Weber Carbs and Fuelish Notions


reliable source of info
Regarding fitting vintage Weber carbs in the normal IR (isolated runner) arrangement whereby there is no common plenum allowing multiple cylinders to share air / fuel mixture...... And a single throttle bore supplies a single cylinder via an IR intake runner...... There are lots of ways to go (both sidedraught and downdraught carbs) with V8 engines of 300 - 350 cu. in. displacement and to a lesser degree, 383 cu. in. versions as well. The problem with fitting Webers to V8 engines of greater displacement is that the size of throttle bore and choke (primary venturi) required for a true IR design for larger cylinder displacement is larger in most cases than the sizes of readily available Weber carburettors.

As an example, if redline RPMs for a street driven V8 engine are in the 6,000 RPM range (give or take a couple of hundred RPMs either way), a 5 litre (300 cu. in.) engine would require a 34mm choke (primary venturi) size, a 5.3 litre (327 cu. in.) engine would require a 36mm choke size, a 5.7 litre (350 cu. in.) engine would require a 38mm choke size and a 6.3 litre (383) engine would require a 42mm choke size. Weber recommends that ideally, the throttle bore be sized at 1.25 times the size of the choke (primary venturi...... Although there is some leeway and in actual practice, a ratio as of 1.166 has been successfully utilized on the OEM Weber 42 DCNF carbs having 36mm chokes as fitted to 1970s Aston Martin Lagonda engines. Ideally, a 34mm choke size would require a 42mm throttle bore, a 36mm choke a 45mm throttle bore, a 38mm choke a 48mm throttle bore and a 42mm choke a 52mm throttle bore. And in the case of my 7 litre (427 cu. in.) Chevy small block stroker engine, a 46mm choke and 58mm throttle bore would be required in such a scenario. Now those are ballpark baseline choke (venturi) sizing for street driven engines in a mild state of tune where mileage and drivability are balanced against all out performance. Different engine combos will require adjustments to fine tune to the particular engine combo. Better breathing heads and intake manifolds, higher compression, larger cams, less restrictive exhaust and the like make more power by enhancing an engine's volumetric efficiency and ability to breathe. Mods such as that will require increased choke (venturi) and throttle bore sizing to accommodate the increased airflow capabilities of modified engines along with corresponding changes in jetting. A true max effort racing engine and / or an engine turning in excess of 6,000 RPMs would require even larger choke and throttle bore sizing than the generic ballpark sizes I stated above, though the tradeoff would be a deterioration of low RPM throttle response and street manners will suffer accordingly if part throttle operation results in airflow moving too slowly through the increased carb sizing.

But there are ways around that. One is to utilize a common plenum rather than a true IR design. In theory, that will not give you the same performance potential, but will allow the use of smaller, more readily available and less expensive carbs, progressive linkage with multiple carbs if need be and its a lot easier to tune once properly jetted and set up. For example, according to Weber's sizing chart, a street driven 383 engine operating at 6,000 RPMs and under utilizing 4 carbs on either a single plenum or dual plenum crossram or an old style log manifold requires only 4 chokes of 29mm (1.14") sizing to reach that RPM. And they can be in the form of dual 2bbl carbs or quad single bbl carbs on straight (non-progressive) throttle linkage, with additional carburettion available in the form of secondary carbs operated by progressive linkage to enhance acceleration if desired. You can buy such Weber ICH single bbl carbs having 29mm chokes for $140.00 each brand new. Or you can buy half as many Weber 2bbl carbs such as 40 DCNF or 38 DGAS having 30mm chokes at $335.00 each.

Current Weber carbs available new with out of the box choke (main venturi) sizing and jetting listed.....

But unless hidden from view by the air cleaner, a set of 4 ICH single bbl carbs atop a common plenum won't necessarily have the visual impact that a Weber IR system has. Which is fine by me because I believe in function over form. But if you are building an induction system for someone else...... Someone who may not hesitate to over-carburrete their engine in order to achieve a certain 'look'...... Or who may over-cam their street driven engine so it will have a certain 'sound' at idle...... Then you have a problem in the making. Because those folks typically are not going to have the patience nor the skill to keep a multiple carb system tuned and synchronized properly. And over-carburretion and / or over-camming will only make the engine harder to tune and less streetable.

The other extreme is one of several variations that I will explore with my 427 Chevy small block stroker engine. And that is using a single Weber 2bbl carburettor (2 chokes and 2 throttle bores) to supply a single cylinder via each IR intake runner. In other words, one of the variations I will construct is an IR intake that mounts 8 Weber 2bbl carbs (1 carb for each cylinder). Yeah, I know...... That sounds pretty crazy and its certainly not the most cost effective nor practical nor easily tuned set up by any means. But over the years, I have scrounged 8 rebuildable Weber 42 DCNF downdraught carbs and I want to see if the 'engine is an air pump' theory holds true in this instance. I have calculated choke and throttle bore sizing and jetting based upon several unrelated mathematical computations and have cross referenced those calcs against Weber sizing and jetting charts for engines having cylinder volume 1/2 that size (only I will be using both chokes of a 2bbl carb to supply a single cylinder twice their displacement)...... And any way I figure it...... In theory, the 42 DCNF carbs should work with corrected jetting and either 30mm or 32mm chokes (31mm is optimal for my engine in theory). That's the good news. The bad news is that in a real world operational environment, I must deal with the difficulty of constructing an IR setup where the carburettor mounting allows the fuel bowls to be positioned towards the front of the engine as they were designed to be...... For an engine having paired intake runner layout such as the 1st generation Chevy small block...... Rather than equally spaced intake runner layout such as the newer LS series GM engines and others. You see, Webers in an IR environment are designed to work on intakes mounting the carbs as close to the intake valve as is possible just as Sir Harry Ricardo related in his engine design theory back in the early 1920s. Its a totally different concept from the tuned runner designs utilizing shared plenums beneath the carbs. With an IR setup, the optimum intake runner length is figured from back of intake valve to tip of bell mouthed trumpets or extensions on the inlet side of the carb airhorn. Which is OK because in a true IR system having no plenums and having carb throttle plates positioned close to the intake valve, the only time the overall inlet tract (including carb) would be at a predetermined 'tuned length' coinciding with max. torque and max. volumetric efficiency at a particular RPM...... Would be at WOT anyway. For at all part throttle operation, even under acceleration...... An IR system (no plenum) will not be operating at 'tuned intake runner length'. And the farther away from the intake valve a Weber carb is placed in an IR design...... The less air flow (velocity), the weaker carb signal, and the more throttle lag and part throttle driveablity issues you will have.

Here are some vintage induction systems that you may find interesting. Most are for early Chrysler hemi engines. I will be exploring several of them over time in my experiments with induction for my Chevy stroker engine......

Offenhauser 5590, Edelbrock XC-8, Weiand 7291 crossram intakes......

Edelbrock X-C8 crossram intake......






The Offy and Weiand designs were quite similar to the Edelbrock X-C8, but the Mickey Thompson version differed somewhat (note carb and runner orientation and addition of cast-in balance passage between carb plenums....... ... 3aUSQ3a101




Info on the Z-28 crossram intakes with removable top plate and aftermarket versions......

Edelbrock X2 six deuce crossram for Pontiac......


Edelbrock X3 six deuce crossram for early Chrysler hemi...... ... 1222735852


Inline 4 deuce intake for early hemi...... ... 1222749378


IR manifolding (no shared plenum) mounting 8 2bbl carbs on early hemi......


Cunningham (4) single bbl intake for early hemi......



Nice homemade crossram intake for Weber side draught carbs (early hemi)......



Example of a new aluminum crossram intake for small block Chevy listed on e-Bay (new 50 or 55 DCO sidedraught carbs fitted with 42mm chokes along with appropriate jetting would work well for a street driven 383 engine)......

$550.00...... ... QQtcZphoto



Fits up to 1986, no port for vacuum, can not use larger valve covers, no large distributors, (no bracket for kickdown on auto-trans)





Example of new Weber 55 DCO cross ram setup for Chevy small block......

$2,750.00...... ... QQtcZphoto


Complete assembled cross ram conversion with new 55mm DCO Weber carbs. Fits Small block Chevy up to 1986.

New intake manifold, carbs and all new linkage.

55 DCO carbs are complete with......

42 mm venturis
F7 emulsion tubes
170 main jets
210 air corrector jets
045 pump jets
60F8 Idle jets

Will not fit Vortec style heads, cannot use tall valve covers, NO HEI, NO kickdown for automatic transmissions.


Example of new Weber 44 IDF setup for Chevy small block...... ... otohosting




Kit includes 4 new Weber 44 IDF carburetors with air by pass metering screws for improved low end response, also extended emulsion tube holders for improved main circuit metering. Brand new individual runner, aluminum intake manifold for improved throttle response. Complete with the best center mount linkage available. Billet aluminum bell crank. also , steel adjustable rods with RH & LH threads for easy adjustment. Ball studs complete with safety clips. Intake system has base line jetting for small block Chevy.

Applicable for Chevy 302 - 305 - 327 - 350

145 main jets - 175 air correctors - F-7 emulsion tubes - 50 idle jets - 4.5 aux. venturi
36 mm main venturi (choke)

Kit is assembled and ready to install. Also included is a synchrometer & Weber tuning manual.


Best regards,



Staff member
Can these systems be streetable without good filtration? All I see them selling is a screen
to keep the big stuff out.


The Grumpy Grease Monkey mechanical engineer.
Staff member

theres a dozen commercially available routes too take, all fairly expensive
the route many people take is fabricating a custom air filter and mount.
I know I took a dozen accurate measurements and purchased and modified
a couple baking pans with my welder and a drill press, an adjustable diameter drill bit
I basically built something vaguely similar to this , in concepts with a rectangle shape that fit the injection
use poster board and tape as a template , until everything lines up and then duplicate the filter box and mount,for the custom air filter , in aluminum, (one more in an endless list of reasons ...why you need a decent welder and drill press and 12" dial caliper)





Staff member
Well that ruins the look for a streetrod, but works in function. What does that assembly cost.... I see
where the seal for the stacks costs $35, but what does the filter cost???


solid fixture here in the forum
Well that ruins the look for a streetrod, but works in function. What does that assembly cost.... I see
where the seal for the stacks costs $35, but what does the filter cost???
David Vizzard used Those massive air cleaner boxes Rick.
He made real power out of almost nothing.