Drilling Webers, A few questions, but possibly a how-to Options

[ThePaintedMan]

Hello all,
So while I wait to finish all the metal work on this car, I thought I would begin to get the carbs and distributor straight so that I can start the thing up and go when its ready. I've got a set of Italian Weber 40 IDFs that I'll probably just go ahead and rebuild while I'm at it. Also, a Bosch 034 SVDA dizzy with vacuum advance. So, the dilemma I have is that these carbs don't have a vacuum advance port already drilled. I know this has been covered ad nauseam in other threads, but I hope that this will be the final one and potentially a "how-to" for others.

By following the Weber diagram that was posted awhile back on the Samba, we can see where the later models included a boss for the vacuum advance port, directly above the brake/manifold vacuum tube. That diagram is below, but unfortunately I do not have any photos of a later Weber 40 with the actual boss from the factory.


If you look at the Webers I have, there is no existing boss to be drilled. The best you can do is look at the diagram and try to use it to mark a hole to drill. The problem is, all references that I have read say that the hole should be drilled just above the throttle plate (butterflies) so that your source of vacuum is throttle-controlled, not the brake/manifold signal, which is on ALL of the time. I have confirmed this with three different carburetor shops in the area, but all of whom did not want to drill the port for me. I was told I could do this myself.

So, I am going to give it a shot. I'm looking for final confirmation from anyone here before I do, however. In the image I posted above, the hole I marked to be drilled is just above the throttle plate, but it differs from the later factory location. Did Weber put their vacuum advance boss in the right place, or am I missing something?

Thanks!
-George

[Valy]

For vacuum advance you want unported vacuum. This means that the vacuum must come from a source prior to the throttle plate.

Make sure that you drill above the throttle plate.

Vacuum from the manifold is not good.
Vacuum from the air filter chamber is fine but not best. You could use this to check the general performance improvement before you drill the carb.

One port form any of the carbs is sufficient and recommended. Don't connect the vacuum ports from different carburetors as there is no advantage in doing so.