Initial Zenith 40 tin jetting help Options

[tornik550]

I have made several post lately about my new engine. I thought it was a 2.2l E with zenith carbs but it actually has 2.7l pistons, cylinders and crankshaft and has stock 1970 2.2l E camshafts. I am going to try to get the engine working for now with the zeniths. What would a good starting point for jets be? The zeniths currently have stock venturis.

[tornik550]

Also, I have no idea what the compression ratio is.

[roblav1]

Unfortunately, a lot of the advice we gave you concerning your engine and getting the Zeniths to work was based on you having a 2.2 liter. Now that you know what is in the engine, all that changes. Your stock venturis will be way too small. The Weber manual and equation shows the venturis should be around 34mm.

Compression is probably about 8.5:1... but your real problem in running the stock Zeniths is that your engine will stop breathing at around 5000 RPM (a swag based on the spreadsheet I made years ago).

Check on the Bird site to see what people have done to Zeniths to make them work. The main venturi size effects everything else, so you'd need to pick that first... then try to find jets and do a lot of fiddling. Here's two threads I found:

http://www.early911sregistry.org/forums/sh...d-design-review

http://forums.pelicanparts.com/porsche-911...nith-carbs.html

If I were in your shoes, I'd sell the Zeniths and buy a new set of PMO's. Expensive... but it will save a lot of hassle, time, effort, and tuning. And they can easily be changed if/when you eventually put S or Solex cams and higher compression pistons in the thing.

It's good you investigated your engine to see what you have.

[Mark Henry]

First thing is buy a wideband Air/fuel meter and install it, you can pick one up new for as low as $150. Then start messing with it.

I still wouldn't be changing anything till you know how it runs. No use pissing money away on the wrong stuff.