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Discussion Starter #1
Curiosity question:
I had Z Therapy delete my SU's heater hoses (or more properly, I asked for a set without the heater hoses), since I'm in the southwest desert. Plus they're ugly too.
I never understood how warming the carb body could improve their performance. I've always been told air/fuel, plus timing, plus spark gets you power. Cold air is more dense and improves performance, right?

What did those Japanese engineers know that I don't - except maybe everything.
 

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Driveability while cold, quicker overall engine warmup, cold coolant flow bypass to reduce water pump cavitation while the thermostat is closed, reduced exhaust emissions while cold. And Nissan put a thermostatic valve in the system so that when the engine is warmed up there is no more hot coolant flow around the carbs.

BTW... The Americans, Germans, Japanese, Italians, French, British, East German, Russian, Yugoslavian, Czech, and Indian car manufacturers all use similar systems on their cars for 60+ years.
 

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Driveability while cold, quicker overall engine warmup, cold coolant flow bypass to reduce water pump cavitation while the thermostat is closed, reduced exhaust emissions while cold. And Nissan put a thermostatic valve in the system so that when the engine is warmed up there is no more hot coolant flow around the carbs.

BTW... The Americans, Germans, Japanese, Italians, French, British, East German, Russian, Yugoslavian, Czech, and Indian car manufacturers all use similar systems on their cars for 60+ years.
Okay John, thanks. Those are all great benefits, although I was really angling for a technical explanation of what is happening at the carb due to the warming effect of the water. For example, fuel atomizes better when the venturi is warmer; or air flows better thru warm aluminum, etc.

Again, it's just something I don't understand.

Am I the only one?
 

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Have the throttle plate ice up on you so it won't return to closed JUST ONCE after holding it at some medium opening to it has sufficient JT Effect to condense incoming air humidity and freeze it at the same time.... Desert Southwest? It can happen there.

Remember this: They designed a car to work anywhere in the world. They wanted their customers to get in, turn the key, and drive across the country in whatever weather existed.

Modify your car to work where you are. Just don't expect it to run in Flagstaff in February... or anywhere north of the 40th parallel in that same time frame.
 

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Discussion Starter #6
Chuckle.
It sounds like this was a icing prevention trick, more than an engine warmup or performance enhancement, which is what I thought.

John said above that there is a thermostatic valve that controls the coolent flow to the carbs, so at cruise there would be no flow, right. Isn't that essentially the same setup that I have? (no hoses).

I wonder if there is air warming from engine heat and headers, before entering the carb throat? I have only air horns.
 

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My 71 had a warm air tube coming from the exhaust manifold to the oem airbox and a manual switch cold/hot in the front to limit non warmed air.
 

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so at cruise there would be no flow, right. Isn't that essentially the same setup that I have? (no hoses).
Not the same thing. Having no hoses to the carbs is different.

When you have no hoses, you have no flow. With the thermostat, there would be the amount of flow required to keep the thermostat at it's control temperature. Even at cruise.

Just like the thermostat between the engine and the radiator.

Of course, every set of hoses I've messed with were plugged completely shut with crud and rust, so in reality, it might the same at this point!
 

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In the later model cars (73-78 240/260's) there were TWO CIRCUITS.
The throttle plate circuit ALWAYS has water through it to prevent icing (same as on the EFI cars, for the same reason, and to keep the PCV goo warm so it doesn't hold the plate open...)
The other water circuit, which heats the manifold for drivability, shuts off once the engine water temperature reaches 177F.
 

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Actually there were THREE distinct parallel circuits, not just two.

One through the carb bodies. Completely encircled the fuel nozzle.
One through the intake manifolds. Same place as earlier years.
One through the balance tube by the EGR valve.

I know the one through the carbs was supposed to have a thermostat but I don't remember about the other two. There was only one thermostat, but I don't remember how everything T'd back together at the rear of the block.

Love that bundle of snakes on the later carbed cars.
 

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Discussion Starter #12
Actually there were THREE distinct parallel circuits, not just two.

One through the carb bodies. Completely encircled the fuel nozzle.
One through the intake manifolds. Same place as earlier years.
One through the balance tube by the EGR valve.

I know the one through the carbs was supposed to have a thermostat but I don't remember about the other two. There was only one thermostat, but I don't remember how everything T'd back together at the rear of the block.

Love that bundle of snakes on the later carbed cars.
Thought this was dead, but since you got me going again...

Tony
1. "which heats the manifold for drivability"
What does this mean, I mean technically what difference does a warm manifold make to fuel/air mixture?

Mr. C
2. "One through the carb bodies. Completely encircled the fuel nozzle"
So again, it sounds like this is designed to improve atomization.
Are we saying a liquid atomizes differently at 50 degrees than at 80 degrees?

I need a physics lesson, and some aspirin.
 

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Liquid forms on the cooler surface (condenses) like you see on the outside of a Coke can. That happens on the inside of the intake manifold where fuel drops out of suspension in the air fuel mixture cause intermittent lean/rich conditions. This is a problem as the engine warms up. Once fully warmed up then its a non issue.
 

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Sorry. Didn't mean to muddy the waters. I was referring only to the 73 and later cars which had a more complicated coolant scheme. You don't have these. The chronology went like this:

70 and 71 had one coolant line that ran through the manifolds. Simple.

72 (stock) had one line that went into the front manifold, ran out of the manifold into the carb body, encircled the fuel nozzle, and then went back into the manifold. It did that on both front and rear manifolds and carbs and then exited the rear manifold and went back to the water pump inlet. You might still have the remnants of some of this on your 72, but if you got SUs from ZT, then you might not have gotten 72's. If you've got your original 72 intake manifolds, then you should have two coolant holes in the face where the carbs mount.

73 and later had the three distinct independent paths I mentioned above.

As for the reasons why? I'm no expert, but I would surmise that because pretty much everything on a car works better warm. Warm fuel would be easier to atomize than cold fuel. A warm nozzle would also be less likely to form a cold spot that would freeze the moisture out of the incoming air. A warm nozzle is at a predictable temp instead of "whatever the temperature is today". They were getting pressures for lower emissions and the fewer variables, the easier that would be.
 

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Are we saying a liquid atomizes differently at 50 degrees than at 80 degrees?
Forgot about this part. Not my area of expertise, but I would expect that the closer you are to a liquids vapor pressure (which is temperature dependent), the easier it is to get it into solution. The "atomization" that occurs at the nozzle probably results in three things. In decreasing physical size, those would be:

Some gasoline mixture suspension in air
Some gasoline colloidal suspension in air
Some gasoline solution in air.

The solution and colloidal suspension are less likely to drop out of the air stream, but the "mixture" suspension is much more likely to revert back to droplets. Cold walls (as John mentioned above) just make everything worse. I would expect that the warmer things are, the more solution and colloid you would have and the less mixture.

That's my non-physicists take on it. Maybe we've got some physicists in the house that can shed some light?
 

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Mr. Completely has the right mixture. Vaporization is T dependent.

The vaporization through the carb occurs more easily because of the reduced pressure as the air is drawn in under vacuum and further reduced as it speeds up to flow through the venturi, and then the air going into the manifold is cooler and returns to the initial vacuum pressure. The air then being cooler and at "higher" vacuum pressure tends to "unevaporate" some of the fuel, which isn't necessarily 100% evaporated through the carb. Thus, the heater when the engine is cold or in cold climates with low incoming air temp affecting vaporization.
 

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73 and 74 carbs DO NOT 'encircle the nozzle' with the water. It's at the back of the carb at the throttle plate, not a bit further. Both the Throttle Plate and the EGR (I don't count EGR water circuit as it's not for the carbs... it's on the balance tube and is just to keep it from going to 800 degrees... not exactly the same thing as the rest of the circuits are doing.)

If you heated the ring at the nozzle, what Dumbstone said would indeed occur: An artificially heated vapor going into a colder area due to JT Effect would condense out of suspension on just about anything it touches... The higher vacuum in the manifold promoted vaporisation, same as when you stick that frog in the bell jar and he boils at ambient temperature. Don't forget the thermostatic damper was sucking 140F+ air into the intakes once fired off. THAT helped vaporise the cold fuel, and that heat kept things happy in colder weather... or hot weather. My 260 is cold-blooded without those little heaters!

The Manifold "Trees" part of the circuit, closed off as John C said.
 

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73 and 74 carbs DO NOT 'encircle the nozzle' with the water. It's at the back of the carb at the throttle plate, not a bit further.

If you heated the ring at the nozzle, what Dumbstone said would indeed occur:
Passageways cast into the carb bodies from the mounting face. One of the rubber plugs removed:


Socked headed set screw plug lurking in the bowl. Cross drilled during mfg and then sealed with this plug:


End result is that the coolant completely encircles the fuel nozzle:
 

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I'm missing something here....

The photo says "one rubber plug removed"...

Mine had both plugs intact. I have been buying up these carbs for years (73 & 74) and haven't yet gotten one without the plugs.

I don't see anything in the "grey book" for remedies on drivability and vapor lock anything about removing those rubber plugs.

If a passage exists, but is plugged...

Is it actually doing anything?

I milled the nozzle out of a set of 73 carbs, I don't remember any dirty mazak being uncovered, I can't remember if I pulled the plugs or not when I welded in the block... Likely I did. Unfortunately the carbs are 7,500 miles away now so I can't just go out and look at them.
 

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Mine has both plugs intact too. I just popped one out of it's hole so you could see down into the cavity where the casting cavity goes. The plug isn't "missing", it's just "removed". The rubber plug that I popped out is sitting right there next to the hole on the carb mounting surface.

You don't do anything with the rubber plugs. You normally just leave them be. I just took it out for that pic.

The rubber plug doesn't go in deep enough to interfere with the water flow. The water flow comes in and out of the two tubes that screw into the holes in the underside of the carb (In those pics, I have paper towels wadded and stuffed into the water line fitting holes to keep crud out). The rubber plugs don't go in that deep.

The plugs don't really do anything... Looks to me like they were trying to cover their bases on how the water was intended to enter and leave the carbs. With those holes in the mounting face they could have sent water into the carb bodies from the intake manifold (like on the 72), but instead they drilled and tapped the two holes on the underside and screwed in the hose fittings.

Those rubber plugs are titts on a boar. Since the gasket between the carbs and the spacer plate covers those holes anyway, you don't even really need them.
 
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