. The way they do this is to dictate an upper limit on the vapor pressure for different locations at different times of the year. How the manufacturers achieve that vapor pressure limit seems to be mostly up to them, including the composition and ethanol content of the fuel. My research turned up two things that always seem to be true: 1) Winter fuel (RFG) is allowed to have a higher vapor pressure than summer fuel, and... 2) California requires a lower vapor pressure than most other states. Of course they are. You remember the difference between "suspensions" and "solutions", don't you? And you mentioned the concept of superheat earlier and I forgot to ask... Why do you think the fuel in the injectors is superheated? Seems difficult to achieve. Have we strayed far enough from the OP's question yet?