Carburetor Conundrum (260Z)

[DadAndLadZ]

53 minutes ago, 240Znomad said:

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 had both dashpots off, both the throttle plates appear to function identically except when the engine is turned on. I don’t know how to check that with engine running tho. I guess it’s time to pull it and see if anything reveals itself from the backside. 

[DadAndLadZ]

1 hour ago, SteveJ said:

What type oil did you use in the dashpots? Is the oil staying in the dashpots?

Just using some motor oil, I think it’s like 10W40. As far as I can tell it’s staying in there, I haven’t seen it leaking anywhere. 

[Captain Obvious]

Something else to try... Start the engine, and while it's high idling, reach under the actuation tab on the front carb and manually pull it up gently to see what happens? In other words, "help" the return spring by adding some more force in the direction that would close the front carb.

And just to be double dog sure, did you ever pop the plastic ball off the linkage to guarantee there isn't something in the linkage holding the front carb open?

If you pop that ball off and pulling up on the linkage for the front carb has no effect, then I agree that it's time to pull that front carb off. It really wouldn't take much of a throttle plate opening to get you to a 2500 RPM idle with no load.

[DadAndLadZ]

1 minute ago, Captain Obvious said:

Something else to try... Start the engine, and while it's high idling, reach under the actuation tab on the front carb and manually pull it up gently to see what happens? In other words, "help" the return spring by adding some more force in the direction that would close the front carb.

And just to be double dog sure, did you ever pop the plastic ball off the linkage to guarantee there isn't something in the linkage holding the front carb open?

If you pop that ball off and pulling up on the linkage for the front carb has no effect, then I agree that it's time to pull that front carb off. It really wouldn't take much of a throttle plate opening to get you to a 2500 RPM idle with no load.

Yeah right before I shot this video I put all my muscle on trying to force that tab while it was running but no dice. And it does this even with all the linkage completely unhooked from the carbs, there is nothing external that I can see putting any kind of load on there. Will just have to pop it off.

Thansk again to everyone for your patience and kindness with me.

[SteveJ]

1 minute ago, DadAndLadZ said:

Just using some motor oil, I think it’s like 10W40. As far as I can tell it’s staying in there, I haven’t seen it leaking anywhere. 

Most people use 20wt oil in the dampers. You should check the level in the carburetors. Also, with the car off, try to lift the piston in each carburetor. The damper oil should provide a fair amount of resistance. I'm wondering if the front carb has enough resistance.

Ace Hardware stores typically carry 3-in-One brand oil that is 20wt.

[DadAndLadZ]

Just now, SteveJ said:

Most people use 20wt oil in the dampers. You should check the level in the carburetors. Also, with the car off, try to lift the piston in each carburetor. The damper oil should provide a fair amount of resistance. I'm wondering if the front carb has enough resistance.

Ace Hardware stores typically carry 3-in-One brand oil that is 20wt.

They feel exactly the same when the car is off. They look the same, adjusted the same, filled with the same oil. But turn on the engine and the front piston opens three times wider than the back one. I intend to get some single weight oil too, but when it’s 100 degrees in the room I can’t imagine this stuff is too thick, and the back carb works fine. Freaking mystery 😕

[240Znomad]

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.

[240Znomad]

7 minutes ago, DadAndLadZ said:

They feel exactly the same when the car is off. They look the same, adjusted the same, filled with the same oil. But turn on the engine and the front piston opens three times wider than the back one. I intend to get some single weight oil too, but when it’s 100 degrees in the room I can’t imagine this stuff is too thick, and the back carb works fine. Freaking mystery 😕

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).

[Captain Obvious]

10 hours ago, DadAndLadZ said:

Yeah right before I shot this video I put all my muscle on trying to force that tab while it was running but no dice. And it does this even with all the linkage completely unhooked from the carbs, there is nothing external that I can see putting any kind of load on there.

Well then it's time to pull that front carb off.

And don't worry about how the front piston is going up farther than the rear. As 240Znomad suggested above, it's cause and effect... The effect is the piston raising. The CAUSE is something else. You fix the cause and the effect will go away.

And don't worry about the damper oil right now either. What you are using is completely fine for now. In fact, at steady state idle, you don't really need any oil at all. Just doesn't matter.

I'm thinking that something within the carb itself is keeping that front throttle butterfly from closing completely. Bent plate, plate installed backwards, plate misaligned badly, complicated choke linkage on side of carb not put together correctly... Something like that.

[Patcon]

If the front carb float bowl level was higher than the rear, wouldn't it run faster due to extra fuel?

[siteunseen]

I don't know if more fuel in the chambers would make one run any different? Too low, most definitely but mine are set higher and it runs good.

He can check that doing the ol runs dirty test Blue talks about. Unhook 1-3 then four through six and listen for any difference. It should run on each carb although crappy but just one carb.

 

[Patcon]

I was thinking not both set high but one set high to the other. Making the front one run rich