Captain Obvious

The all-thread rod I've seen availably commercially (read "hardware or big box home improvement store") is crap. It's nickel plated low carbon steel and is, well, crap. I'm not surprised that people are snapping it or pulling threads out. If you want to try some proof of concept stuff, I think I've got some acme threaded rod in the shop. Probably 3/4-8 or 3/4-16. Don't know the composition, but I can guarantee it's a stronger alloy than the all-thread from the H/W store! About how much would you need? What are you going to do for an acme nut? Haha! Yeah... Good luck with that! Must be Pennsylvania salted roads, or something, but I don't care how nice your thrust bearing is... The pins I've removed weren't coming out in one piece.:ogre:

As mentioned previously, if it's a 73, it's supposed to have both an air pump and an EGR system. So why am I in this thread then? Because reading through the service manuals, it appears that 73 and 74 is identical. So if you're looking for EGR parts, you can probably use 73 or 74 parts. Also, I think everything from 70 to 74 had air pumps, and I believe they are all the same. So if you're looking for an air pump, I don't think it matters what year. So does this stuff have to work, or just be present and look like it's connected properly?

Is there a hole in the side of that thing to vent to atmosphere for when it is not holding the throttle open? I've got a 74 260 and the throttle opener is a little more complicated because it includes an electronically actuated purge vent. My first thought on your oscillation is maybe the foam was a damper for the spring? How fast does it oscillate? Does it "hunt" or "buzz"?

Haha! From all of them I've dissected... They did! Seriously though, the sealing between the inlet and outlet on that power valve is this goofy flat rubber washer and every one I've opened up, that washer is either brittle hard and dried out or gooey and partially dissolved. I don't know what those washers are made out of, but I suspect it's a poor choice of material for today's fuels with ethanol included. Datsun even mentioned the possibility of leaky power valves in their documentation a couple of times. That document on the HJL38W was interesting. Very much halfway between the round tops and the flat tops in operation. One of the most interesting things is the idle air adjustment control just like on the HMB46W. They don't reference it anywhere in the text that I could find, but on the parts breakdown, it's #12. They meter and split the air at the rear carb and pipe to the front carb (which is the opposite to what they did on the 260Z), but the concept is identical.

Yes, the two holes in the upper left and upper right are the high pressure side of the suction piston. The two smaller holes closer to the square mouth (one in the center and another on the upper left) are mounting screws for the vacuum break actuator. I took the actuators off and just reinstalled the screws into the holes so I wouldn't lose them. One of them was a little longer than the other and sticks down into the carb throat. It does interfere with the choke plate, but it's not supposed to. With the vacuum break device installed, those screws aren't long enough to stick down that far.

Yes, I found that post over on zcar.com, and I disagree with his analysis. He got the first part right about how the vacuum actuation works, but I contest the rest of it. I will point you back again to that interesting read on popular hotrodding.com: Mixture Requirements . . . When the vehicle is cruising, the mixture needs to lean out considerably if good mileage is to be achieved. With most carbs, we are likely to be dealing with a power-enrichment circuit, activated by a vacuum-sensitive power valve. This usually takes the form of a vacuum diaphragm, which senses how much intake manifold vacuum is present. Opening the throttle causes the intake manifold vacuum to decrease to near zero. This allows the power valve to open what can best be described as an additional main jet that supplies the extra enriching fuel. This additional main jet in any Holley-style carb is commonly known as the power valve restriction channel, or PVRC for short. This description above is exactly what my analysis led me to understand as to the operation of the flat top power valve. If you take a look at the pictures from that article of a typical "accelerator pump", and a typical "power valve", the biggest stand-out difference to me is the check valves in the accelerator pump. The power valve has no check valves in it which means it cannot "pump". There is nothing to prevent it from simply pushing a diaphragm load of fuel back down into the float bowl. Here's an accelerator pump: And here's what the power valve looks like (functionally): The fuel is pulled through by vacuum, not pushed through by the diaphragm. I'll take some detailed pics of the device when I get the chance and I'll try to show you why I don't think it has any transient function, but I didn't get to that today.

If I get the chance, I'll take some pics of the vent holes leading to the suction piston. Being as how they are hidden up on the roof of the flat top bore above the choke plate, you don't normally see them unless you're specifically looking for them. Speaking of those vent holes... At the risk of getting kicked out of the SU section for heresy, I consider this another improvement with the flat top design. You see... On the round tops that vent hole is completely on the other side of the intake horn in the air cleaner, while on the flat tops, it is a true differential pressure right across the suction piston. Probably doesn't matter, but it's just "purer" with the flat tops. You mean the volumes of the respective float bowls, right? If that's the case, then yes, I should be able to measure that (crudely) and let you know what I find out.

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