Weber Jetting Survey

[jhirasak]

Hyde,

Thanks for sending your latest jetting spreadsheet to me. Just as a point of interest, I would like to mention the jetting and other changes that I had to make to a set of brand new 48 IDA Webers (out of the box from Redline) to get them to run smoothly on my car. They are as follows:

1. Adjust each float needle height to 25 mm by sanding the needle seat gasket (found a difference of up to 1 mm between the four carbs)
2. Adjust each float height setting after normalizing needle seat height
3. Adjust each carb air flow with butterflys closed so they indicate within 1 unit of each other reading on the air flow syncro meter (the general recommendation is to twist the throttle shaft to get the two throats to read the same value. I chose to drill small holes in the butterflys to get the same reading on all of the carbs)
4. Decrease Idle Jet holder from 120 to 100 to cure off idle transition hesitation
5. Adjust idle mixture screws to approximately 1 1/8 turns from closed (different taper on idle mixture screw than earlier Webers? Italian Webers might prefer 3/4 turns vs Spanish Webers like mine which might prefer 1 1/4 turns)
6. Change Emulsion Tube from F7 to F11 to correct for main jet transition richness (found problem by use of wideband O2 meter)
7. Change throttle linkage from as-provided single cross link to a bell crank arrangement to correct for left bank to right bank throttle position error (with single cross link arrangement, if both bank flow the same amount at idle, they would flow significantly different at 1800 to 2000 rpms, linkage geometry problem)
8. Change accelerator pump exhaust valve from 00 (blank) to 50 to correct for fuel dripping into the cylinders after hot shutdown (problem made worse by recent addition of alcohol in pump gas)

For anyone planning to install Webers on their car, I would highly recommend that they perform the first three steps listed above [u]before[u] they ever install them on the car for the first time.

John

[Gaz64]

Originally Posted by jhirasak

Hyde,

Thanks for sending your latest jetting spreadsheet to me. Just as a point of interest, I would like to mention the jetting and other changes that I had to make to a set of brand new 48 IDA Webers (out of the box from Redline) to get them to run smoothly on my car. They are as follows:

1. Adjust each float needle height to 25 mm by sanding the needle seat gasket (found a difference of up to 1 mm between the four carbs)
2. Adjust each float height setting after normalizing needle seat height
3. Adjust each carb air flow with butterflys closed so they indicate within 1 unit of each other reading on the air flow syncro meter (the general recommendation is to twist the throttle shaft to get the two throats to read the same value. I chose to drill small holes in the butterflys to get the same reading on all of the carbs)
4. Decrease Idle Jet holder from 120 to 100 to cure off idle transition hesitation
5. Adjust idle mixture screws to 1 1/8 turns from closed (different taper on idle mixture screw than earlier Webers?)
6. Change Emulsion Tube from F7 to F11 to correct for main jet transition richness (found problem by use of wideband O2 meter)
7. Change throttle linkage from as-provided single cross link to a bell crank arrangement to correct for left bank to right bank throttle position error (with single cross link arrangement, if both bank flow the same amount at idle, they would flow significantly different at 1800 to 2000 rpms, linkage geometry problem)
8. Change accelerator pump exhaust valve from 00 (blank) to 50 to correct for fuel dripping into the cylinders after hot shutdown (problem made worse by recent addition of alcohol in pump gas)

For anyone planning to install Webers on their car, I would highly recommend that they perform the first three steps listed above [u]before[u] they ever install them on the car for the first time.

John

John,

I beg to differ on point 8.

The exhaust valve with a STEEL inlet checkball is OPEN at rest so will not allow pump well pressure to overcome the pump nozzle outlet check ball, even after hot engine shutdown.

The nylon ball exhaust valve MIGHT have your mentioned phenomenon, but more than likely the carburettor is percolating. Solving the percolation issue is usually the best way.

[jhirasak]

GAZ64,

In my case, the pump exhaust valves that were provided with my new Webers all had a 00 bleed. This is a blank or closed exhaust valve and the gas inside the pump chamber has nowhere to go except out of the pump jet and into the throat of the carb in the event that the gas was heated above the vapor pressure of the alcohol that is currently being added to pump gas. By using a bleed valve which has a .50mm opening, any slowly expanding gas inside the pump chamber is allowed to return to the float bowl instead of dripping into throat of the carb.

Additionally, I have doubled the gaskets at the base of the carbs to reduce the amount of heat transfer from the manifold to the carbs. I also took the precaution to add an electric fuel pump that I could shut off some time before stopping the car to reduce the level of gas in the carbs. Some or all of these measures now appears to have corrected the problem of raw gas dripping into my engine after hot shutdown.

John

[Gaz64]

John,

I still fail to see how fitting an exhaust valve of 00 (no bleed) causes your concern. The inlet ball is OPEN at rest, which will not allow pressure buildup within the pump chamber.

Similar to early Holley float bowls. They had a hanging steel inlet ball which seated by pump pressure. It had a .011-.013 open clearance.
The late Holley bowls have a viton rubber inlet valve AND a .010 vapor bleed back to the bowl. Without the bleed (or blocked), the pump can discharge as the engine heatsoaks.

[jhirasak]

Gday Gary,

I think that when the gas in the pump chamber starts to percolate, it provides sufficient pressure to seat the inlet check valve and, without a bleed outlet, the gas and expanding vapor bubble pushes the gas inside the pump chamber out of the jet into the throat of the carb. That is a guess on my part as what was going on. In any event, after I changed to a bleed valve with a .50mm orifice, my fuel dripping problem became a bad memory in the past.

Since you live down under, do you have alcohol added to your pump gas by mandate of the government? The justification by the authorities was that this was added to our gas to reduce emissions problems. I am not sure about the validity of their explanation regarding the reduction of emissions but I am sure that it has caused us some corrosion and driveability problems with older cars.

Cheers Mate,

John

[PaulProe]

Thanks, Hyde, for the copy of the jetting survey. A question for you and the others on the forum:

There are only 5 entries for the big block cars, mine being the sixth. Are there not more guys who have Webers on a big block? How about posting your specs so we can get a wider sampling

Paul

[*13*]

Originally Posted by PaulProe

Thanks, Hyde, for the copy of the jetting survey. A question for you and the others on the forum:

There are only 5 entries for the big block cars, mine being the sixth. Are there not more guys who have Webers on a big block? How about posting your specs so we can get a wider sampling

Paul

At the time I put this together, I requested info. What you see, is what I recieved.
What is needed is a web page where people can submit their specs & it will automatically post into a chart. I certainly don't have the resources. It may get a little more attention, though.

[Gaz64]

Originally Posted by jhirasak

Gday Gary,

I think that when the gas in the pump chamber starts to percolate, it provides sufficient pressure to seat the inlet check valve and, without a bleed outlet, the gas and expanding vapor bubble pushes the gas inside the pump chamber out of the jet into the throat of the carb. That is a guess on my part as what was going on. In any event, after I changed to a bleed valve with a .50mm orifice, my fuel dripping problem became a bad memory in the past.

Since you live down under, do you have alcohol added to your pump gas by mandate of the government? The justification by the authorities was that this was added to our gas to reduce emissions problems. I am not sure about the validity of their explanation regarding the reduction of emissions but I am sure that it has caused us some corrosion and driveability problems with older cars.

Cheers Mate,

John

Hi John,

I see your point, and if it works for you, then you must have had nylon check valves in the 00 pump exhaust valves. The nylon balls seat faster then steel.

We have had ethanol blended fuels for some time now, it's a lottery what you get now.