240Znomad

@ckurtz2 I never really figured out what was (or is) causing the problem. I haven't driven the car for a while now but I'm guessing that when I do get it fired back up I will still have the problem.

It's engine vacuum that causes the dashpot to open so if the throttle plate is closing, the dashpot should not be rising. The fact that the dashpot rises suggests to me that the throttle plate is opening and the only way to open the throttle plate is using the throttle linkage. Several things on the throttle linkage can affect the throttle plate but the main 2 (besides the gas pedal) are the idle adjustment screw (which you have unscrewed), and the choke (which will open the throttle slightly when the choke is on).

If both throttle plates are fully closing and operating correctly then something must be opening the front one when the car is supposed to be idling. I'd start looking at the throttle linkage as Captain Obvious just mentioned. Try helping the throttle close completely. It's really hard to tell from pictures, but to me it looks like there might be a gap in the front linkage where the main throttle bar connects to the front carb. I have attached a picture of that part on my '71 240Z (same carbs as yours) showing what mine looks like and as you can see there is no space between the 2 small plates. On your video it looks to me like there might be space in there suggesting the front carb throttle plate is partially open.

In my experience, with the idle adjust screws completely unscrewed the engine should barely if not even idle(ie. the throttle plates are basically fully closed and there isn't enough air/fuel mixture to run the engine). It seems to me that that front carburetor is letting a ton of air (and fuel) into the engine which suggest that the throttle plate at the back of the front carb has a problem (bent, wrong part, installed incorrectly, who knows) or that something in the linkage is holding the throttle partially open (bent part, incorrect installation). I think I'd start by removing that front carb and checking the throttle plate to make sure it's installed correctly and closing fully. It's easy to take off, just remove the choke cable, fuel line, and 4 screws that hold it to the intake manifold. If that looks ok, I'd start looking at the throttle linkage. You should be able to remove the dashpot on the front carb and with a flashlight and maybe small mirror you could make sure that the throttle plate on the front carb is closing completely in the "no throttle" position.

I just modified my original post slightly but I'll mention it again hear. It looks like someone has modified the throttle linkage, probably to make the round top carburetors work when they switched from the flat top carbs to the round top carbs. That last piece of throttle linkage (with the single plastic connector on the end) should actually have plastic connectors on both ends. In addtion, on the top end (where there is no plastic ball), there should be an additional adjustment screw (I think they call it a fast idle screw) that appears to be missing. I don't use it (unless I'm tuning and want to keep the car at high idle) and normally the fast idle is completely out (unscrewed) so you shouldn't have a problem running without it. I have attached a picture of that throttle piece from my '71 240Z (stock setup) so you can see what the original round top linkage looks like. By disconnecting that throttle linkage you should be able to get the carbs idle set properly (using the idle screws and a flowmeter). Then you can go back and try to figure out how to fix that modified linkage. I'm sure there are lots of people on this site with '73 and '74 Z's running round tops (it's a very common thing to see) that should be able to help you with the proper linkage setup.

I worked on a friends 240Z a few years ago after he bought it and it had a similar problem. What we discovered was that even with the idle screws completely unscrewed the idle was still too high (between 1000-1500 RPM). The throttle plates were not being allowed to fully return to the "idle" position (ie. nearly closed) because the throttle linkage from the gas pedal was keeping them "opened" even when the gas pedal was not being pressed. He had a '73 240Z with round top carbs. The 1973 240Z's and the 1974 260Z's came with "flat top" carburetors and not the "round top" carburetors so someone has replaced the original "flat tops" with "round tops" from a 1970-72 240Z, similar to yours (ie. a '74 260Z with '70-72 carbs). From what I've seen, the flat top carburetors have a thinner insulating spacer (the reddish brown plastic looking thing) between the carburetors and the intake manifold) than the round top carburetors and this was affecting the throttle linkage and keeping the throttle plates partially open even when the idle screws were completely out. I would start by disconnecting the main throttle linkage rod that connects the main throttle bar to the carbs (the little 4-5" or so rod with the 2 plastic end pieces). That should disconnect the carbs from the gas pedal and allow the throttle plates to fully close. You should now be able to use the idle screws to adjust the idle speed of the carbs. Once you get that set, you can start to figure out where the problem is in the throttle linkage that is keeping the throttle from returning back to "fully" off. I'm guessing (because I've never worked with the flat top carbs before as I have a 240Z with round top carbs) that the throttle linkage from the flat top carb setup is slightly different in some way from the throttle linkage from the round tops and I'm guessing that you may have the the throttle linkage setup from the flat top carbs. Let me know if this helps. I just took a second look at your video and it looks like someone has modified the little throttle rod (with the plastic ends) and yours only has one plastic end. That's the part you want to remove to get the carbs disconnected from the throttle linkage. On the round top carb setups there is a main (or fast idle) screw above that linkage and yours appears to be missing that entirely.

The problem I found with the modern replacement pumps was that the valves (inlet valve pulling fuel from the tank and the outlet pushing fuel to the fuel rail/carbs) would not seal properly. Because of that, it couldn't create a strong vacuum to pull the fuel from the tank. Typically they would work for a few days before failing. Fortunately, the size and hole pattern on the aftermarket one I was trying matched the size and hole pattern of the OE Nikki pump that failed for me (the old Nikki pump rubber diaphram became old and brittle and cracked) but the valves were still good and working. I took the bottom half (containing the rubber diaphram) from the aftermarket pump and the upper half of my OE Nikki and mated them together and it seems to work well.

I tried a similar test myself using a vacuum pump when the new GMB pump was failing. I hooked up a vacuum pump to the GMB inlet side (side coming from fuel filter and tank) and when I applied a vacuum, it would not hold. I took the GMB pump back to the store where I purchased it and showed the test to the guy at the counter. He exchanged the pump for me. Much like you, I went through 2 GMB pumps and still have the 3rd (brand new) sitting in my garage as a spare. I tried the Nikki/GMB clone idea before installing the 3rd GMB pump and so far it has worked so I haven't had to try my spare GMB unit.

When I got my 240Z running a few years ago (after sitting for 28 years), I replaced the original Nikki pump (mechanical) with a new GMB pump as the rubber diaphram inside the Nikki pump was dried and cracked. The new GMB pump worked initially, but soon (just a few rides) it stopped pumping fuel and I saw the same issue you did, after turning the engine off, the fuel in the fuel filter would slowly drop. I purchased a new GMB and the same thing happened, it worked for a few rides, then quit holding a vacuum (ie. sucking in fuel). I figured the one way valves on the new GMB pumps were failing so I took the bottom half of the new GMB pump (side with the rubber diaphram) and the top of my old original Nikki pump (side with the two one way valves) and mated them together. It has so far been working well.

I replaced my old, original Nikki pump a few years ago when I got my 240Z out of storage. I replaced it with a GMB. It worked fine initially, but after a few months (probably no more than 50 miles of driving), it stopped working. I got another GMB, and it too worked for a few days and then quit. I used a vacuum pump to determine that the problem was the "one way valves" that allow fuel to enter the pump, and exit the pump, were failing to keep a good seal. The rubber diaphram (driven by the lobe on the cam) that is used to "pull fuel in" and "push fuel out" was just fine. I took the good GMB bottom (with rubber diaphram) and used my old, original Nikki top (with the original two valves) and mated them together to create one good pump. It's been working for over a year, granted I don't drive the car much, but that is longer than either of the GMB pumps. Needless to say, I am not a fan of the GMB pumps.

I have a 4/71 Z as well and mine has the ash tray and lighter in the center console.

That model number on ZCarDepot is different than the aftermarket one I tried using but it looks very similar, ie. has the OE look. The specific model that I've had problems with is a GMB brand, and the number on the box is 550 8040, I still have the box with the top half of the pump sitting in it. I'd definitely spend a little time checking the fuel pump as your problem sounds eerily similar to mine. If the fuel filter is going dry, it's got to be either no fuel coming from tank (ie. no fuel, blocked line, etc.) or fuel pump not drawing fuel.

When I got my Z running a few years ago (after sitting for 28 years), I replaced the fuel pump before I ever tried to start it (original Nikki pump). After removing it, I opened it up to check it's condition and sure enough the rubber diaphram was cracked and broken. I replaced it with an aftermarket pump that had an "original" look. It worked, but within days (I don't drive the car much) I started have issues where it would die and then not start. I noticed that the fuel filter would always go dry when the car died or the engine was off. I used a pressure gauge to check the fuel pump and quickly discovered that the fuel pump valve (inlet side that draws fuel up from the gas tank through the fuel filter) wouldn't hold a vacuum. So I tested the valve on my old Nikki pump that I just replaced and sure enough it would hold a vacuum. I went and purchased another aftermarket pump (same exact model) and it did the exact same thing, worked briefly and then the inlet valve started leaking (would not hold a seal) so I took it back for another replacement. About this time I starting thinking that the new aftermarket pumps were of poor quality so I decided to try something. I mounted the top of my old Nikki pump (which contains the 2 one way valves) to the bottom half (containing the rubber diaphram and lever arm) of one of the new aftermarket replacement pumps and lo and behold, it worked. I have been running this setup (old original top half of Nikki pump with bottom half of aftermarket pump) for over a year with no problems (granted I don't drive it much). Try using a vacuum pump on the inlet side of the new fuel pump, you should be able to create a small vacuum that holds. Do you have any pictures or a make and model of the aftermarket pump you used? I'm really curious if it's the same one I've had problems with.

From reading your last post, I get the sense that you may have replaced the fuel pump recently as you mentioned "other than when the old fuel pump that was on it stopped working." In your opening post you say that the float bowls keep going dry which would suggest that either the fuel pump is not working properly or the float valves are getting stuck. Based on your comment that the fuel filter was also dry, that would suggest that the fuel pump may not be working. What is your fuel pump setup, original mechanical style or electric? If mechanical, and it was replaced recently, what did you replace it with. I had a very similar problem recently when I got my old garage stored '71 Z back up and running.

When I got my clutch working (after sitting for 28 years), I had a lot of travel in the pedal as well before it starting to move the slave cylinder push rod. It turned out that the clevis pin that connects the clutch pedal to the master clutch cylinder was all worn out. Instead of being a nice round hole it was worn down to a long oblong/oval hole. In addition, the clevis pin that fit into that hole was worn completely down to almost nothing. I removed the clutch pedal and took it to a shop to have the ovalled out hole welded up and re-drilled. Then I replaced the clevis pin with a new one. Mine was worn so badly that I was probably losing half the pedal travel before it actually starting to move the master cylinder.