adding a 34 ADF, vacuum question

[TJC]

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......

But for the same engine speed, the plex 201 still cannot advance spark at low loads, right? If so, that was the point I was trying to make between getting a plex (which I'm sure is an awesome addition) and using a ported vac for the Marelli dizzy.

As far as the DMS vs DMSA, one other thing I always thought the "A" brings you (at least with that model Weber) is the ported vacuum. I mean, at least according to the Weber diagrams for the DMS and DMSA. So, I made the leap that the "A" for "America(n)" emissions also means a ported vac for EGR and/or vac advance dizzy (as fuel economy and emissions are directly related).

BTW, what's the secondary throttle limiter?

[bradartigue]

Plex worked just like the vacuum dizzy but didn't require the vacuum. Same basic advance curve - probably had more curve actually. It was sold for years by the vendors in a kit with the coil pack. You could convert your points type or replace your vacuum type. I ran one on an FI car and two carbed cars and it worked great, it now lives with Troy Mckaskle on his 72. I would buy the entire car from him to get that Plex back! (well, maybe not just for the plex it's a hell of a nice car).

I think "A" just meant "conforms to USA standards" - and could mean lots of things depending on the years.

DMSA/DHSA had ported vacuum on the Coupe only (for example) and not the Spider, and not to pull a distributor. ADHA had it on the 79/80 but ADFA did not.

Throttle limiters were the funky little devices mounted to the DHSA body and DMSA body that kept the throttle from closing if the clutch was in and the gas was out and the car was in 3 or 4th gear. They also kept the choke plate from moving.

[TJC]

Plex worked just like the vacuum dizzy but didn't require the vacuum.

Wow, I didn't know that. I'm fuzzy on it's function, though. Could you explain how the Plex, without the use of a ported vac signal, would know when the engine is at a low enough load to boost the advance curve above that of the engine speed based curve? Today you just use a throttle position or manifold pressure sensor, but back then I thought vacuum was all we had.

Throttle limiters were the funky little devices mounted to the DHSA body and DMSA body that kept the throttle from closing if the clutch was in and the gas was out and the car was in 3 or 4th gear. They also kept the choke plate from moving.

Ah, yes. The books calls this the "Fast Idle Circuit", I believe. I actually hooked mine back up because I liked the idea of not going so lean on decel and throwing NOx out all over I-696. For my car, it also relieved some of the decel popping I get with my overly large non-OEM exhaust a PO installed.

[bradartigue]

It works no differently than a regular mechanical distributor. The engine is statically advanced to a certain position (on FIATs it is often 5 or 10 degrees BTDC) and as it accelerates a plate with weighted "arms" spins outward. Springs help regulate the arm movement.

The vacuum types were part emission control; by using the drop in manifold pressure you make the distributor dependent upon actual engine load as opposed to engine speed.

If you run the vacuum type it should be hooked to the right place in the dizzy; if you run a Plex or points you don't need the vacuum.

[TJC]

The vacuum types were part emission control; by using the drop in manifold pressure you make the distributor dependent upon actual engine load as opposed to engine speed.

Yeah, I just thought you were saying the Plex replaces the vacuum advance. But, from what I'm hearing, it doesn't, it just eliminates it.

Vac advance isn't so much an emissions function as it is for highway fuel economy (and with a car with a 4.30 rear, I'll take all I can get). Safely advancing spark (i.e., at low loads only) allows the engine to run closer to its MBT, enabling a higher fuel conversion efficiency thereby lowering specific fuel consumption.

[TJC]

Safely advancing spark (i.e., at low loads only) allows the engine to run closer to its MBT, enabling a higher fuel conversion efficiency thereby lowering specific fuel consumption.

Sorry, I just realized how didactic that sounded (or is it pedantic.....well, anyway). Forgive me, I was still at work and was a little too eager to show off my engineering skills (or lack thereof). It's very rare I do that at work, everyone seems so much smarter than me......I did go to college, right? Maybe I shouldn't have joined a frat and paid more attention in that engine class Prof. Heywood was teaching (the class was about beer, right?).

I must've gone to college, I remember my parents complaining of large amounts of government paperwork. I should've paid more attention just for their sakes at least. It's like when my father used to scold me about my braces. He spent a lot of money to straighten my teeth, then I don't have the decency to wear my retainer. Now, 25 years later, my teeth can get me back into the pages of The Big Book of British Smiles.

Wow, that was a rant. This is what happens when the first thing I do after geting to work (but before my coffee) is read the Fiat boards.

[mic57]

After reading this thread I'm totally confused, maybe someone can help me out.
I'm trying to straighten out the mess I inherited when I bought my 79 spider 2000 this year. It came with a 34ADF carb on the stock manifold and all of the vacumn lines but no air pump. I installed the single plane manifold and hooked up the remaining lines as per Brad's diagram but some things are not making sense.
1. The line from the bottom of the vapor cannister is shown going to the high idle vacuum motor, I thought this hose was to connect to the fuel bowl vent?
2. Where is the fuel bowl vent? if there is none can't I just eliminate the hose?
3. should I connect the high idle motor and if so to what?
4. the port that the top of the cannister connects to I have seen shown as the PCV valve, is this true for the ADF?
5. Can anyone explain the PCV system on this car.

Sorry for the long thread and the spelling but I would really like to understand what I am doing.

[So Cal Mark]

some of the 79s didn't use an air pump, they had a reed valve-pulse air style of air injection
The pcv system is different than most cars; there is no pcv valve. One large hose connects to the air cleaner on one end and the oil separator on the other end. Fresh air travels down this hose to the sump as well as crankcase pressure traveling up the hose to the air cleaner. There is also a vacuum hose that connects to the air cleaner/crankcase hose tee. That small amount of vacuum is enough to keep negative pressure in the crankcase. Any restriction in that hose or oil separator can cause positive pressure in the crankcase, resulting in lots of oil leakage.

The evap cannister should have 3 hoses to it; one from the tank, one from the carb bowl vent and one vacuum hose. It also needs a fresh air intake.

[jaubine]

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!)

Hey Brad, et. all -

I've drilled out the hole, far enough to open up to the veritcal opening in the carb body, and seated a nipple into the hole:



My question though, is looking at the throttle body, I see a little indent where that vertical hole lines up, but I dont see where this is going to get any vacuum? See below:



The "dimple" sits right over the hole (above, right of the triangle marker) on the side of the throttle body, so I'm hesitant to drill that out, as it would just pass right into the hole below.. There's no channels that lead from the hole to anywhere, and there's nothing in the gasket/separated between the throttle and carb bodies that would indicate that the hole would go anywhere other than where this "dimple" sits..

So, where does the dizzy get its vaccum from?

[So Cal Mark]

that hole should make a 90 bend and open into the venturi above the throttle plate. The gasket will need a hole in it at the appropriate spot

[jaubine]

that hole should make a 90 bend and open into the venturi above the throttle plate. The gasket will need a hole in it at the appropriate spot

Mark, I'm not sure I'm following... The hole you're describing is to the right of the dimple on the throttle body (in yellow box); that hole does not connect (as far as I can visibly tell) to the port I tapped on the carb body in any way:



On the side of the throttle body, that hole (the one from above in the yellow box) capped with a brass stopper just above the mixture adjustment screw. the one to the left of that hole (pictured below) is the one that the dimple lines up to..



Also the gasket does have a hole in the proper spot, but it just stops at the dimple in the throttle body.



Still very confused

[bradartigue]

Was this resolved? Every ADF I've had has the vacuum port where you put it, with air being drawn from that passageway I can see cast into the carb. It does not go through the gasket, it makes a 90 degree bend into the adjacent pumping and into the lower throttle body below the plates.

Did you spray something through it to see if anything comes out the other side? It certainly should.

[sambalee]

Gonna pick this up as I have the exact same issue. I just got hold of a Weber 34 ADF and need to add the vacuum port but as suggested, the hole to add the port to doesn't seem to go anywhere - I'm also confused so any help appreciated.

It might be as well to get one of the Chinese copies.

[msjulian]

I have the same issue.. I don't see where the vac adv port would get vacuum. It just dead ends after the hole in the gasket.

[Nanonevol]

My recent purchase has this added by a PO. Curious as this one also seems to make a dead end and I could not blow air through it.