Choosing Dual Weber IDFs
Fiat Twin Cams
By Mike Richmond

For passionate Fiat twin cam owners, nothing inspires so much anticipation and fear as the prospect of bolting on two Weber carburetors. The Weber mystique, the promise of real Italian horsepower and the fear of forever fiddling combine in a swirl of emotion. Should you take the plunge?

How much does it cost

A bolt-on kit with an intake manifold, new Weber 40IDFs, jetted to your application, with the carbs properly linked together in the center, with free-flow air cleaners costs about $1000 new. They are available from many of the popular Fiat mail order suppliers. One notable exception is Bayless, who advocate the 40DCNF instead.

Why IDF vs. DCNF

Both models are dual-downdraft, and each model in a dual carb setup will feed each cylinder from one throat in the carb. The DCNF is smaller front to back which makes fabricating a center linkage easier (not an issue if you buy a new kit or the original 40IDF-13/15 setup used). The disadvantage of the DCNF is that the float bowl is oriented optimally for transverse engines like an X1/9, not an inline engine like a 124. Fiat spec’d the 40IDF for their dual carb application on the European 1608 Fiat 124 Spider.

How much power will I get

About 25% more than a stock single carb, which is very restrictive. You are literally doubling the venturi area when you go to dual carbs over stock. Compared to fuel injection, the picture is muddier, since stock specs show that FI adds 20% to the twin cam. And if your engine is bad shape and needs a rebuild, that’s where you should put your $1000. Especially since you can add high compression pistons or trade up to a 2 liter short block.

Are they loud

With free-flow air cleaners, they are when you open them up. But so is Pavorati. The original turtle-back air cleaner produces a mellow bass note that is not substantially louder than a single carb. The free flow air cleaners are much cheaper than a turtleback (by 8X even assuming you can find one of the two new ones rumored to still exist- and used ones command ~$250). Free flow air cleaners also allow you to see the carbs.

What about a bigger single carb?

It’s certainly cheaper and quieter. For example, a new 34ADF (the big brother of the 32ADFA on 1756cc twin cams) is about $400 new. If you do a lot of city driving, this may be a better idea because low RPMs and idling are less efficient in a dual carb setup. You have a lot more area for the same amount of air leading to low air flow in each barrel at idle and low RPMs. The air velocity through the venturis is too low in a dual carb setup for optimal air/fuel mixing. To get good drivability, your dual setup will end up with richer idle jets to compensate- more fuel, more emissions. On the highway, the opposite effect occurs. RPMs are in the sweet spot, pumping losses are lower and mileage can be higher. I get 27MPG highway with my 40IDFs at 65MPH in a 1756cc engine with 9.8:1 compression. In the suburbs I get 20MPG. If emissions testing is strict be careful about going to dual carbs. Want a good combination of fuel economy, power and emissions? Convert to fuel injection, but it’s not as much fun.

I saw a dual IDF kit in a catalog. Is this the original factory setup?

If you are looking for 100% authenticity, you won’t find it in new parts. The original Fiat factory manifold (called the waffle-top manifold because of the grid pattern cast on it’s top) was made by Cromodora and is no longer available new. The advantage of this manifold is that it has coolant jackets like the single carb manifolds do, for faster warm up and for cold days. The original factory air cleaner (called the ‘turtle back’) is an enclosed black or gray steel air cleaner with an oval element. It is rare, although NOS examples have been reported available from Faza. The advantage of this air cleaner is that it is much quieter than free flow air cleaners. The major disadvantage is cost, and the fact that the carbs are virtually invisible when installed making adjustments more of a chore. The original linkage for connecting the center linkage between the two carbs to the accelerator pedal is also NLA, and even if it was, would only apply to 1608 models with a special accelerator pedal. This was a mechanical vs. cable linkage and might win Concours points but is actually a poorer design since it can wander as the engine moves around on its mounts. Finally, the carburetors that are available new today are 40IDF-70s and these are different in several details from the 40IDF-13 and 40IDF-15 that were designed for the Fiat twin cam application.

What’s the difference?

You can thank the VW dune buggy crowd for the fact that 40IDFs are available new at all, and if you go looking for used IDFs you may find a set configured for VWs for <$250 used. Add a manifold to fit your Fiat (~$100 new for the U.S.-made variety) and you may be off to a bargain setup. But whether new or used the typical VW configuration is different than what Fiat engineered:

Carb Model


40IDF-13/40IDF-15 on the 1608 124

Carbs link in center



PCV valve for original air cleaner.



Vacuum port for vac. adv. distributor



Venturi size



Main jet



Air corrector



Idle jet



Cold starting device



Air horns



* all of these items are addressed by the supplier when you buy a new 40IDF kit for the Fiat twin cam. The carbs will have been modified to link together in the center and they will have venturis and jetting for your application.

If you get a ‘good deal’ on used VW-style 40IDFs, you can buy all the venturis and jets needed to match the twin cam. But you will still have to find a manifold and modify the right side of one carb and the left side of another to link them in the middle. The difference in time and money spent on jets means that a set of used -13/-15s on a used manifold will command $200-$300 more than the VW-style IDFs used at $250. Any used dual carb setup will often require a rebuild kit at $40/each.

Are they hard to keep in tune?

Not if you stick with a tried and true formula: set them up and tune them. Then leave them alone. Replace the needle/seat and clean them every 24,000 miles.

Are they hard to tune the first time?

They shouldn’t be, but most people make the mistake of failing to install them right the first time (especially used ones) or they change too many things at once. Install them right and be scientific and you will have them humming after a couple of weekends. In the meantime, you will learn to curse in Italian. Of course, that is a topic for another article.