34 ADF Vacuum Port for Distributor

[volzke]

Hello. I finally sourced a used 34ADF to rebuild. It will be part of my conversion on a '79 2L, changing out the 28/32 and dual plane intake manifold for a 1800 single plane and 34 ADF. As I have viewed the threads, most advise that tapping a vacuum port for the distributor advance is needed on the 34ADF. However, a few threads have instead stated the vacuum line can simply be hooked into the intake manifold. I was also lucky enough to get some photos from Warren on how he did his '79 conversion. He did not tap the 34ADF, but rather hooked the vacuum line to the intake manifold. Curious on the groups thoughts on this. Is it really necessary to tap the port into the 34ADF? Or, would there be any 'downstream' impacts to think about by simply connecting the line to the intake manifold? Many Thanks.

[geospider]

the only thing I could think of is: is there a vacuum difference between the 2 ports. since the port on the carb is at the base, I would doubt it is much different, but???
also, make me think you could then adjust the nut on the advance itself on the dizzy.
interesting question.

geo

[wetminkey]

The 34ADF does NOT have a vacuum port for distributor advance?!
Manifold vacuum is not the same as timed vacuum. A timed vacuum port will provide no vacuum at idle, while a manifold vacuum port will,...

[blazingspider]

The biggest difference between taking vacuum from a port on the intake manifold as opposed to a ported vacuum source is the fact that there will be vacuum at idle when using vacuum from the intake manifold and there will not be any vacuum at idle when using ported vacuum.

Assuming your vacuum capsule is the 10 degree version, and you're timing set to 10 degrees BTDC at idle; when you apply intake manifold vacuum, you're timing at idle will now be 20 degrees BTDC. That will require you to adjust your timing.

If you stomp on it, the throttle plates on the carb will be wide open and vacuum from the intake manifold will drop to zero which means you're now only using whatever advance curve is applied from the mechanical centrifugal set up on your distributor. As a result, your timing will fall back so to speak to whatever the mechanical advance is set for.

At part throttle openings there's not much difference between intake and ported vacuum and you will get the benefit of advancing the timing an additional 10 degrees when at part throttle.

[volzke]

Thanks for the insight guys. I called Mike over at Pierce manifolds today. He hadn't had anyone inquire about work on a 34 ADF in quite some time. We looked up the full number and he determined in his "Weber Bible" (as he calls it), that my unit is a 78 racing spec carb. He said that was the reason the vacuum for distributor was not ported. Exactly as was stated in the last post, he advised the racing carbs are timed differently to account for vacuum difference at idle. In either case, I'm going to ship it over to pierce manifolds and have him tap and install a ported vacuum. That way it is set up correctly for my installation.

[wetminkey]

Wow, that's cool, volzke!
Glad The folks at Pierce can get you fixed up with timed vacuum on your 34 ADF. They are a good bunch,...
Todd.

[Warren]

Hello: I never had any issues with the port on the intake manifold at least in my 64 year old view.

One thing I should do is replace the idle speed jet with a smaller one as engine RPMs are a little high with the auto transmission in Park and the idle screw is backed out all the way. Everything ok in Drive and it does not diesel in Park when shutting off but it could stand a little lower RPM.

[volzke]

Operation rebuild Italian Carburetor appears to be a success! Didn't do the big job of swapping out for the 1800 manifold yet. Simply wanted to verify that the carburetor I rebuilt this summer would bolt on and run with the original manifold. Starts up like a charm! Spent the afternoon hours smoothing out the idle. Overall it seems to be running strong!
34 ADF authentic Italian Weber Carburetor, vintage 1978 (not the silly Spanish or Chinese versions), installing on our '79 Spider. I had Pierce Manifolds look up the base stamp in his "carb bible" (as he calls it) when he was tapping the distributor port, and he verified this unit is 131 racing series European spec unit not provided originally to North American cars. Ciao!

[Blue792000]

Hi Volzke,
I have the same situation I believe. I have a Weber 34 ADF 450 5M. Could you repost your pic? I am interested in where the port was installed.
Thanks!