built 3.0L Weber IDA403C jet sizing?

[911ducktail]

Hi

carbs are kinda magic to me, I'm looking for some help figuring what I need to change after rebuilding a 3.0L

1981 3.0L
10.5:1 JE Pistons
964 Cam
Converted to twin plug

the carbs were on it when it was a "stock" (no mods other than webers)

Idle - 60
Main - 145
Venturi is marked 4.5 - the threads Ive searched all seem to size them in MM?
Air Corrector - 180
Emulsion tubes - F3
pump jets - 50


also not exactly sure how to size the choke? The bottom inner diameter is 40mm, the top inner diameter is 34mm

pic of venturi

[1QuickS]

The main venturi is in the bore of the throttle body and are typically identified as to size when viewed from the top of the carb. PMO venturis are ink-marked so this can be removed with little effort. If so then you will need to measure inside diameter of the main venturi best performed after removal of the venturi. There is one grub screw that is safety wired that secures one main venturi. Since you have the auxiliary (booster) venturi removed it is easy to then remove the grub screw & draw out the main venturi. Your 145 main fuel jet implies a 34mm main venturi which is what you measured, the 40mm diameter is the throttle bore size which is 40mm.

Your new engine will probably want jetting like this:

• 36mm main venturis
• 155 main fuel jets
• F3 emulsion tubes
• 170 main air correction jets (180s are OK)
• 60 idle fuel jets
• 50 accelerator squirter jets

36mm main venturis are the largest that 40IDA3C Webers can run, larger venturis will need to be installed in 46mm Webers.

The picture you provided shows the auxiliary (booster) venturi which is the type you want. Be sure the hollow wing is installed to be aligned with the fuel coming through the emulsion tube well. It is easy to reverse these during installation and eliminate main circuit fuel supply for that cylinder.

Go to my website for basic tuning info.

[911ducktail]

Thank you for the information Paul

I noticed Pelican doesnt sell jets/venturis - they dont even seem to sell the rebuild kit they used to. I ended up buying the ones I have from weber in italy. Considering Italy is in quarantine is there a recommended vendor to purchase jets and the venturi from?

[kent olsen]

Pierce Manifolds

[1QuickS]

PMO gaskets are superior to any others on the market. They also make all the jetting & venturis for the Webers. The float needle valves were pressure checked by me for PMO and I was seeing 8% failure rate until recently. Go to this web site to purchase:

Weber Carb Parts, PMO Carburetors and Parts, Porsche Posters, engine oil testing

The kits are sold with pressure tested needle valves and standard gaskets but the main body gaskets by PMO are sold separately and are worth getting.