Overview
At some point during the ownership of a Fiat 124 Spider you will be faced with a decision about fuel delivery. If you own a 124 built between 1966 and 1980 then you likely have one of several Weber dual-barrel carburetors installed on an aluminum Fiat intake manifold. If you are wondering what your options are and what the differences are, this document is for you.
If you own a 124 built between 1980 and 1985 and that car has Fuel Injection then this document is probably not for you, unless you intend to remove the FI system and install a carburetor (a procedure that may leave you with less performance, higher emissions, and reduced fuel economy.)
Carburetors and Manifolds
Part One: 124 Carburetors
Fiat used Weber as the primary manufacturer of carburetors for the 124 series. There is an exception: if you had lived in Europe during the production of the 1608 BS series Spider, you may have the optional dual Solex or Dell'Orto carburetors. Chances are, you lived in the U.S., bought your car in the U.S., and it has a single Weber carburetor.
For the sake of simplicity, I will group the Weber carburetors used on the 124s into two major categories and several minor categories. Major categorizations are derived from the type of throttle used to operate the secondary barrel. These were operated either mechanically or by vacuum:
- A mechanically operated secondary literally opens the secondary barrel with a series of levers attached to the primary barrel. At a certain point in the opening of the primary, the secondary begins to open. This "point of operation" can be altered by the owner and tailored to fit specific needs.
- A vacuum operated secondary serves the same purpose as the mechanical type, but the secondary barrel is opened by means of a pump. The pump is activated by intake pressure and operates an "arm" which connects to the secondary mechanism and opens or closes the barrel. The point of operation can be tailored as well, fitting the needs of the owner.
The minor categories are based upon the type of low-temperature "choke" used on the carburetors. In both cases the choke is external and above the primary barrel. The exception to the rule is dual carbs, which utilize an internal choke mechanism not discussed here. Fiat Spiders used either a mechanical or automatic (water-type) choke:
- A mechanical choke operated the choke plate by means of a lever and cable mechanism. The cable ran into the driver's compartment and, when pulled, operated a lever which closed or opened the choke.
- An automatic choke operated the choke plate with a spring and lever mechanism. A coil spring, located in a canister on the side of the carburetor, would expand or contract from the heat of engine coolant entering the canister. This expansion would cause the choke plate to open or close. All the driver had to do was press the gas pedal fully to the floor once, prior to starting the engine cold, to choke the carb.
Therefore we will group our carburetors by the type of secondary and choke they utilized. Understand these two components (mostly the secondary operation) will help you decide which carburetor to choose when you decide to bolt one on.
Part Two: 124 Intake Manifolds
Deciding which intake manifold to use is as important as deciding which carburetor to use. Unlike carburetors, which many options and sizes exist, Fiat manifolds really don't give us much room for decisions. Let's take a quick look at what is available before anything else.
There are basically two types of single-carb intake manifolds used on the Spiders. Each one was designed to accommodate a specific carburetor, but each one can also accommodate most of the carburetors from any other model year. This allows you to mix and match manifolds and carburetors to create the solution that best fits your car. However, only two of the manifolds are really worth considering, but we'll get to that in a little bit.
The two basic intake manifold types:
- Dual
Plane:
Easily identified by the two distinct plates in the
carburetor. Designed to accommodate vacuum-operated carburetors,
the dual plane is not a good choice for other carbs. Small
tubes are fairly restrictive.
- Single
Plane: Considered
the best of the manifolds, the 1800cc manifold had large
intake tubes, a single-plane, and few emissions provisions (those
that exist can be blocked off with a few bolts). This
type will fit any of the carburetors I discuss, but a vacuum
operated carburetor may not operate properly; the lack of a
high pressure secondary plane, while desirable for all
mechanical carburetors, may cause the vacuum type to fail.
- 2000cc Type: Possibly the worst manifold designed for the 124 series, unless you have to run this manifold, get rid of it. Emissions-laden and highly restrictive, the 2000cc type literally strangles the engine. If you ever wondered why the 2000 Spider only had 81 HP, it's this manifold and the carb (ADHA) that came with it. If you are unfortunate enough to require it, your best bet will be to ensure that everything is working perfectly and keep it very clean - ADHA parts are hard to find and expensive.
Identify your Carburetor
You may not be running the stock carburetor (which isn't always a bad thing) or manifold. Weber carburetors are stamped on the base plate (where it attaches to the manifold). Usually under a plate of crud by this time, it can be found by cleaning up the base.
Regardless of what carburetor you have, it's a good idea to know what your car had before it was altered. I have a comprehensive table, including all of the stock jetting, on my main carburetor page, but here is a reminder:
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This table represents the common series for each model year. This does not account for the occasional "oddball" where, for example, Fiat may have installed the 34DMSA carburetor and single-plane manifold on a 1973 Spider. This kind of thing was not unheard of during end-of-production model runs.
Note that Weber typically places an "A" after the model type to signify carburetors designed with emissions controls. For example, an ADF would be a fairly standard carb with only the charcoal canister connection. The ADFA adds other emissions control stuff, such as PCV ports, etc. to the mix.
Selecting the Perfect Carburetor
In order to select a carburetor that suits your needs, you have to ask yourself some questions about your intended use for the car. If you plan on building up a 180HP wheel-burning monster then please stop reading - because this document doesn't deal too much with heavy customizations. If your desire is to improve reliability, increase performance, and end up with a smooth-running Spider, this section is all for you.
Good and Bad Swaps
As mentioned previously in this document, you can mix and match the various model year carburetors to suit your needs. However, you probably don't want to do that. Over the years, Fiat selected some good carburetors and some very bad ones. The DMSA, for example, was used by virtually every sports car manufacturer of the mid-1970's. Holley copied it for several U.S. vehicles. It was reliable, easy to rebuild, parts were plentiful, and it had "no surprises." Today virtually any car parts store can get you every part for a DMSA.
On the flip side, it was amazing that any car outfitted with the ADHA even ran. Loaded with emissions control pieces, the poor ADHA was hardly larger than the carburetors mounted to 817cc Fiat 850's! Today the ADHA is nearly impossible to find parts for and few mechanics will even touch them.
The point here is that you would never want to take an ADHA and mount it on a car that was previously equipped with the DMSA. This would be a downgrade in both reliability and performance. However, you might want to take a DMSA and the single-plane manifold and mount it on a 1608 Spider with the vacuum-operated DHSA. This would be an increase in performance and reliability.
Choosing the Right Manifold
This is the easy choice. Select your manifold using the table below:
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Why?
In order to use the DHSA or ADHA carburetors properly you should use the dual-plane (stock) manifold that the carburetor was originally mounted on. This will ensure that the secondary operates properly and, since the carburetor is not going to start producing more power, you really don't need a bigger, better flowing manifold. Keep in mind, though, that using the DHSA or ADHA and stock manifold on an engine with high compression pistons and high performance cams is a wasted effort; you should upgrade to a different carburetor and manifold.
The 1800cc Single-Plane should be used when installing any other Weber carb on any Fiat twin-cam that is running a carburetor with a mechanically operated secondary. It is the least restrictive (hence highest performance) of all of the manifolds produced, it mounts any of the stock and optional high-performance single carbs, and has few emissions control provisions. You can literally remove your old manifold and bolt one of these on in a matter of minutes!
Choosing the Right Carburetor
This is the hard part (for me, the poor author). Finding the right carburetor means that you must find a solution for your budget, your performance needs, and your reliability concerns. This means you also need to pick a carb that has the most number of available spare parts and a good number of people around to help you tune it if you need them.
I cannot tell you what to buy, but I can provide suggestions on carbs I have purchased, rebuilt, installed, and used on cars for myself and friends.
Vacuum-Operated Carburetors: The DHSA and ADHA
The DHSA and DHSA2
The DHSA was installed on the original 124's up to 1971. Difficult to find parts for and generally notorious for secondary vacuum leaks, the DHSA is not a popular upgrade nor is it recommended as a candidate for a rebuild.
The DHSA2 and later models were used from 1971 through 1973 and offered larger primary and secondary barrels. Difficult to find parts for and often hard to rebuild correctly, unless absolute originality is required, this carb should be removed and replaced with a later, mechanically-operated model.
Pros:
- Maintains originality.
Cons:
- Hard to find parts for, hard to rebuild, difficult to diagnose.
- Most examples of DHSA carbs have passed 30 years of hard, every day use.
The ADHA
I can only assume that if you continue to use the ADHA it's because you have to pass California emissions and you own a 1979 Spider 2000. That's OK, just keep the two together and very clean. ADHA's need that 2000cc manifold to pull the secondary properly, and the local California emissions control person wants to see all of that original stuff.
Pros:
- Maintains originality
Cons:
- Too many to list. Some examples: extremely restrictive, hard to diagnose, hard to find parts for, poor manufacturing.
Mechanical Carburetors with Manual Chokes
The 32DMS / DMSA
The DMS/DMSA series was, and still is, extremely popular. Inexpensive, even when purchased new, a DMS carburetor will bolt right on to your Fiat, link up, and run. It has a mechanically operated secondary and choke, and even the DMSA has few emissions control provisions.
Pros:
- Impressive performance increase over stock, but retains fuel economy.
- The DMS series is a great replacement for pre-1975 Fiats that shipped with the DHSA carburetors and dual-plane manifold. You can retain the originality of the mechanical choke yet gain a good amount of performance (the DMS is a more powerful carb than a DHSA). It will also fit under one of the stock Fiat air cleaners if you intend to do so.
- Anyone can learn to install and tune a DMS carburetor.
Cons:
- If you intend to use the DMS on a later model car (where the throttle lever is cam-box mounted) then you will have to order a different throttle "pull" assembly for the carb. This is easy to replace and costs about $10.
Mechanical Carburetors with Automatic Chokes
The 32ADFA
The 32ADFA is probably the most prevalent Fiat Spider carburetor on the "used carbs" circuit. Large, well-built, and reliable, the 32ADFA bolts right up to the Single-Plane manifold (used on the 1800cc cars) and can therefore be installed (with the manifold) on any Fiat 124. It has a mechanically operated secondary and automatic choke, and like the DMSA has few emissions control provisions.
Pros:
- Impressive performance increase and retains fuel economy.
- Automatic choke retains originality for stock and later model cars. Also fits under stock Fiat air cleaners for those wishing to maintain originality.
- A great performance increase for all Spiders not originally equipped with the ADFA, including the 1979 and 1980 cars.
- Vehicles will still pass 49-state emission control laws.
Cons:
- If you intend to use the ADFA on a later model car (where the throttle lever is cam-box mounted) then you will have to use the throttle "pull" assembly from your ADHA.
- If you intend to use the ADFA on a pre-1975 model year car, you will have to install the heater pipe (runs under the exhaust manifold) from a 1975+ model year Fiat 124.
The 34ADF
Expensive, but one of the best carburetors made for the 124. Solid and extremely reliable, the 34ADF was provided (by Fiat, actually) as a bolt-on performance improvement for 1975+ Fiats. Nearly identical in manufacture to the 32ADFA, the 34ADF lacks the emissions control ports of the ADFA and has larger primary and secondary barrels, improving performance throughout the entire r.p.m. range. Requires the Single-Plane manifold to operate efficiently. Has an automatic choke.
Pros:
- Impressive performance increase over any stock Fiat carburetor.
- Available new.
- Parts interchange with ADF and ADL series carburetors - and these parts are easy to find and cheap.
- Can be used under Fiat air cleaners, although a "free flowing" type is suggested.
- Vehicles will still pass 49-state emission control laws.
Cons:
- Expensive - average price is $400 to $500, not including the Single-Plane manifold (if needed) which average about $75.
- If you intend to use the ADFA on an pre-1979 model car (where the throttle lever is manifold mounted) then you will have to use the throttle "pull" assembly from your existing ADFA or DHSA carb.
- If you intend to use the ADFA on a pre-1975 model year car, you will have to install the heater pipe (runs under the exhaust manifold) from a 1975+ model year Fiat 124.
The 36ADL and 38ADL
Similar to the ADF series in most respects, the ADL was designed for the Lancia Gamma 2000cc and 2800cc cars (neither of which were sold in the U.S.A.) Hard to find, the ADL series can add serious performance where a single-carb is required. ADL carburetors, like the ADF, have a water-activated automatic choke.
Pros:
- Available new.
- Tremendous performance increase over stock.
- Parts interchange with ADF and ADL series carburators.
- Vehicle will still pass 49-state emissions.
Cons:
- Really need a compression increase to be effective. A change in camshafts would also help.
- Few examples exist in the U.S.
- If you intend to use the ADL on a later model car (where the throttle lever is cam-box mounted) then you will have to use the throttle "pull" assembly from your ADHA.
- If you intend to use the ADL on a pre-1975 model year car, you will have to install the heater pipe (runs under the exhaust manifold) from a 1975+ model year Fiat 124.
Other Types
I excluded the "pros" and "cons" for these types because (a) I've never run a DFEV and (b) this document is not about to extend for another 12 pages on how to select, install, and tune twin carbs!
The DFEV
Our deviant from the manual/water choke fold is the electric choke DFEV, available new from parts vendors and with two huge barrels. A more modern design than the ADL or ADF, the DFEV offers the same basic benefits (and the same difficulties when mounting to a pre-1979 engine).
Note that the DFEV requires a positive lead (energized by the ignition key) to operate the electric choke.
Twin Carburetors
Many owners opt for twin-carburetor configurations. This is the best way to ensure that your car will require lots of attention and will burn a lot of gas! However, the benefits are great - massive increases in torque, the ability to fine tune the fuel/air mixture per cylinder, a loud purr when opened, and a definite increase in your personal knowledge of carburetors.
Having owned Weber IDFs in the past, I can assure you that they are a heck of a lot of work and a heck of a lot of fun.
Conclusion
I hope this document helped you decide which carburetor to run on your Fiat. Generally speaking, I would suggest that everyone run a 32DMS or 34ADF on a 1800 Single-Plane manifold, at least 9.1:1 compression pistons, and a 4-2-1 exhaust manifold.
And you had to read the whole document just to get my conclusion!