Captain Obvious

The work sounds like it's going well. I like the part about the thermostat housing... It's always something! That 3.5 psi is "engine running, sitting still, with return line in operation", right? About the thermostat, I wonder how much of a difference in required ratio the thermostat would make. By that, I mean... Is it conceivable that the change in temp from 130 up to 180 would change the optimum fuel/air ratio enough that maybe the carbs were working as designed all along, but it just wasn't enough fuel for an engine that was perpetually "semi-cold"?

Hmmm... I'm no expert on this stuff and I would gladly defer to someone else who's been through this before, but I guess I would come out of the gate with this: Mechanical pump still attached, but outlet routed back to inlet (because there's still residual fuel inside and you don't want it spraying on your cooling fan). And the reason I would take the mechanical pump out of the loop is because it may provide an unwanted restriction to the electrical pump, and in theory, the electrical pump should be able to provide all the fuel the engines needs, even at WOT high RPM, right? Electrical pump mounted as close to the tank as possible (because the documentation for those cube pumps says that, while they are self priming and can do 12" lift, they are really pusher pumps). Regulator located after the last carb pickoff point from the rail, and then outlet of the regulator returning back to the tank. Question... Are you running the original fuel rail? If so, what (if anything) are you planning to do with the original fuel rail "regulator" restriction orifice? If you're installing a real regulator elsewhere, I would remove that orifice. You don't want a pressure drop there anymore. You could drill it out or cut an inch off the end of the hard return line tip. And... "Good luck. We're all counting on you." PS - Crap! I was out late last night and didn't check the forum when I got in... Just looked at the posting time, and you're already on the track!

Just found one more interesting piece of info. On this website (which looks like a Chinese second source to the 40106 pump) http://www.aliexpress.com/store/301124/209377070-452432241/Facet-Red-Top-Electric-Fuel-Pump-40106-squre-shape-.html It lists a curious spec "Shut-off pressure : 4-6psi" The implication to me is that these pumps, being solid state and all, have actually got some pressure sensing technology in them and the pumping action is actually controlled to maintain the pressure between the two pressure points listed. In other words, the implication is that the 40106 pump will turn off when the outlet line pressure is above 7 psi, and will turn back on again if the outlet pressure is below 4 psi. And if you stay in the hysteresis band between those two limits, the pump will always run. So, if you've got a pressure regulator between the carbs and the return to the tank you just pick the Facet pump that has your regulators pressure at approx the middle of it's operating range. This is all inferred, of course.

After much digging and cross referencing**: NAPA 610-1077 contains Facet pump number 40106 NAPA 610-1078 contains Facet pump number 40104 Slightly more info for those two pumps can be found here: http://www.facet-purolator.com/index.php?option=com_content&task=category&sectionid=7&id=16&Itemid=31 PART # MAX-MIN PSI GPH 40104 4. - 1.5 25 40106 7. - 4 32 I still don't know what the specs mean, but here... This graph should clear it all up. This is what happens when Marketing asks Engineering for some documentation that they can put on the website that couldn't possibly cause anybody to actually be able to be held accountable for anything at anytime.:mad: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ **Here's the NAPA page for the 610-1077 - https://www.napaonline.com/Catalog/CatalogItemDetail.aspx?R=BK_6101077_0334661226 on that page it lists the mfg #NAP12SV. Here's a pdf catalog from Facet/Purolator - http://www.facet-purolator.com/images/facet_guide_2004.pdf At the end of that catalog it lists the solid state cube pump P/N FEP 12SV At this point, it's pretty clear that the NAPA 610-1077, Facet FEP 12SV, and Purolator PRO 12SV are all the same thing. Then on this page - http://www.facet-purolator.com/index.php?option=com_content&task=category&sectionid=5&id=38&Itemid=62 there's indication that the FEP12SV and PRO12SV are clamshell kits containing a pump number 40106 and other associated items. Similarly (without listing all the intermediate steps), the NAPA 610-1078 is a clamshell containing the 40104 pump.

Don't you hate ambiguous specs. I did a little digging and came away more confused than when I started. I don't know for sure what those numbers mean...:mad: What are the part numbers for the pumps you bought?

Cool. So... What happens if all the investigation into fuel delivery doesn't turn up the problem? How can you tell if the fault is actually ignition, and not fuel delivery? Other than shotgunning it and replacing modules to see if that makes any difference, is there any other diagnostic work ahead of time that can be done? I've done no work with the Z ignition to date, other than plugs, wires, caps, etc... maintenance stuff. Thankfully, I've never had to do any diagnostic work, and consider myself blissfully ignorant. Maybe the spark is timed correctly, but doesn't have the strength to fire through the densest gasoline cloud that occurs under your problem conditions. You wouldn't see it under light load conditions. Could rev fine all the way to redline while sitting still, but might not work so well once on the track and you're pulling all that fuel and air into it? I'm not trying to be pessimistic... Just thinking about contingencies now because once you're at the track, it's all willy-nilly fire drill, right?

That sounds like a great plan. I sure hope you find the issue!! Couple of quick comments before it's too late... Why are you going to remove the mechanical pump? The 260 originally had an electrical pump in back feeding the mechanical pump up front. And, just because you're not using it, doesn't mean that the pump has to come off the side of the block, right?. You could leave it in place and plumb around it. You could even get fancy and pipe the outlet back to the inlet! But if/when you do remove the mechanical pump, you should be able to easily fashion a block off plate out of some sheet steel or aluminum. There isn't anything complicated about the shape. Flat works fine. Cut to rough shape, drill two holes, cut some gasket material, smear some goop, and you're done. I'd hate to see you spend out of your $500 for something like that! And... If you're going to put a thermostat in, you should probably have a working temp gauge.

Hi Jeff, Of course, armchair troubleshooting is fraught with potential incorrect diagnoses, but from your description, I agree with the assessment that it's a problem getting fuel TO the carbs, not a problem inside the carbs themselves. Have to start with the simple stuff like filter screens though, right? So next? A fuel pressure gauge between the pump and the carbs is a must at this point. And I wouldn't block off the return to the tank until you know what the pressure is. You don't want to overwhelm your float valves and blow fuel out the overflow tubes especially if you put an electrical pump in series with the mechanical. You should be able to test the "sucking air" theories by running without a gas cap. I haven't completely digested the original tank venting system details, but I believe the original system is "supposed" to run at a slight vacuum. That vacuum is controlled at the stock filler cap. No filler cap - No possible vacuum - No pressure differential - No sucking air. (Of course, unless you're creating a pressure drop elsewhere with a clogged filter or something.) Just remember on left handers that you've got no filler cap. The ID of the tube between the float bowl and carb bodies is 3/16 ID. If you want to do more than check the bowl levels and actually run the car with some clear stuff, I would recommend the gasoline grade F-4040-A Tygon. http://www.tygon.com/tygon-f4040a-tubing.aspx. How about some Tygon between the float bowl and the carb body and an engine bay cell phone video cam taped to the fender well?:bulb: If you see bubbles coming through that tube, you're sucking the bowls dry. At least you'd have the "what?". All you would need to do is the "why?". I don't have any Tygon, but I do have a plenty of suitable Buna-N black opaque 3/16 ID tubing if you need some. If you're worried about kinking or collapsing at the bend, you could probably put on a length longer than usual and include a strain relief loop. PM me if you want to talk about it. As an aside... 29 replies in 300 views. Now that's what I'm talkin bout! Glad I could break the ice.

Hi Jeff, I'm sure you know already... "It could be lots and lots and lots of things", right? But from what you've described, I would guess fuel delivery issues. Sounds like you're getting enough fuel under all conditions except at high RPM full throttle conditions when, of course, you need the most of it. Only thing that seems to contradict that is this part: Could you put a pressure gauge on the fuel rail so that you can see it as you drive, and take a run with it installed? Have you checked to make sure that the inlet filter screens in your SU's are clean? You might also pull the needle valves and make sure crud isn't partially plugging them too. I've seen paper towel fibers get through the fuel lines and gather like a wood pulp wad in the needle valve. They built up gradually and can act intermittent. As an aside... Wow! I'm surprised you don't have fifty responses with help by this point. I would have expected a post like yours to draw in all the experts from world around!

No, no... Don't worry about the float volumes. Those measurements I took were with needle valves and floats installed and working as used in application. I was thinking about it and was giving myself a headache trying to decide if there was validity in measurements without floats and Archimedes appeared to me in a vision and made fun of me because I couldn't figure it out.:stupid: So I just bit the bullet and did it with valves and floats installed. I didn't spend a lot of time getting the float levels spot on, but they're pretty close. (That's one of the "shortcuts" I allowed myself to take.)

Hi Bruce, I'm sorry, but I'm not sure I understand your question... What we're trying to show here is the volumes of the two float bowl chambers. This spawned from another thread that was discussing differences in the design of the venturi bridge between the round tops and flat tops. At request from Blue, I made the measurements above. I wasn't sure if it would have been better to post this in the original thread, but since it really had nothing to do with the content of that original thread, I thought it would be better posted in a new thread. Here's the original thread where the request came up: http://www.classiczcars.com/forums/showthread.php?43440-SU-Bridge-Evolution-Round-Top-vs.-Flat-Top-(Comments-welcome)

Upon request, I have measured the fuel capacities of a typical round top and flat top carb. Note that due to shortcuts I took to make the measurements easier, I'm not claiming accuracy to the cc here, but the results should be fairly accurate. Here's the round top: And here's the flat top: The bottom line is that they are close, but it appears that the flat tops have a greater capacity in the float chamber.

Please don't get me wrong, I'm not poo-pooing the idea. I agree that acme threads are a better choice than standard threads for an application like this. I just came in here to offer you some lengths of acme threaded rod for the cost of shipping if you wanted to mess around with the design before you spent real money on material of known lineage.:classic:

Yeah, those spindle pins can truly be a pain. What I'm saying is that I've got some acme threaded rod if he wants to give it a try, but I can tell you from experience that I've run across some spindles that weren't coming out in one piece using a "puller style" tool no matter what kind of all-thread you used. Sometimes the forces holding the spindles in are greater than what the threaded ends of the spindles themselves can withstand. If that's the case, the spindle itself will yield before it will budge. BTDT.

Oh. Right. So you aren't running the original intake manifolds, balance tube, or air cleaner, right? That makes things more difficult.

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!