Thoughts on 40 DFI carb?

[ScotcH]

I finally pulled of the dual 40 DCNF carbs and intake (what a fucking pita that was!), and though I had it all figured out

I picked up an intake and a weber 40 DFI carb ... it has synchronized dual 40mm barrels. The intake is form a 1400 that has been ported to match the 40mm carb, and also port matched to the head.

However, since I have an 1800, the intake does not cover one of the coolant ports on the head, so I can't use it (without mods). I do have a couple of single plane intakes that I could port for the carb.

The question is, should I bother? I also have a 34 DMS that I can rebuild, and just slap it on there without any work ... thoughts?

Also, the dual carbs used a cable setup but without the top mounted linkage, which I'll need for the new single carb. Are these available, or junk yard only?

[lanciahf]

I think the 40 DFI might be too much carb for around town driving, probably great on a track.

Never a big fan of the americanized 34 DMS, I like the 34 ADF carbs, nice and simple.

John Erskine from DC Fiats should be able to help you with your linkage needs.

Whats wrong with the DCNF setup? I hear they are fine for street use.

[carl]

No personal experience but I hear most folks who tried a 38DGAS never got them to work right on a spider. Like your carb, this is a two barrel carb with the throttle plates synchronized to open at the same time. I would assume your 40 DFI would have the same set-up problems. 34DMS is a nice carb and bolts right on to a spider manifold since it was the stock carb for the 74 coupes and spiders.

carl

[ScotcH]

Ian, the DCNF setup was meant for a lateral mounted engine, so in turns they lean out (bowl placement I'm told). Also, it's 4x 40 mm barrels, so I don't think the 40 DFI can be any worse than that for streetability, lol

I slapped it on tonight, just to see what would happen. I ported the ~10mm spacer I had to match the DFI barrels on one side. The intake is still stock (unported), so certainly not ideal, but it fired right up, idles fine, and revs nice. Certainly a good start. It's a super simple carb, just one fuel line going in, and nothing else. No vac, and since my engine is totally stripped of any emission stuff, it was dead simple to install.

Now to score the throttle linkage and do a basic setup on it.

[AndyS]

curious where you found the 40mm dfi. I have had no luck locating one to try out on my modified 1800 engine. that was a common conversion done many years ago on many Fiats. Please let me know if you know where to find another one.

Only thing i read was they do not have an accelerator pump, so you have to set them up rich in order to get them to run right.

[manoa matt]

It "should" work, but is not idealy suited. You will likely experience some tunability issues. You may switch your driving style to suit the carb and make it work and if it gives you more power it's shortcomings are often overloked. However, it will be running lean or rich or alternating throughout its range, and either is not a good thing for the longevity of the engine. The 40 dci is a rare carb and rebuild kits may be hard to find, it does have an accelerator pump circuit though. A more suitable and tunable carb would be a Weber 36 DCD, since it is progressive and had replaceable choke tubes, but they are also rare. Baltobernie has a 36 DCD carb.

Most people are under the impression any carb can be tuned to an engine if the primary venturies are close to a stock carb size, and fine tuned with the jets. The real secret is in the secondary venturies aka choke tubes. Without the right choke tube size the vacumn signal is off and the jets are not being utilized the way they are intended.

[ScotcH]

curious where you found the 40mm dfi. I have had no luck locating one to try out on my modified 1800 engine. that was a common conversion done many years ago on many Fiats. Please let me know if you know where to find another one.

Only thing i read was they do not have an accelerator pump, so you have to set them up rich in order to get them to run right.

Guy here locally had one on his 71 spider ... he sold the car, and had a bunch of performance parts left over. I might be tempted to sell the DFI, so let me know if you're interested. This is my wife's car, and I think a stock DMSA might be the way to go. I'll be trying that one next, just have to run a return fuel like (the 40 DFI doesn't need one).

[bradartigue]

There was always a lot of talk on the DCNF having problems when being mounted inline but I don't know anyone who actually owned them and had issues. They were, compared to the brutish IDF, a work of art, compact and tune-able beyond anything the IDF was capable of. Whoever made them fuel starved must have turned the cars on two wheels...

There is no reason for a DFI to not work, but simply put two 40mm synchronized barrels dumping fuel into your engine is the very definition of "over carburetion." While I know, from working with the DGAS, that the low end is an absolute riot (you can break U-Joints...), I can assure you the engine will suffer long term, as you bathe the bores with unburnt fuel. It is also very unlikely the performance curve is very good relative to a progressive carburetor like an ADF or ADL (both of which the point of convergence of the barrels can be adjusted by the way).

Now with a modified engine, I don't know. With the right camshafts and pistons none of this is an issue, but I'd still argue it is a lot of work to just run a different Weber.

[AndyS]

just sent you an email if you decide you'd be interested in selling the 40mm dfi.

I also have a brand new, not rebuilt factory carb that works well for the street if you wanted to trade. I have to check, but I think its a manual choke DHSA 28 - 36, and has good street response with the small primary, and really kicks in the top end power when the 36mm secondary opens up. chokes on that carb are 23 primary, and a very large 27 secondary-biggest of any of the factory carbs. works with the factory air cleaners of course.

[RoyBatty]

Now with a modified engine, I don't know. With the right camshafts and pistons none of this is an issue, but I'd still argue it is a lot of work to just run a different Weber.

Hmmmmm,
"Ar[ti]gue...
argue, ar-ti-gue.....
Interesting.
Guess we can't have one without the other.

[AndyS]

I just purchased the carb and am going to give this 40 dfi a try on my performance build 1800. I will report back after it arrives and i get it on the car and spend some time sorting and rejetting it as needed.

[AndyS]

well, I got the 40 DFI here now, got it cleaned up and replaced a few gaskets. Put it on my car today. Had to modify my linkage to work with it, and will have to modify the choke cable if I end up getting it to run better than what I have on there now.

first attempt I got it to idle well, and it pulls good off the line, but then it leans out at the mid and higher rpms. I have a digital A/F gauge that I clip to the exhaust to read the a/f ratios. was seeing 15 + to 1 when I started to get on it, so I backed off before I melted something and ordered some richer jets.

Spoke to Weber expert Mike Pierce who runs Pierce manifolds about my set up. He told me basically the 32mm venturis are really too big to run right with our cars with a plenum manifold, and that is why I am needing very large main jets to get the mixture right. That said, he told me I may be able to get it to run great on my car, as the theory and practical don't always go together. Mike also said they have a powerful twin pump jet circuit that might help make up for the too large venturis. I know guys in Australia who ran these carbs on built up 124 engines with great success and swore they were the best carb they ever ran on their cars, so I am not giving up yet.

there were 195's main jets in there and 190 air corrector jets (that is what these carbs came with from the factory - they were used on some vintage Euro Fords) I ordered a few smaller size air corrector jets,(smaller air correctors richen the top end without affecting the lower rpms much) but also ordered some larger 205 main jets, as I know it needs more fuel still. Some of the Saab V4 rally guys who run these carbs needed to put 205's in there to get them to run right. will have the richer jets next week, and will report back after i changed them out and run it with the a/f meter again.

[AndyS]

ended up borrowing some weber calibrated drill bits, and made my own jets so I could try to get the A/F ratio right this weekend when I had time to work on it.

I ended up going up 1 size on the mains that were in there, from 195 to 200 (these carbs run larger main jets than most Weber's would-190 is stock). The main change I made that did the most good was going from 180 on the air correctors to 135, and that took care of the lean condition I was getting at the higher rpms.

Like any performance carb set up, its hard to get the A/F ratio to be right under all conditions (that is why EFI was invented). Right now, mine runs rich at light throttle, but has great numbers just about everywhere else -- mid 12's to mid 13's.

Now for the final verdict: This set up on a modified 1800 Fiat twin cam is incredible once you get it jetted right. The bottom end torque is better than any set up I have ever run on the car, including twin 40 IDF's and a 36 DCD. Mid range and top end are also very strong, and the power curve is very smooth with no flat spots at all, and it pulls to redline effortlessly in 2nd and 3rd gear. A few of my Fiat and Alfa friends have already driven the car as well, and were blown away by the way it pulls so hard everywhere. It is also docile enough to use in traffic, and is not like an on off switch when getting on the throttle. I am very satisfied with this set up. The intake noise also sounds incredible.

Drawbacks? Main drawback is trying to find one of these carbs anywhere, and having it be rebuildable and not totally trashed. they are like 30 years old now and were never common carburetors here in the USA especially. Came on some English Fords, Ferrari 330's and 365's, and FAZA sold them. I am also guessing with that double pumper accelerator pump and the big venturis and jets and running rich at light throttle, my gas mileage is going to take a hit. Can't have it all though, and in a weekend car like this, I will take power like this over a few MPG.

[AndyS]

few photos. wanted to show how unassuming this 40mm weber was. looks like a factory carb. Also a photo of the front of my 1967 AS series modified Spider

Car started easily on first try this morning without the choke attached. 2 pumps of the pedal, wait a couple of seconds, fired right up and idled. Took it on the freeway today, and it cruises at 75 MPH effortlessly, and still pulls hard when you step on it at that speed.

[ScotcH]

Looks great and clean setup ... maybe I should have spent a bit more time with it!