Setup for Zenith TIN 40 carbs on 2.7RS spec engine

Hi,

I'm trying to find a starting point for a correct setup for a set of carburetors that is going to be installed on my new engine. Have searched this forum and others for a while, but I'm still pretty much in the dark. I suppose Zeniths and Webers should have corresponding setup.

Engine:

Modified 2.4T block
2.7 NiCaSil sylinders, 9.5:1 comp.
36 mm inlet and exhaust ports.
S cams

I have 34 mm venturis, but from I have read so far I realize that they may be too small.

In this thread there is a tip from Paul Abbott for a 2.7 engine with S cams and 9.5:1 compression ration, but the porting size is not stated.

https://www.early911sregistry.org/fo...-40-IDA-3C-3C1

"Carbs are set up based upon displacement and at what RPM the engine makes its peak HP; a 2.7 with S cams and 9.5:1 compression would like something like this: 34mm main venturis; 150 mains; F3 emulsion tubes; 55 idles; 180 air corrections."

Advice will be much appreciated.


Einar Irgens

Hi Einar,

Running a modified 7R case with S cams and TIN 40.

Port venturi 48

Venturi 35,5
Mains 160
Idle 60

BR

Dirk

36mm is what I would recommend today, 34mm suggestion was ten years ago and is still valid but 34mm does provide a bit more mid-range power, you will still get to 7300 RPM from low RPM with 34mm venturis, perhaps a little quicker than with 36mm venturis but 36mm venturis would be quicker from 5000 RPM to 7300 RPM.

There are two versions of Zenith progression patterns for the ROW Zeniths and Zeniths are not identified as to which version you have. There are two more versions unique to the USA market, four versions in all. You may see a midrange transition lean issue and more pronounced with 36mm venturis. This may be tuned out and most effectively by adjusting the size of the air correction jets used for the low speed circuit. This is a modification as these jets are not tunable as delivered.

I recommend you select which main venturis size you prefer and then see if you have a 3000 RPM lean transition and then tune that out.