Tuning Twin downdraft carb manifold

[Twink80]

This just popped on ebay UK today

 

[Slotman]

Would certainly like to own that! But doubtfull they would ship internationally...I've purchased parts internationally before though and it always went smoothly paying thru western union..

 

[Twink80]

Would certainly like to own that! But doubtfull they would ship internationally...I've purchased parts internationally before though and it always went smoothly paying thru western union..

Well if you dont ask……
Eurocarb is another good supplier of Webber and Dellorto spares and tuning parts

 

[Twink80]

Would certainly like to own that! But doubtfull they would ship internationally...I've purchased parts internationally before though and it always went smoothly paying thru western union..

This is the Webber carb manual. Its got a great section on the OE fit to lots if car makes and the details of the chokes, jetting, emulsion tubes etc

 

[Twink80]

Would certainly like to own that! But doubtfull they would ship internationally...I've purchased parts internationally before though and it always went smoothly paying thru western union..

I messaged the seller, no problem if you want it shipped to the US!
I’m not sure what import taxes would apply, if you can get the seller to label as a gift or sample maybe no taxes?

 

[124BC1]

This is the Webber carb manual. Its got a great section on the OE fit to lots if car makes and the details of the chokes, jetting, emulsion tubes etc

There are lots of books/manuals available on Weber carburettors, probably more on this make of carb. than any other.
I've got the Haynes manual, here's a few others, including some official Weber factory training (I was trained on Weber carbs in my early years):-



Also, Weber used to provide information sheets (in 4 languages) for each of their carb models listing which cars this carb was originally fitted to, explaining with diagrams, how each circuit in that carb operates, plus giving the dimensions of each carb and the operating arcs of throttle butterflies and linkage levers. Here's a couple :-



Another couple of interesting books for those interested in tuning/modification of engines:-



I'm somewhat reluctant to recommend technical manuals to members on here, those who might benefit the most from such manuals probably already have them or similar and those who don't yet have, might not 'get their money's worth' if they did buy them - I don't want to be encouraging others to spend and possibly be left feeling they have wasted their hard-earned money!

 

[Twink80]

There are lots of books/manuals available on Weber carburettors, probably more on this make of carb. than any other.
I've got the Haynes manual, here's a few others, including some official Weber factory training (I was trained on Weber carbs in my early years):-

View attachment 463325View attachment 463326View attachment 463327View attachment 463328View attachment 463329

Also, Weber used to provide information sheets (in 4 languages) for each of their carb models listing which cars this carb was originally fitted to, explaining with diagrams, how each circuit in that carb operates, plus giving the dimensions of each carb and the operating arcs of throttle butterflies and linkage levers. Here's a couple :-

View attachment 463330View attachment 463331View attachment 463332View attachment 463333

Another couple of interesting books for those interested in tuning/modification of engines:-

View attachment 463335View attachment 463334

I'm somewhat reluctant to recommend technical manuals to members on here, those who might benefit the most from such manuals probably already have them or similar and those who don't yet have, might not 'get their money's worth' if they did buy them - I don't want to be encouraging others to spend and possibly be left feeling they have wasted their hard-earned money!

A great collection!
Aside from balancing the carbs and an initial stab at chokes, emulsion tubes and jets, you are better off going to a rolling road dyno so the carbs can be properly calibrated. It gets expensive very quickly guessing which components to change x4 and the engine needs to be fully loaded, wide open throttle.
I did the basics and the car ran but I learned a huge amount from a trip to the dyno. You get a baseline run then any changes you can see the effect on power and torque. Mostly its wide open throttle not transients. You should get a power and torque curve which you can bore your mates with over a beer!

 

[124BC1]

Agreed, setting up on a rolling road is definitely the way to go! (if you can afford it, that is).

In the old days (i.e. what is now the last century, so makes me feel very old), they used to recommend doing 'plug chops' after a run on w.o.t., to check mixture strength, often on the public road - never had much success trying this method, had some close shaves though.

In the mid '70's, I spent some time in a motorcycle workshop that had a 'rolling road', it was a revelation to see what could be achieved when setting up the carbs on Japanese 4 cyl. m/cycles and how they went afterwards.

Here's another couple of books (front and back covers this time to show the contents) from my 'library':-

 

[124BC1]

------------------------ You should get a power and torque curve which you can bore your mates with over a beer!

Or you can go all modern, install aftermarket F.I., then you can bring your laptop to the pub and bore your mates by showing them your 'maps' -
just don't spill any beer on the laptop, it won't like it

 

[124BC1]

Would certainly like to own that! But doubtfull they would ship internationally...I've purchased parts internationally before though and it always went smoothly paying thru western union..

Here's an inlet manifold for you, closer to home (@MrFiat.com):-

And you could then get the dual Weber 40 IDF carbs from VickAuto.com

Or there's a full conversion kit (manifold plus dual Weber 40 IDF's) at VickAuto.com

 

[Slotman]

Wow! Thanks for the tip! I've seen the kit but hard to pull the trigger for $1600..the 2 carbs and the intake are more reasonable and are something to consider! Thank you

 

[124BC1]

Wow! Thanks for the tip! I've seen the kit but hard to pull the trigger for $1600..the 2 carbs and the intake are more reasonable and are something to consider! Thank you

Please note, I'm not trying to spend your money..
Personally I don't think that conversion is worth the money versus what you might gain.

Often you end up with a car that is less pleasant to drive, needs more re-tuning and uses more gas, plus is easy to flood the engine.

The way I see it, is that when you are driving in stop/go or slowly moving traffic, you need an engine that is smooth and progressive, not a wild animal that gets unleashed every time you press the loud pedal, and when you're cruising on the highway at a constant speed, you just need sufficient power to maintain your speed and hopefully get good gas mileage, the dual twin choke carb set-up is unlikely to meet any of these requirements...

I suspect that some of those who've paid for such a conversion are trying to convince themselves and others that it was worth the cost. Probably only worth it if you are also significantly modifying the rest of the engine. Iirc, the U.S. spec engines always produced less power that the Euro spec ones - if I wanted more power, I'd think I'd try to duplicate the Euro spec. (e.g. U.S. spec 2 litre engine = c. 85 bhp, Euro spec 2 litre engine = 120 bhp...).

As regards the cost of such a conversion using either new parts or paying what others now ask for old second-hand parts (in dubious condition), I'm astonished. Back in the day when this dual set-up was the stock set-up on the 124 t/cam over here, many people paid to have their cars converted to the earlier single single twin-choke carb because they were fed up with them, easy to flood when starting from cold, even easier to flood starting when hot, going out of tune, surging terribly when accelerating if a little out-of-tune (some used to refer to the car 'kangarooing' down the road), not helped by the low ratio 1st gear coupled with the lowered final drive ratio on these cars and the poor fuel economy.

And the same people didn't want to retain the dual carbs and manifold set-up, so lots of them were junked, nobody wanted them, I gave several complete set-ups to friends for free, just to get rid of them, I was literally tripping over them in my storage shed. And when I say 'complete set-ups', I mean including the large so-called 'turtle-back' (because of it's shape) air cleaner unit, just try finding one of these nowadays (and it's likely asking price..).

My advice, stick with that nice new upgraded carb that you have already fitted.

 

[Slotman]

Wow! I always appreciate your wisdom and advice !! A very good read!

 

[Twink80]

Please note, I'm not trying to spend your money..
Personally I don't think that conversion is worth the money versus what you might gain.

Often you end up with a car that is less pleasant to drive, needs more re-tuning and uses more gas, plus is easy to flood the engine.

The way I see it, is that when you are driving in stop/go or slowly moving traffic, you need an engine that is smooth and progressive, not a wild animal that gets unleashed every time you press the loud pedal, and when you're cruising on the highway at a constant speed, you just need sufficient power to maintain your speed and hopefully get good gas mileage, the dual twin choke carb set-up is unlikely to meet any of these requirements...

I suspect that some of those who've paid for such a conversion are trying to convince themselves and others that it was worth the cost. Probably only worth it if you are also significantly modifying the rest of the engine. Iirc, the U.S. spec engines always produced less power that the Euro spec ones - if I wanted more power, I'd think I'd try to duplicate the Euro spec. (e.g. U.S. spec 2 litre engine = c. 85 bhp, Euro spec 2 litre engine = 120 bhp...).

As regards the cost of such a conversion using either new parts or paying what others now ask for old second-hand parts (in dubious condition), I'm astonished. Back in the day when this dual set-up was the stock set-up on the 124 t/cam over here, many people paid to have their cars converted to the earlier single single twin-choke carb because they were fed up with them, easy to flood when starting from cold, even easier to flood starting when hot, going out of tune, surging terribly when accelerating if a little out-of-tune (some used to refer to the car 'kangarooing' down the road), not helped by the low ratio 1st gear coupled with the lowered final drive ratio on these cars and the poor fuel economy.

And the same people didn't want to retain the dual carbs and manifold set-up, so lots of them were junked, nobody wanted them, I gave several complete set-ups to friends for free, just to get rid of them, I was literally tripping over them in my storage shed. And when I say 'complete set-ups', I mean including the large so-called 'turtle-back' (because of it's shape) air cleaner unit, just try finding one of these nowadays (and it's likely asking price..).

My advice, stick with that nice new upgraded carb that you have already fitted.

Great article
The quickest and easiest upgrade fir US engines is to fit Lancia Beta pistons to raise the compression ratio. They are hard to come by and Im not sure if they are still made new. They are cast so only ok for stock ish engines. Anything higher needs forged which I think are still available but pricey!
You can also change the cams for std Lancia as they hsve slightly higher lift.
I have fitted twin 40 idfs which sound great. As I dont have wild cams fitted the engine has huge amounts of torque and pulls strong to 6500rpm, about the rpm limit fir cast pistons. Its got great driveability, is smooth and progressive. Fuel economy depends how you drive it and ranges from 30mpg (UK galls) to much less giving it full beans! The roads and speed limits dont alliow thst too often!
Guy Croft wrote an excellent book on tuning the Twincam and I met up with him to discuss what I was proposing.
I wanted ti buiild the best performing 2lit engine using standard Fiat/Lancia parts. I had seversl Lancia Beta coupes in my youth 1.6 and 2.0, they were fantastic engines do the 2.0 Fed spec FI spider engine was a real disappointment, it wouldnt rev past 5000rpm and felt lifeless, not at all appropriate for a Sports Car

I have since learned that you can keep the fuel injection and raise the compression ratio and you will get most of the European oerformance back (more squeeze, same fuel). You just have to find the pistons! Equally there is no point putting Twin 40’s on a low compression engine if you want more performance

Renovating my Spider has been a 15 year plus job and done in stages as I found the oarts, had the time and knew what I was trying to achieve

My other bit of advice would be to try and write a spec of your ideal car that reflects how you intend to use it. Eg you don’t need massive brakes, stiffer springs and poly bushes if your thing is enjoying a cruise round on a sunny day. Spend the time fuxing rattles and squeaks and improve sound deadening in the floor to make the driving experience more refined.

 

[Twink80]

Agreed, setting up on a rolling road is definitely the way to go! (if you can afford it, that is).

In the old days (i.e. what is now the last century, so makes me feel very old), they used to recommend doing 'plug chops' after a run on w.o.t., to check mixture strength, often on the public road - never had much success trying this method, had some close shaves though.

In the mid '70's, I spent some time in a motorcycle workshop that had a 'rolling road', it was a revelation to see what could be achieved when setting up the carbs on Japanese 4 cyl. m/cycles and how they went afterwards.

Here's another couple of books (front and back covers this time to show the contents) from my 'library':-

View attachment 463339View attachment 463340 View attachment 463341View attachment 463342

I have a copy of the Race and Rally car source book with a blue cover, great book.

 

[124BC1]

It wasn't just the lower compression ratios used, nor the very different (and inferior as regards engine performance) valve timing used on U.S engines, as @Twink80 has already said, but also little things like setting the ignition timing at 5* btdc @ idle, compared to 10* btdc in Europe - interesting that their early cars used 2 sets of contact breaker points, one set which was switched in for starting (these being timed @ 10* btdc) and then the other set was allowed to take over for normal running (these being set @ 5* btdc). I wonder if the air injection into the exhaust manifold might have caused the crack to occur in smahaley's manifold - perhaps the car had a misfire or over-fuelling issue and the air injection resulted in the damage - cast iron exhaust manifolds are usually quite long-lasting (ime).

Here's another few books form my 'library' (front and back covers to show the contents) - they describe the differences between U.S. and Euro. spec. Fiat engines and lament that the Americans, despite being by far the main market for the 124 Spider, were 'short-changed' as performance due to emissions regulations :-

 

[124BC1]

Great article
The quickest and easiest upgrade fir US engines is to fit Lancia Beta pistons to raise the compression ratio. They are hard to come by and Im not sure if they are still made new. They are cast so only ok for stock ish engines. Anything higher needs forged which I think are still available but pricey!
You can also change the cams for std Lancia as they hsve slightly higher lift.
I have fitted twin 40 idfs which sound great. As I dont have wild cams fitted the engine has huge amounts of torque and pulls strong to 6500rpm, about the rpm limit fir cast pistons. Its got great driveability, is smooth and progressive. Fuel economy depends how you drive it and ranges from 30mpg (UK galls) to much less giving it full beans! The roads and speed limits dont alliow thst too often!
Guy Croft wrote an excellent book on tuning the Twincam and I met up with him to discuss what I was proposing.
I wanted ti buiild the best performing 2lit engine using standard Fiat/Lancia parts. I had seversl Lancia Beta coupes in my youth 1.6 and 2.0, they were fantastic engines do the 2.0 Fed spec FI spider engine was a real disappointment, it wouldnt rev past 5000rpm and felt lifeless, not at all appropriate for a Sports Car

I have since learned that you can keep the fuel injection and raise the compression ratio and you will get most of the European oerformance back (more squeeze, same fuel). You just have to find the pistons! Equally there is no point putting Twin 40’s on a low compression engine if you want more performance

Renovating my Spider has been a 15 year plus job and done in stages as I found the oarts, had the time and knew what I was trying to achieve

My other bit of advice would be to try and write a spec of your ideal car that reflects how you intend to use it. Eg you don’t need massive brakes, stiffer springs and poly bushes if your thing is enjoying a cruise round on a sunny day. Spend the time fuxing rattles and squeaks and improve sound deadening in the floor to make the driving experience more refined.

As regards availability, I'll repost some websites in the U.S. that I provided recently that are major stockists of124 Spider parts - some of these stock pistons of various compression ratios (cast or forged), some can even supply rebuilt engines, cyl. heads, and manual transmissions:-

There are also 124 Spider parts suppliers in the U.K. (an internet search will find most of these), but here's a couple in Europe that are worth perusing:-

As regards different cams, I've seen mention that the cams fitted to the 124 Sport 1438cc engine were better that those fitted to the larger capacity engines, but there's not much difference in valve timing figures and minimal if any difference in lift - I don't have any info on the Lancia cams, which is a little funny as I worked for a Fiat+ Lancia dealer and worked on lots of Beta's (Saloons, Coupes, Spiders, HPE's) and remember them fondly - terrible pity that they withdrew from the U.K/Ireland and didn't stand firm against their critics, could have pointed out that other makes of cars rusted just as badly, if not worse. Oh, well.
I did play around with some Piper cams with reasonably good results, Also tried having just the base circles ground down on standard cams to increase lift (i.e. leaving the valve timing more or less unchanged) without much noticeable improvement but a bit of a palaver setting them up to work - had to even machine the cam-boxes to set the valve clearances, sufficiently thick shims not being available, might have overdone the grinding of the base circle.. Did a bit of reading on cam design/valve timing/ acceleration/deceleration rates of valves and the problems that can arise. It's a massive and complicated subject and imho modifying cams is probably best left to 'the experts'. Also read the famous book - The Scientific Design of Intake and Exhaust Systems - made me feel very ignorant!

You seem to have gotten your dual twin-choke carb set-up to work ideally, so well done to you and your 'rolling-road' specialist, unfortunately, many others have been disappointed, probably due to incorrect set-up/jetting, inability to adjust/balance the carbs themselves or find someone capable of doing so, which is why I urge caution to those thinking of spending quite a bit of cash on such a conversation to be aware of the possible downsides.

I was going to purchase Guy Croft's excellent book shortly after it came out, but having read a friend's copy, I decided it was too much race/competition modifications oriented for my needs at the time so didn't buy. Given how much these books are now being advertised for sale at, I wish I had bought one when it was c. Euro 38 (iirc, books are dearer here in Ireland than in the U.K.), saw a couple of ad's from the U.S. on eBay? recently @ $500 - 600!!

I fully agree with your advice about not overdoing things on a car, just set it up to suit your intended usage and include the little modifications that make it a nice place to be - I believe classic car ownership and driving, should be a pleasure not an uncomfortable ordeal.

Interesting that you mention 'polybushes', there was a feature/write-up in Practical Classics magazine in the U.K. (April 2025 issue) about a guy that restored a Fiat 124 Spider from the 'States (this car also had a broken oil pump and a dented sump, sound familiar to you?), he had fitted 'polybushes' and now intended to revert back to standard bushes... In the same magazine issue, the editor has now gotten 2 x 124 Spiders and intends to make one good car out of 2, there's quite an amount of body repairs needed, it'll be interesting to see how things progress in future issues. I'd scan and upload the relevant pages from the magazine but I don't want to possibly contravene copyright rules or upset anyone at the magazine. But issues of this magazine can be viewed digitally/online, (will require a digital subscription c. $62/year for U.S. ,13 issues) if anyone is interested. To order a digital subscription :-

Practical Classics Magazine

Practical Classics is an original classic car magazine that has been providing help and inspirations for car enthusiasts since 1980. Become a subscriber today!

www.greatmagazines.co.uk

 

[124BC1]

Sorry, I forgot to repost the list of 124 Spider parts suppliers in the U.S. (there's probably others I haven't found yet!) :-

Midwest-Bayless Italian Auto

Midwest-Bayless Italian Auto

www.midwest-bayless.com

FIAT | Parts for classic Spiders, X1/9, and more! | Vick Autosports

FIAT 124 124 Sport Spider (1968-’78) 2000 Spider (1979-’82) 124 Sport Coupé 124 Sedan & Special 124 Special T (DOHC) 124 Familiare Wagon X1/9 FIAT X1/9 (1974-’82) Bertone X1/9 (1983-’88) 850 850 Spider 850 Coupe 850 Sedan & Special 128 & Yugo 128 Sedan & Special 128 Rally 128 Coupé & 3P 128...

vickauto.com

Fiat 124

MrFiat

mrfiat.com

Auto Ricambi FIAT 124 Spider Parts - New and Classic FIAT Spider Parts

Auto Ricambi FIAT 124 Spider Parts - New and Classic FIAT Spider Parts. We also carry parts for the 2012-on FIAT 500

autoricambi.us

__________________________________________________________________

Several U.K. suppliers of 124 Spider parts can be found using an i/net search

_________________________________________________________________

Some EUROPEAN suppliers of 124 Spider + other Fiat parts :-

Fiat 124 Spider Classic Car/Oldtimer Spare Parts - riAuto

Fiat 124 Spider Classic Car/Oldtimer spare parts for engine, drivetrain, chassis, electrics, interior, car body and fluids

www.riauto.ch

Holtmann_NiedergerkeGmbH

Ersatzteile klassischer italienischer Oldtimer Automobile Marken: Fiat, Abarth, Autobianchi, etc. Spezialist für: Fiat 124 - 500 - 600 - 850

fiatparts.de

Parts catalog Fiat 124 Spider AS - BS - CS - CS0 - CS1 - DS

Parts catalog - Carparts Fiat 124 Spider AS - BS - CS - CS0 - CS1 - DS

www.martinwillems-webshop.eu

https://oldtimer-shop.eu

Arnold Classic Shop

Fiat Oldtimer Ersatzteile im Arnold Classic Shop kaufen. Parts und Zubehör für Fiat 124 Spider und Coupe, Fiat 850, Fiat 600, Fiat 500, X 1/9, 131,…

shop.arnold-classic.com

https://shop.arnold-classic.com/en/fiat/124-spider/technics (<This 2nd site has 124 Spider parts lists/diagrams, so very useful, it also gives piston dome sizes for the oversize pistons).

 

[Slotman]

Wow, very helpfull!...I just ordered from ricambii yesterday for the first time getting the timing belt kit, and I treated myself to something I had been wanting, a "test pipe" haha to "test" my exhaust system with out the catalytic converter...my converter looks to be in excellant shape so I will preserve it in event the test needs to be over...I have ordered from Bayless, and a few times from vicks and had a great experience every time so hoping ricambi will be The same! I do appreciate the European contacts as well, thanks for sharing the list!

 

[Twink80]

Sorry, I forgot to repost the list of 124 Spider parts suppliers in the U.S. (there's probably others I haven't found yet!) :-

Midwest-Bayless Italian Auto

Midwest-Bayless Italian Auto

www.midwest-bayless.com

FIAT | Parts for classic Spiders, X1/9, and more! | Vick Autosports

FIAT 124 124 Sport Spider (1968-’78) 2000 Spider (1979-’82) 124 Sport Coupé 124 Sedan & Special 124 Special T (DOHC) 124 Familiare Wagon X1/9 FIAT X1/9 (1974-’82) Bertone X1/9 (1983-’88) 850 850 Spider 850 Coupe 850 Sedan & Special 128 & Yugo 128 Sedan & Special 128 Rally 128 Coupé & 3P 128...

vickauto.com

Fiat 124

MrFiat

mrfiat.com

Auto Ricambi FIAT 124 Spider Parts - New and Classic FIAT Spider Parts

Auto Ricambi FIAT 124 Spider Parts - New and Classic FIAT Spider Parts. We also carry parts for the 2012-on FIAT 500

autoricambi.us

__________________________________________________________________

Several U.K. suppliers of 124 Spider parts can be found using an i/net search

_________________________________________________________________

Some EUROPEAN suppliers of 124 Spider + other Fiat parts :-

Fiat 124 Spider Classic Car/Oldtimer Spare Parts - riAuto

Fiat 124 Spider Classic Car/Oldtimer spare parts for engine, drivetrain, chassis, electrics, interior, car body and fluids

www.riauto.ch

Holtmann_NiedergerkeGmbH

Ersatzteile klassischer italienischer Oldtimer Automobile Marken: Fiat, Abarth, Autobianchi, etc. Spezialist für: Fiat 124 - 500 - 600 - 850

fiatparts.de

Parts catalog Fiat 124 Spider AS - BS - CS - CS0 - CS1 - DS

Parts catalog - Carparts Fiat 124 Spider AS - BS - CS - CS0 - CS1 - DS

www.martinwillems-webshop.eu

https://oldtimer-shop.eu

Arnold Classic Shop

Fiat Oldtimer Ersatzteile im Arnold Classic Shop kaufen. Parts und Zubehör für Fiat 124 Spider und Coupe, Fiat 850, Fiat 600, Fiat 500, X 1/9, 131,…

shop.arnold-classic.com

https://shop.arnold-classic.com/en/fiat/124-spider/technics (<This 2nd site has 124 Spider parts lists/diagrams, so very useful, it also gives piston dome sizes for the oversize pistons).

Thanks
Great list of suppliers and really useful to compare prices