SU Bridge Evolution: Round Top vs. Flat Top (Comments welcome)

[240260280z]

Interesting read on popular hotrodding.com:

source:http://www.popularhotrodding.com/tech/0611phr_the_secret_life_of_carburetors/index.html

The Accelerator Pump

Under idle and cruise, a considerable amount of vacuum exists in the intake. This vacuum reduces the boiling point of the fuel, causing it to vaporize much easier under the prevailing high vacuum conditions than under low vacuum. This useful characteristic helps fuel distribution considerably during idle and cruise. When running down the freeway at 2,000 to 3,000 rpm with 15 inches of vacuum, a lot of fuel being drawn into the engine is vaporized well before it reaches the cylinders. Standing on the gas pedal completely changes the situation. When the vacuum transitions rapidly from a high value to near zero, fuel held in vapor form now condenses into liquid onto the manifold walls. Although a fresh charge of air is entering the engine and carrying its associated fuel, the engine, for a moment, still goes very lean. This is due to the fuel that was contained in the air suddenly clinging to the manifold walls, and for a moment at least, going nowhere. This causes an enormous flat spot that the engine simply will not drive through. To offset fuel condensing on the walls, an accelerator pump system is added. This squirts additional fuel into the intake to cover the would-be hole. A basic schematic of a typical pump system is shown in Fig 11 (see p. 91). In this example, a piston is shown injecting the fuel, but most often, the function of the piston is carried out by a spring-loaded diaphragm such as in a typical Holley carburetor. Calibration of the accelerator pump system is carried out by jets to control the rate at which it goes in; various springs, cams, and diaphragm sizes are used to control the amount that is injected and the duration of the injection phase.

[240260280z]

Here is a shot of the roof with your jet depth tool:

It looks like a screw stop for the choke valve in centre.

I also see two drilled passages (with light from above) on each side of the walls that seem to go up to the piston area .... but what is the hole in the front upper left? Is that associated with the vacuum break?

Edited September 7, 2011 by Blue

[Captain Obvious]

Yes, I found that post over on zcar.com, and I disagree with his analysis. He got the first part right about how the vacuum actuation works, but I contest the rest of it. I will point you back again to that interesting read on popular hotrodding.com:

Mixture Requirements

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When the vehicle is cruising, the mixture needs to lean out considerably if good mileage is to be achieved. With most carbs, we are likely to be dealing with a power-enrichment circuit, activated by a vacuum-sensitive power valve. This usually takes the form of a vacuum diaphragm, which senses how much intake manifold vacuum is present. Opening the throttle causes the intake manifold vacuum to decrease to near zero. This allows the power valve to open what can best be described as an additional main jet that supplies the extra enriching fuel. This additional main jet in any Holley-style carb is commonly known as the power valve restriction channel, or PVRC for short.

This description above is exactly what my analysis led me to understand as to the operation of the flat top power valve.

If you take a look at the pictures from that article of a typical "accelerator pump", and a typical "power valve", the biggest stand-out difference to me is the check valves in the accelerator pump. The power valve has no check valves in it which means it cannot "pump". There is nothing to prevent it from simply pushing a diaphragm load of fuel back down into the float bowl.

Here's an accelerator pump:

And here's what the power valve looks like (functionally):

The fuel is pulled through by vacuum, not pushed through by the diaphragm.

I'll take some detailed pics of the device when I get the chance and I'll try to show you why I don't think it has any transient function, but I didn't get to that today.

[240260280z]

Here is a flat-top/round-top-ish manual for a Hitachi carb HJL38W:

http://www.ratdat.com/images/posts03/HJL38W.pdf

[Captain Obvious]

I also see two drilled passages (with light from above) on each side of the walls that seem to go up to the piston area .... but what is the hole in the front upper left? Is that associated with the vacuum break?

Yes, the two holes in the upper left and upper right are the high pressure side of the suction piston.

The two smaller holes closer to the square mouth (one in the center and another on the upper left) are mounting screws for the vacuum break actuator. I took the actuators off and just reinstalled the screws into the holes so I wouldn't lose them. One of them was a little longer than the other and sticks down into the carb throat. It does interfere with the choke plate, but it's not supposed to. With the vacuum break device installed, those screws aren't long enough to stick down that far.

[240260280z]

Yes, I found that post over on zcar.com, and I disagree with his analysis. He got the first part right about how the vacuum actuation works, but I contest the rest of it.

snip

The power valve has no check valves in it which means it cannot "pump".

But what if Nissan engineers designed a "leaky" check valve using pressure differentials and orifices to get the job done...we need to go exploring

[Captain Obvious]

But what if Nissan engineers designed a "leaky" valve

Haha! From all of them I've dissected... They did!

Seriously though, the sealing between the inlet and outlet on that power valve is this goofy flat rubber washer and every one I've opened up, that washer is either brittle hard and dried out or gooey and partially dissolved. I don't know what those washers are made out of, but I suspect it's a poor choice of material for today's fuels with ethanol included. Datsun even mentioned the possibility of leaky power valves in their documentation a couple of times.

That document on the HJL38W was interesting. Very much halfway between the round tops and the flat tops in operation. One of the most interesting things is the idle air adjustment control just like on the HMB46W. They don't reference it anywhere in the text that I could find, but on the parts breakdown, it's #12. They meter and split the air at the rear carb and pipe to the front carb (which is the opposite to what they did on the 260Z), but the concept is identical.

[240260280z]

Another fun day Bruce!

FYI: We took apart a power valve and looked it over closely (Thanks SteveJ that carb is finally under the knife). The power valve it is just a valve that opens off throttle when the manifold vacuum drops. It allows additional fuel from the bowl to be drawn through a small jet by vacuum. The nozzle is low just after the bridge so it will most likely spray fuel when the throttle opens quickly just before the piston starts to rise and possibly after the piston rises (IF there is sufficient vaccumto draw the fuel).We will have to measure when the weather is good.

We also noticed a circular step in the back of the throat of the flat top (but not in the round). It looks like a weber choke is in there:

Edited December 4, 2011 by Blue

[SteveJ]

I'm glad I could contribute to the cause. One man's trash is another man's treasure.

[Captain Obvious]

Steve, It's definitely treasure. Thanks for helping the cause.

I haven't measured needle profiles yet, but I will post results as soon as I get the chance.