Weber info about Aux Venturi

[priobe]

Hello All,

I have a question in regards to the aux. venturi.

On my webers I received them (used) with the aux venturi cut. I have not really found a reason as to why this was done.

Does anyone know as to why someone would have made this modification.

The aux venturi tube is completely cut so that it is even with the carb body. I am thinking it might have been done for hood clearance.

Here is my theory, and would like to get your feedback.

The aux. venturi creates a vacuum which signals the main ,emulsion, and air corrector when to come in. I understand that the air corrector and the emulsion tube does this as well, but the aux venturi is what cause the intial vacuum for this circuit.

By cutting the aux tube of the venturi will cause less vacuum which mean the main jet would take longer to start.


I am just guessing

[Jac Mac]

You are probably correct in saying they were cut for hood clearance, another case of someone doing a mod without thinking of the possible ramifications.

The signal in the secondary venturi is created at the bottom end nearest the butterfly/throttle plate, not at the top, however if the top end has been shortened & no attempt to radius the cut this will affect the signal as the air quality/flow will now be different at this point to when the aux venturis were longer. With the square cut the air entering the aux venturi at higher flow rates cannot make the right angle bend around the 'cut' & tries to make its own larger radius which has the effect of reducing the actual size.

Jac Mac

[priobe]

Here is a picture of the Aux. venturi. You can see how much was cut.

[barry950]

I had to cut the aux. venturis down to match the 1" air horns necessary for hood clearance on a 289 with 48 IDAs and De Tomaso cylinder heads. (similar to Gurney Weslake). There appears to be no adverse effect as the engine pulls strongly from 3000rpm to 6500rpm with no holes in the torque curve. It makes 369 lbs ft of torque at 5000, and 403bhp at 6200 on 93 octane fuel.

[priobe]

I noticed a difference at idle when I replaced the aux tubes in my 427 fe

I even had to adjust the jets

I guess the every motor is differne

[patty442]

Mr. Priobe. It does make a difference. Real difference in driveability at your cross over point, especially. 3500 to 6500 rpm is way past that point and does not make a difference that other contributors can feel in a 400 h.p. 289.. Cutting them off shorter is usually for hood clearance issues but
some "smart" guys in California think there is a performance gain. We don't because we haven't seen it. We have worked extensively with auxillary venturis and their effect on overall performace, performance in specific RPM ranges, and under different loads conditions. We have worn out two 675 h.p. FE motors on the dyno, testing. We have installed and tested many, many combinations in a borrowed 4000 car before installing the final results in our original 3000 race car. We have tried original 4.5, 4.0, and 3.5 auxillary venturi and have seen the changes in main circuit activation in both timing and strength. We have tried 4.5's cut at 1/4 inch intervals. Almost to the nonsense cuts that you pictured. We even found a 3.0 set which received the same cycling on both the dyno and our test car. As our 4.0, 3.5, and 3.0 auxillary venturis are made from unobtanium, my best suggestion for you is to stay with the 4.5 uncut and go from there. I must add that we do NOT use the 4.5 in our final set up. Hope this helps

[Rick Parker]

I think the Aux Venturis on all the Daytona Coupes are cut along with utilizing shorter stacks for hood clearance. This will change the signal strength and require a different jet package and probably a different Emulsion tube too. An engine of a given size utilizing a roller cam will generally be able to use a smaller fuel jet (all things being equal) because of faster valve opening events compared to a flat tappet cam. There are lots of variables when tuning these carburetors. More than one way to achieve the same results too.

[priobe]

Hi All,

Thank you all for responding to this thread, however it is quite old (2008), not sure how this was resurrected. But I have learned a lot since 2008 and still continuing to learn. Patty and Rick, you guys are both spot-on, especially with the roller cam.