Interesting Weber jetting

[Eljaro]

I had Cobrafan1 asking a few questions about my latest Weber setup and I want to share these with all of you.
One emulsion tube I had been using was a modified F7 tube, with a self made sleeeve covering the lower holes and making the middle ones aireate the fuel further up, since the air bubbles would go up between the emulsion tube and the sleeve and come out at the top of the sleeve (tested this in a glass of water blowing air into the top) I also drilled 8 new holes at the very top and 4 holes below in order to lean out the lower midrange.



Since I have always been testing other emulsion tubes to find the absolute best here is the one I was using when the crankshaft broke. It is a F11 tube with two extra holes at the top. Worked well also, but I was not able to go back to the modified F7 E-tube to make comparisons. The crank is broken for now.



And I can warn that if you have well set up Webers, a light flywheel and a not so good crankshaft, the Webers will twist that crank to death. Happened to me. Stay away from eagle cast.

[Eljaro]

Latest tuning done to my webers.
After installing a wider hood scoop to make more space for the air horns, I did not make a new wider scoop cover, so had again the hesitation at mid throttle. Installed a cover and the hesitation is gone.
I was shooting out black smoke on acceleration, so installed a .35 bypass and the black smoke is much reduced, so will need to go to a .50 bypass.
A slight bog came up when using the bypass, so installed a larger idle jet, .65 before, .75 now, and the bog is gone. Idle air is 120, idle screw 1 turn out.
Installing the bypass has another positive side to it.
Apparently the fuel trapped in the pump circuit drains now into the fuel bowl and so does not drip into the throats when I stop the engine and the carbs soak up heat.
No more fuel dripping since the bypass installation.
Engine runs great, no weak spot anywhere.

[carmine]

Hello Eljaro

Good to see you running again

[Eljaro]

Carmine,
followed your tip on the idle jet when going with a pump bypass and it works. Will have see if the .50 bypass will do without a bog or if it needs a larger idle jet. Larger idle jet makes low speed action smoother and A/F meter does not show richer than before, rather leaner.

[carmine]

Good to here it worked for you so far

The extra idle fuel and idle air will also make your engine run cooler temps

Best regards
Carmine

[dallas_]

Originally Posted by Eljaro

I was shooting out black smoke on acceleration, so installed a .35 bypass

No more fuel dripping since the bypass installation.

Glad to hear you are back in action. I'm not familiar with the bypass, can you elaborate on that?

Thanks,
John

[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.