Late 260Z Smog Pump adjusting arm

[Captain Obvious]

I'm not thinking that shaking the car exacerbates the boiling. I'm theorizing that the boiling (bubbling, emulsifying, however you want to describe it) is occurring all the time, whether you are shaking the car or not. Theory is that those bubbles are passing into your carb bowl and positively pressurizing the air pocket above the fuel level. When the car isn't shaking around, that vapor passes harmlessly out the bowl vent, but when you shake the car and get the fuel in the bowl sloshing around, you can temporarily close off that bowl vent hole. And when that happens, the positive pressure sends a little glurp of fuel up and out of the vent.

In other words... The boiling isn't changing. The blockage is.

When the car is sitting still, the vent hole is not blocked. But when the fuel is sloshing around, you are intermittently blocking that hole with a wave of fuel.

I also suspect that if you're seeing fuel coming out of that vent hole, then I'd be pretty sure that you're getting fuel pulled out of that "axillary nozzle for starting" (your choke enrichment jet) as well. Might not be as visible as fuel coming out of the vent hole, but that nozzle pulls from the same source and it requires much less vertical lift than that vent hole. if it's coming out of one, I bet it's coming out of both.

As for you not hearing bubbles back at that tank, the theory is that the front carb swallows most of the bubbles, the rear carb gets most of the remainder, and tank return gets very little? Maybe that's why you don't hear bubbles back at the tank?

And as for the thermostat I was talking about. There's supposed to be a small thermostat in series with the carb coolant lines. It lives back by the bulkhead wall near the rear of the block. Where the coolant lines turn the corner towards the passenger side of the head. On page EC-2 of the 74 manual, they call it the Water Control Valve" and it's a temperature control valve. Most of them are clogged (or removed) by now. I'll see if I can dig up some more info about it.

[Captain Obvious]

Thought of another way to potentially test for pressurization of the float chamber. Maybe you could put a piece of tubing over that vent tube outlet and attach a deflated plastic bag to it? With the car hot enough to be potentially causing the problem, but sitting still, the bag should NOT inflate. But if the bag inflates, then it's an indication that you have gas (vapor, not gasoline) trying to escape from the float chamber.

Normally, you have fuel flowing into the carb, and fuel flowing out of the carb, and the whole thing should be in pretty much equilibrium. But what I'm thinking might be going on is that you are not in equilibrium. You might have excess vapor that is trying to escape.

Baggie over the vent tube outlet would probably be quicker and easier than clear fuel line?

Fire extinguisher handy at all times!!

[tunesnxs]

That sounds like an easy test. I could do that easy enough.

The water control valve is passing water last time I flushed out the lines (about two weeks ago) so I'm assuming it's at the very least, not clogged. Now whether it's working as advertised, well... that's another story entirely.

[Captain Obvious]

Note that I've not actually tried that "inflate the bag" test technique. I was just thinking that if the float chamber is getting supplied with an overabundance of vapor, then that vapor needs to go somewhere.

My first thought was trying to inflate a balloon, but I don't know how much pressure it takes to blow up a balloon. So I figured that a completely deflated plastic bag applied to the vent would require less pressure to inflate than a balloon. In fact, I would guess that a plastic baggie would take darn near zero pressure to inflate until it was plump. But by then, you'd already have your answer.

I'm interested in getting to the root of the problems you're having because the reports of heat related fuel problems with the flat top carbs run far and wide, but I've never been convinced that it's actually a carb problem. I've never personally had any heat related issues with the flat tops, and this is the first instance where "replace" is second choice behind "fix". Most people just give up and replace.

[tunesnxs]

Ok, after today's tests, which included re-routing  the fuel lines completely, and bypassing the fuel rail, I think I have hit on the cause. I was looking at the inside of the float chamber, studying the interior structure, holes, etc. I looked through the sight glass and noticed where the fuel level would normally be, and I saw that it sits just below the bulge of the water passage inside the chamber. I thought, what if the water going through that passage, which comes directly from the engine block, was heating that water passage "bulge" just enough to boil the fuel when it gets sloshed enough to touch that bulge? So, I took my trusty vice grips and clamped the water line.  Sure enough, after getting the engine back up to temp, I couldn't get the problem to occur again! 

Googling "boiling temp of gasoline" it seems that if the water in that carburetor passage, which comes directly from the engine, is slightly hotter than 180° that bulge in the float chamber will get hot enough to boil the fuel when the fuel touches it. That however leads me to wonder, how this design ever worked in the first place!

I'm now left with a couple of options. 

1. I currently have too much coolant in the radiator.  It should be 50/50 Coolant and water. Because of the high coolant/low water ratio, the temp runs slightly hotter than normal. Somewhere around 185-190°. Getting the coolant correct could help stabilize the temperature and help the carburetors run normally.

2. Install a lower temperature thermostat, which would lower the water temperature in the cooling system and again help stabilize the carburetor temperature, causing it to operate more normal.

3. Close off the water line that goes to the carburetors from the thermostat housing, thus eliminating the over temp in the float bowl of the carburetors.

Sorry I didn't get to the test you were suggesting with the bag. I had it all planned out too, but as I sat studying the body of the carb, it started to make a little sense to me. 

[zKars]

Good detective work.

I believe there is a valve in that hot water line that is supposed to stop the coolant flow after it reaches something near operating temp. Dollars to donuts its staying open the whole time.

[Zup]

I think you guys are on the right track and you are not alone.

In the Downloads section of this site under "S30 Manuals and Tuning Guides" there is this Technical Service Bulletin originally submitted by Jarvo2: 

Fuel-1973 240z 1974 260z fuel system modifications.pdf

This is a comprehensive document that deals with many of  the 73/74 flat top issues and gives a systematic, step by step approach to resolutions.

It is not "big" on the why of the issues, but is pretty good on how to go about the modifications Nissan felt were necessary---it may be of some help to you.

As the good Captain is aware, I too am smitten with a case of "boatanchoritis"---- currently hoarding 4 sets of said, as I attempt restoration to full functionality for my 73.

Give it a read if you are not familiar with it.

Edited May 11, 2016 by Zup

[Namerow]

That 'manifold thermostat' valve is easy to test.  Just pull it out of the coolant line, put a short length of hose on one end, then dunk it in a pot of boiling water for a minute or so.  Then pull it out of the water and see if you can suck air through the hose.  If you get no resistance, then the valve is stuck open.  An overnight soak in 'CLR' or some other calcium/lime remover might free it up. A new valve is probably going to be unobtanium.  If you can't free up your existing valve, don't just throw it out and re-connect the hose, as this will guarantee that hot coolant flows through the intake heating passages all the time.  Instead, you'd need to block the line (e.g. put a solid plug in the hose before re-connecting).  There's been some debate -- unresolved, IIRC -- about the wisdom of permanently blocking the line.

[Captain Obvious]

Interesting findings and I hope it's that simple, but call me skeptical. Wouldn't be the first time someone thought they had a problem licked only for it to return again in the future. 

To test your theory, get the car back to stock. Fuel rail back where it belongs, water passages as they were a couple days ago, etc. And then recreate your shake test. Shake the car as you did before at hot idle, and verify that fuel glurps out of the bowl vent tube like it did before. Has to be repeatable.

Then once you are positive that you have the car in the state where the fuel overflow will occur, shut the motor off, give it ten seconds or so for pressures in the fuel lines to stabilize and then repeat the shake test.

If the theory about flash boiling the fuel by contact with the coolant passage inside the carb is correct, you will get a glurp of fuel out the vent tube even with the engine not running. But if you do NOT get a glurp with the engine off, then you're chasing a red herring.

Reason being, if the overflow is caused by flash boiling fuel that is coming in contact with that internal heated passage, it shouldn't matter whether the car is running or not. Ten seconds after you shut off the engine, that passage is still at the same temperature, and the fuel level in the bowl is still at the same level. Results shouldn't matter whether the engine is running or not.

Does that make sense?

[Captain Obvious]

10 hours ago, Zup said:

"boatanchoritis"

I like it! DId you make that up?

"Anchoritis" kinda flows too.

[tunesnxs]

I will update on the valve cleanout and the "ten second shake test" tonight after I get off work. I'm feeling fairly confident about the whole thing now! ....which is probably not a good thing to even mention. As soon as I begin to feel confident, the car can sense it and whatever is next on the car's "I'll break next" list will go. ...and it never gets cheaper!

[Patcon]

tunesnxs

I assume you're in Orlando Florida, if that is the case I see no need for any carburetor heating at all, unless someone can point out something I'm missing...