Jetting recommendations, 2.4SS: Zenith? Weber?

[teenchy]

Cross-posted from Pelican, and alluded to here: https://www.early911sregistry.org/fo...g-displacement

My engine build is under way, being done by a very reputable builder in the region. The story thus far:

Stock 2.2T -> 2380 (2.4) via 87.5 Nickies x 66 crank. 10.5:1 JE pistons, will be shimmed down to 9.8:1 such that can stay single plug.

Case has received the necessary upgrades: squirters, oil bypass mod, shuffle pins, ARP bolts.

Heads ported to S specs.

Cams were to have been 2.2Es, but since heads ported will likely end up with Solex/mod-Solex/DC30.

Ignition is stock CDI box rebuilt by Partsklassik and a Partsklassik coil.

A sport muffler of some sort (jury still out on Dansk, Porsche Classic, Scart).

The engine builder has suggested I source a set of Weber 40IDAs to replace the Zenith 40TINs. I'd like to think the Zeniths can be appropriately jetted, that any Webers I bought would still require rejetting, and that the cost savings from rejetting the Zeniths can be reapplied elsewhere.

I've searched here and on Pelican, read and reread every thread on topic. The appropriate jetting would appear to fall within a range:

Idle: 55, 57, 60
Mains: 135, 140, 145, 150, 155, 160
Air correction: 170, 180
Venturis: 32-34mm (builder recommends no larger than 32)

Have seen numerous references to the 1970s-era Stoddard "sport kit" which included 34mm venturis and other jets, but the size of the jets is never mentioned.

I know this is going to take some fettling but I'd be grateful for any recommendations. Thanks for your input.

[targatuesday]

Listen to your mechanic. Don’t spend all that $$$ on your engine then hamstring its potential with a fuel system designed for emissions not performance.

I spent a ton of time researching this topic on the bird forums and all the info is there. Rebuilt my Zeniths using bigger venturi, plugged the emissions ports, changed all the jets, used the correct o-rings and gaskets vs the wrong items included in the rebuild kits. Had my mechanic drive it and dial it in. In the end I could definitely feel a difference. The engine did feel like it had slightly more torque across the rev range. But we never were able to eliminate 100% of the popping and spitting so many complain about with Zeniths. Especially on deceleration downhill or at hot idle. The design of the zeniths also mean fuel percolation can be an issue with ethanol fuel on a warm day. Carb fires are no fun.

In the end I switched to PMO Carbs and only wished I had done it sooner. The car is such a pleasure to drive. Great performance, no pops or crackles.

If you were doing a stock rebuild or a concours restoration it might make sense to rebuild the zeniths. They are not bad carbs but even with mods don’t seem to be the sporting choice. With the engine you are building you would probably be much happier with Weber or PMO carbs.

[RickWhite]

+1 My zeniths are sitting on the shelf. Webers are so good, I am pulling off EFI & PMO combo and using IDAs as an interim step before a rebuild & back to MFI in a year or two. The Webers are very easy to tune to the specific engine, and to tweak depending on driving conditions. I am pretty sure of all the stuff we buy for the cars, the webers will hold their value the best if you are worried about the "investment."

[Richy]

From my experience , carburetors will not be the issue , the engine will never run as it could because of the large deck height required to lower the C.R. from 10.5 to 9.8 . The difference will be small . Typical I think , build a bigger displacement engine that is not really optimized , but everyone is satisfied because it will run relatively strong .

[teenchy]

From my experience , carburetors will not be the issue , the engine will never run as it could because of the large deck height required to lower the C.R. from 10.5 to 9.8 . The difference will be small . Typical I think , build a bigger displacement engine that is not really optimized , but everyone is satisfied because it will run relatively strong .

You think 2.2 -> 2.4 is that much bigger? There has to be some element of cost containment here. Next thing you know we're talking twin plug and adding another $5-8K to the build.

[Richy]

No , I do not think 2.2 - 2.4 is that much bigger , i'm concerned about adding shims to lower the compression ratio , not the best way to lower the CR , the deck height is too big and the squish pattern in the combustion chamber is compromised . Your "very reputable " engine builder should have a better plan ,

[teenchy]

No , I do not think 2.2 - 2.4 is that much bigger , i'm concerned about adding shims to lower the compression ratio , not the best way to lower the CR , the deck height is too big and the squish pattern in the combustion chamber is compromised . Your "very reputable " engine builder should have a better plan ,

Not naming names. Tell me what you would do.

[Richy]

Get the correct pistons from CP or Wossner or have the J&E piston domes machined .

[72targa]

Title updated per author

[Alaskan911]

Get the correct pistons from CP or Wossner or have the J&E piston domes machined .

Would the 9.5:1 JE Pistons be a better fit for this build? Only reason I ask is that this motor "recipe" is very similar to what I have in the works.

J