Captain Obvious

Hahaha!!! Well, I couldn't keep a straight face!!

Here's a couple pics of stuff I have here. Idle screw not touching the linkage: This carb closes down pretty far, but I've seen them close down even further. So I took a look at the linkage over by the stop screw, and it looks like on this carb, the tang on the linkage may be contacting the carb body a little bit even when the idle stop screw is backed out beyond contact. Take a look at yours and see if maybe you've got something like that going on with yours. Might just have to bend the tank down a tiny bit to provide a little more clearance?

Well I have no idea if the M66 is the correct needle for your application, but I did a few minutes of web searching and came up with a couple hits: https://ratsun.net/topic/57757-needle-modification-for-l20b-sus/ http://www.team.net/html/datsun-roadsters/2000-07/msg00225.html https://ratsun.net/topic/52225-hitachi-side-draft-su-needles/ There was some mention that the M66 run rich at idle, but then go lean at higher throttle positions. So yours sound like they are following that pattern... At least the idle part.

OK, so 0.090 nozzles. Do you know what needles are in there (and what needles are supposed to be in there)? And even if they are appropriate needles and nozzles, it could still be a problem there. They might have worn from rubbing due to incorrect alignment. Or corrosion over the years eating away at the brass. Those kinds of issues would be most pronounced at idle since the gaps are (supposed to be) so small.

Kinda sorta, but in theory, only on transition. Running steady state, the oil viscosity should not have any effect at all. The impact of the oil would be most pronounced on increased throttle position, like when you punch it.

You should be able to check the float needle valves by looking at the bowl level. Really easy if you have the drain plugs on the bowls. Not as easy, but possible without those drain plugs. That said, I suspect the problem is elsewhere. So, a couple questions to start: Are those carbs supposed to use a 0.100 nozzle, or a 0.090 nozzle? And have you verified that you have appropriate needles installed? Maybe if you ran the two cars into each other real hard, things would even out? 🤐

The little jiggly bits on the bottom of the damper stalk form a check valve which is dipped into the carb damper oil. That check valve is provides significant hydraulic restriction when the suction piston is rising (when you are opening the throttle), but it provides way less restriction (because it's a check valve) when the suction piston is lowering, such as when you are decelerating. Hence, the oil really only has an impact when OPENING the the throttle, not closing it. Yes, it will have SOME impact in both directions, but the raising direction is way more dramatic than the lowering direction.

If you have the idle stop screws turned out far enough that they aren't making contact at all, then I would expect the butterflies to close down further than what I see in your pics. It takes a surprisingly small amount of gap to allow the engine to idle. Are you sure the stop screws aren't holding those carbs open at all? Are you sure there aren't carbon deposits on the butterfly edges or carb body throat preventing those from closing further? I'll take a pic here of what I would expect on a well aligned butterfly when fully closed. Won't be able to do that tonight, but should be able to get to it tomorrow.

Do you have one of those cheapie smoke machines? Use one of those to look for leaks?

Heavier oil wouldn't do anything about that. The oil really only has an impact when OPENING the the throttle, not closing it. But curiously, usually when you close the throttle abruptly, they go very rich, not very lean. Are you really sure you're really lean? I'm thinking that maybe your A/F meter is misleading you... It might be misfiring because it's so rich it won't ignite (and your A/F reads lean because there is unused oxygen in the exhaust stream?). In any event, that's less of an issue than running lean under steady state conditions. Whether it goes rich or lean on deceleration is a transient condition that isn't as important as what it does when you're driving steady.

Ummmm..... Yeah. You have a car hobby, so good luck with that. Hahaha!!!

Doubtful. I've seen it sometimes get the butterflies to hang open when the throttle is closed slowly, but if you blip the throttle, they would settle to where they belong. I would really doubt the vacuum would win a battle against the return springs and actually pull the butterflies open.

First question... Are you sure the linkage isn't hanging up somewhere? If you disconnect the throttle linkage rod from the balance tube, does the problem still persist? I assume it does, but I have to ask. I'm also concerned that you tightened up the carb mounting bolts and got multiple hundreds of RPM change. That should not happen. So maybe your gaskets aren't sealing well for some reason? Next, with the butterflies completely closed, the engine should not run. Regardless of whether there is a vacuum leak or not. If you're running at 1500 RPM, then you're pulling enough air through the carbs to pick up enough fuel to support that. In other words.... Your butterflies are not sealed as well as you think they are. Those butterflies should be able to close "pretty much completely" when the idle screws are backed off enough not to contact the linkage. If they don't close to pretty much seal off the carb throat, then I suspect they butterfly plates are not installed correctly.

Well, he was supposedly possessed at the time. 😀 Haha!!

I like it!!!* * (Spoken like Chevy Chase)

Understood. Looks great and I'm sure the next pieces will be even better than the first.

I think that looks fantastic. As for making a bunch of changes to your fixture... How many times are you ever going to use it again? Just once for your other floor pan on the car? Only you can judge your level of perfectionist (and my level is pretty high) and unless it looks way worse in person than in the pics, I'd say ship it! 😀

Durn. I should have warned you about that. At just 1/4 inch thick and the wide area you're trying to form, I was worried that top plate would be too bendy. So... you don't have to go through the effort of welding the support to the top plate, you just have to use something thick enough that it won't bend (much). For example, a 1" square bar that runs the length of the oval? Or now that the center has bent, you could use two pads of thicker material and an "unbending" bar or rod to span the gap between the two to bend the two ends simultaneously? Not sure if I'm doing a good job describing this with just words. Let me know and I'll try to whip up some sketches.

Hahaha!! You just posted the wrong video!!

Yes, it matters, and sorry to contradict @SteveJ , but the correct location would be the location you have circled in yellow. The location circled in yellow is "ported vacuum" which is what the distributor wants. The other port (red) is direct manifold vacuum at all times, which is not what you want.

You think the brake issues are related to the balance tube? What are you doing instead for brake vacuum if you aren't using this balance tube?

@Mike This bot is still at it. I hit "report" on this thread and the previous one as well.

Have you put a vacuum gauge on the balance tube to see how the vacuum looks? I'd try it at both your more centralized PCV connection, and also at your brake booster connection on the end. Just to see if you can glean any useful info from the gauge?

Or maybe.....

Oh, and what caught my eye first was the mention that it was "For a ’72 240Z". Like the year matters for what film would work. So by saying it out loud, I guess I'm risking making the bots better, but that's what really started me wondering.