Question re vac advance connection to early Zenith carb

[Tierfreund]

Hi all

I need help with an issue thats been haunting me. I´ve converted my 71 3.0S E3 back from tripple webers to the stock zenith carbs. Now on the 71 euro E3 that means the earlier simple carbs without TN-starter and a simple distributor with vac advance only. No emissions control equipment on either.

Now the trouble I´m having is figuring out where to connect the vac advance.

The carbs have three vacuum hose connectors each. One at the engine intake side, one at the side that points to the brake booster and one that point forward in the direction of the radiator.

I know that the connections at the brake booster side get joined from both carbs via a y-piece and then connected to the air filter housing to join the engine breather pipe.

What I don´t know is weather the vac advance gets connected to one of the connections pointing to the engine, to one of the connections pointing to the radiator or to both carbs, again connected via a y-piece (and if, which connector...)

The setup would probably be found in the US on the 2800CS and / or very early 3.0CS (before legislation added emissions control equipment and later the TN-starter).

Any suggestions or pictures? I´ve been scratching my head for quite a while now and the blue books only explain the connectors on the later carbs with TN-starter which has quite a different throttle body.

Any help is appreciated.

 

[bengal taiga]

nt

 

[Tierfreund]

Thanks for the info, but I´ve bee through all that. The picture shows the later type carb with the mixture bypass system. That´s also the type and unfortunatley the only type pictured in the blue books when it comes to the vac advance connection.
The older type without mixture bypass that I have is thoroughly explained and there is even a complete instructzion in the blue books on how to rebuild tehm and tune them, but unfortunately now info on how to connect the vac advance.

The connectors on the throttle plate of the older type are in a very different position, so it´s not at all obvios. I have no problem finding the connector but I can´t find any info on which of the two candidates would be vor advance and which would be for retard (so for my distributor should be plugged up)

Anybody out there know? I´ve searched the tech section, the e12er forums, the e9er forums, all info I could lay my eyes on
My google is smoking...
That info is somehow unobtainable through regular research. I need an old hand or an expert to help me out here...

 

[bengal taiga]

The carbs have three vacuum hose connectors each. One at the engine intake side, one at the side that points to the brake booster and one that point forward in the direction of the radiator.

I know that the connections at the brake booster side get joined from both carbs via a y-piece and then connected to the air filter housing to join the engine breather pipe.

What I don´t know is weather the vac advance gets connected to one of the connections pointing to the engine, to one of the connections pointing to the radiator or to both carbs, again connected via a y-piece (and if, which connector...)

Let's try this again. I deleted the prior post since it attempts to answer a question you did not ask.

It might be helpful if you posted a picture of the carbs and/or at least a model number. I would bet Euros to Sacher Torts that other car makers used the same carb in one form or another. Mercedes is a likely suspect. But even without a picture or a diagram, the answer is probably in your finger.

There are exceptions to every rule; however, most distributor vacuum advances function off of vacuum found at closed throttle or part throttle. Chances are, one or more of the ports are located above the throttle plates or at throttle plate level. Any one of those should work. You should be able to feel the vacuum with your finger as soon as the throttle is opened. If there is substantial vacuum at idle, you are tapping into manifold vacuum, and, as I said in the other post, that is typically used for vacuum retard pots. Its that simple.

If you don't want to use your finger, there is always a simple hand held vacuum gauge like the one pictured below which you can use for any number of other things including carb synchronization and actually reading the vacuum produced at each port.

The fact that there is more than one port is of little concern because additional ports may be designed for multiple uses including "anti stall dashpots" or uses that were contemplated in other markets.

Make sure the unused ports are capped. I would expect that tapping into one port on the forward most carb is all that is necessary for the vacuum advance to function. Unless you have dead cylinders under that one carb, I do not seen the need for tying the vacuum into both carburetors since the vacuum from that one set of three cylinders should be sufficient.

Maybe I am missing something but just plugging the distributor vacuum hose into the the available ports ought to provide immediate evidence of what works and what doesn't. I am surprised the Blue book nor the manuals seem to deal specifically with your carbs. As you can guess I am located in North America. I have seen many Euro market cars including some early E3's and E9's but do not recall seeing the carbs you describe. Is it even possible you have carburettors that are not original to your car? Many carbs other than the original can be adapted to work.

Again, if I have misunderstood your post, I am sorry. A picture or two might inspire others to offer a better approach to the problem.

 

[Tierfreund]

Thanks for your help, I think we´re getting there. Actually I´ve taken some pictures of the carbs to go with the question, but hesitated to post them since I feel they are actually more confusing than helping.

Let me try narrow the question down:

I´ve found the potential connectors. The only trouble is I have two candidates. They both go into the throttle plate and both have vac. (Now why would I hesitate to use my fingers to check? They´re not really big enough to suck my finger in... )

Candidate 1:
Goes into the throttle plate and has it´s vac pickup in the throttle body between the first and second stage throttle butterfly. Thus there is manifold vacuum on it even at idle

Candidate 2:
Goes into the throttle plate and has it´s vac pickup just above the first stage throttle butterfly (when it´s closed). Thus there is no or very little vacuum at idle but full manifold (or even more due to venturi effects) vacuum as soon as the throttle is opened even slightly.

So it boils down to:
Do I need/want vac advance at idle or only above idle when there is load on the engine?

Thanks again for taking your time to help me here. I really appreciate the info and help this forum provides.

 

[bengal taiga]

Candidate 1:
Goes into the throttle plate and has it´s vac pickup in the throttle body between the first and second stage throttle butterfly. Thus there is manifold vacuum on it even at idle

Candidate 2:
Goes into the throttle plate and has it´s vac pickup just above the first stage throttle butterfly (when it´s closed). Thus there is no or very little vacuum at idle but full manifold (or even more due to venturi effects) vacuum as soon as the throttle is opened even slightly.

So it boils down to:
Do I need/want vac advance at idle or only above idle when there is load on the engine?

Per your description, I vote for No. 2.

(Do not wish to seem evasive, but - unless you are running Nitrous (N02) you really have very little to loose by experimenting with each choice. see pm)