Interesting Weber jetting

[Eljaro]

I found this explanation in Understanding a Weber Side Draft Carburetor Through a Fictional Supposition.. Forum memeber BLUE explains it better than I could possibly do:

1. The Bleed-Back/Spill jet sits at the bottom of the fuel bowl.
2. When the throttle valve is closed (idle), fuel enters the bleed-back valve, flows past the ball bearing valve and fills the pump chamber as the rod is pushing the piston is up high in the pump chamber and the spring compressed. Weights atop the two other ball bearing valves keep them closed and prevents fuel from flowing out of the two jets.
3. When the throttle is opened quickly, the rod drops and the spring pushes the piston down. This forces fuel backwards. The ball bearing in the spill jet is forced upward and closes this valve preventing fuel from back flowing into the fuel bowl. The pressure also lifts the other two ball bearings and corresponding weights (opening these valves) and allows the squirt of fuel to shoot out each pump jet.
4. The diameter of the pump jet holes, the length of the piston's excursion, and the spring tension affect how much fuel is squirt and the time duration of the squirt.
5. When the throttle is opened slowly, the rod also drops but the fuel squirts slowly back into the fuel bowl as the ball bearing valve in the bleed-back/spill valve is designed to "bleed" on slow throttle transitions. In fact the progression circuit is designed to be the key supplier of fuel during slow throttle transitions...however some fuel will inevitably go through the pump jets. Size of the bleed back valves, accelerator ball bearing weights and piston spring pressure are the key factors in how "leaky" the pump jets are in slow to medium transitions and also can be customized for normal fast transition squirts.

My explanation for the carbs not dropping fuel into the throats when engines is turned off is:
When engine is turned off and heat up the carbs the fuel in the pump circuit expands slowly and drains into the bowl instead of dripping into the carb throats.

[Gaz64]

I'll add a little more.

At ANY throttle opening (steady throttle), the inlet valve is open. The ball to be has forced up on it's seat from fuel pressure within the pump cavity.

The compressed piston spring determines pump pressure. Pump rod length, pump jets and bypass orifice size determine duration of shot.

The inlet valve can have no bypass 00, or 0.35, 0.50, 0.60 etc.

The inlet valve can also have a steel or plastic inlet ball. The plastic ball seats faster because it is lighter.

All other circuits of the carb should be optimised on their own with no input from the pump circuit. Then and only then should the acceleration circuit be addressed.

As can be seen on Eljaro's car as example, the pump circuit can be backed off, there is seldom need for a closed bypass.

[dallas_]

Hmmm..... any chance you could post some photos?

We are talking about IDA's right?

Here is my pump rod and jet. I don't see an inlet valve.

[carmine]

Inlet valve or bypass valve located under float.
You have to remove float to get at it.