adding a 34 ADF, vacuum question

[dlbj]

Just FYI about the vacuum advance. I re-connected from the port that is on the carb vs. the one off of the manifold and ran it around. There wasn't really any difference. Now part of that could be that the engine is about to undergo a rebuild and right now it is a little out of tune. After I get the rebuilt engine in I will try it again to see. I'm sure that if there is a deference to be had I will notice it given what I plan to do the engine.

[So Cal Mark]

sorry it took me so long to reply to Denise's question. using manifold vacuum for the advance can be a big advantage with a perf camshaft. If you need to raise the idle speed, turning the idle speed screw opens the throttle. That bypasses the transition circuit and cause fuel to dispense from the main discharge nozzle. That results in an erratic idle and run-on when the key is turned off. By using manifold vacuum to the adv, the timing advances and raises the idle speed. Most perf cams like more advance at low speed anyway. If you merely advance the static timing, you end up with hard starting.

[mdrburchette]

Thanks Mark. Does it scare you that it makes sense to me?

[So Cal Mark]

Denise, what is most scary is that you understand my incoherent ramblings

[bradartigue]

I'm flipping through this and noted a couple of things:

- Those pictures are of an ADF without the vacuum port tapped.

- The reason why there is no difference in the advance when you move the hose from manifold to carburetor is you're making all of your connections on the wrong side of the air stream.

The ADF series has a port cast into the carburetor body above the throttle body. If you look at an ADF you'll see it has two distinct parts, the bottom has the butterflies and then a small gasket, then the carburetor body. The bottom is referred to as the throttle body. Anything above the throttle body is not ported; anything above is. When the butterflies open the pressure drop pulls air from the distributor vacuum line into the carb, yanking back on that advance plate.

You can port an ADF by tapping the correct port. It is located above the triangle (cast into the throttle body) on the very bottom of the carburetor. It is cast into the flange and is on the same side as the mixture adjustment screw. You can have a carburetor shop - like Pierce Manifolds - do this if you aren't comfortable (you can ruin you carburetor if you drill to far!)

So that's it. If you need a more precise picture I can find one for you.

[bradartigue]

Here is a diagram of where the port should be:

http://www.artigue.com/fiat/Pubs/WeberADFPorting.pdf

[dlbj]

Hey Brad, I'm a little confused with a couple of things you said so I was hoping you could clarify. Your post says

The bottom is referred to as the throttle body. Anything above the throttle body is not ported; anything above is.

Obviously a typo there, but I think that you mean that the throttle body is ported and the carburetor body is not. Is that what your meant to say?

Also, thanks for the link for where the port should be but when I click on the link I get a 404 error.

Thanks again for your input,

- DJ

[So Cal Mark]

below the throttle plate is manifold vacuum, above the plate is ported vacuum.

[dlbj]

Thanks Mark. Then based on what you said, the quote was "anything above the throttle body is not ported; anything above is" and it should be that "anything below the throttle body is not ported; anything above is". So looking at this picture I outlined the triangle that Brad was talking about in yellow on what I think might be the ported tap candidate in red.

[So Cal Mark]

yes, that's above the throttle plate so it's ported vacuum.

[Hvnfn]

If you do not mind me asking, where did you pick up the new 34 ADF?

[bradartigue]

You figured out what I was trying to say...and your picture of the port with the red circle around it is exactly where it is supposed to go.

Distributor vacuum is ported above the throttle plate to that tiny cast-but-not-drilled in your carb port. Years ago the vendors went ahead and ported them, I guess the demand wore off.

If only they still sold the Plex 201 kits as new we wouldn't need bother with this porting nonsense...ah well.

[dlbj]

Thanks for your help Brad.

For HVFN: I got mine from Bruce's Parts Bin
http://www.fiatparts.com/default.htm

[TJC]

If only they still sold the Plex 201 kits as new we wouldn't need bother with this porting nonsense...ah well.

As I have an everyday driver and not a racer, I actually like the ported carbs. That little "A" at the end of my DMS means that my elect. dizzy can take advantage of as much spark (at light loads) as it's designed to in order to boost efficiency. Plus, it's not a complicated system and it doesn't rob power, it just makes light load running a bit more efficient.

[bradartigue]

A plex 201 was the same distributor without the vacuum porting, it was all mechanical. That's what I mean by not having mess around with porting.

That carb wasn't designed for the distributor port, that isn't what the "A" means anyway. A meant "America" or more precisely "American Emission Controls" and they were different in California and the USA.

DMS A had a choke limiter, secondary throttle limiter, and a high idle device. As did the DHS A.

ADF A had a high idle device and an idle stop solenoid

ADH A was like the ADF A but it was tiny and pneumatic

DHS A was the same as the DMS A but had a pneumatic secondary.