Venturi and Jet Advice for Weber 40?

[N882928]

I am getting some conflicting info. on the set up for my weber 40s. I know nothing is a hard rule with carbs because of all of the factors that impact them but I need a good starting point. So, the engine is a 2.7 from a 77, CIS cams stock. One source says 32 venturis, 135 main jets, 180 airs, 55 idles and F26 emulsion tubes. The second source says 34 venturis, 145 or 150 main jets, 165 or 170 airs, F3 emulsion tubes.
Some other info on the engine that may or may not matter here. Compression will be 9.5:1. Head ports measure 35. Manifold to head measures about 32. So, what is the right set up for this engine? Or where should I start? I was leaning toward the 34 venturis because the head ports are 35. Any thoughts? Thanks.

[Steve@Rennsport]

The first question I have is: What size are the intake manifolds you have?

These should be 40mm at the top and 35mm at the bottom.

Jetting depends on many things, but two in particular here:

1) Engine RPM range (camshaft profile)

2) How the car will be used; street or track.

With the mild CIS cams, you want the top suggestion someone offered you. The second one is appropriate for a 2.7 with Solex or S cams; neither of which you can use with stock CIS pistons.

[N882928]

Thanks for the reply Steve. To clarify:

1. The manifolds I have are 40 at the top but only 32 at the bottom. It did occur to me that I should open those up to match the heads. Any concerns with doing that if I use the old grinder to open the bottom of them to 35?

2. The pistons are not stock CIS. They are 9.5:1 JEs. Is this a factor in selecting jets or is the cam the main factor? The car is to be mostly street with an occasional track day.

[Steve@Rennsport]

Originally Posted by N882928

Thanks for the reply Steve. To clarify:

1. The manifolds I have are 40 at the top but only 32 at the bottom. It did occur to me that I should open those up to match the heads. Any concerns with doing that if I use the old grinder to open the bottom of them to 35?

2. The pistons are not stock CIS. They are 9.5:1 JEs. Is this a factor in selecting jets or is the cam the main factor? The car is to be mostly street with an occasional track day.

1) Those manifolds need to be opened up to 35mm, however it should be done by someone with a flow bench to ensure that all branches are flowing the same amount of air. All PMO manifolds are done in this fashion.

2) If your JE's are the standard double valve relief design, you will not have any issues with a streetable camshaft. Just be certain they are not CIS-type domes.

[N882928]

Thanks Steve. Flow bench for the manifolds is a good point. Do you guys do that or do you have any recommendations?

Yes the JEs are double relief. They are definitely not CIS, those are out and on a shelf.

[Steve@Rennsport]

Originally Posted by N882928

Thanks Steve. Flow bench for the manifolds is a good point. Do you guys do that or do you have any recommendations?

Yes the JEs are double relief. They are definitely not CIS, those are out and on a shelf.

I do those sorts of things, however I'm just buried at the moment and would not be able to get it done for awhile.

Where do you live?

[Lapkritis]

For the 2.7 CIS, the PMO manual calls for:

Chokes 32
Mains 135
Air 180
Idle 55
Emulsion F26

The setup with F3 emulsion is for the 2.7E or S.

Set fuel pressure for 4-5lbs when warm, ignition advance set to 32-35 or less... usually lower is safer to start and to get to know your fuel source/quality.

[Lapkritis]

I read a bit more and I see you're running S sized ports. You might be closer with the F3 emulsion setup then. There's one way to find out...

[Steve@Rennsport]

Originally Posted by Lapkritis

I read a bit more and I see you're running S sized ports. You might be closer with the F3 emulsion setup then. There's one way to find out...

FYI,....emulsion tubes are designed & configured for intake manifold vacuum characteristics. Port size doesn't change that unless they are massively bigger and thats a totally different problem,...

F26's are appropriate for CIS cams, SC cams, and often times, 964 cams.

F3's are the correct item for E, Solex, S, and Mod-S cams.

F24's are the ticket for most race cams such as 906, GE-80, RSR Sprint, GE-100, etc.

[Lapkritis]

Originally Posted by Steve@Rennsport

FYI,....emulsion tubes are designed & configured for intake manifold vacuum characteristics. Port size doesn't change that unless they are massively bigger and thats a totally different problem,...

F26's are appropriate for CIS cams, SC cams, and often times, 964 cams.

F3's are the correct item for E, Solex, S, and Mod-S cams.

F24's are the ticket for most race cams such as 906, GE-80, RSR Sprint, GE-100, etc.

Port diameter difference is 3mm... my experience is they like to run a bit fatter on the mixture. I imagine we would be hard pressed to nail it perfectly on a keyboard but these are good starting points. Properly adjusting the linkage for the banks to match should be given as much detail in my opinion.

[1QuickS]

3.5 psi is the factory specification for fuel delivery pressure. Also, check float levels when engine is running.

[N882928]

Thanks to all for the input.

Steve, we are in Columbus Ohio. If you know anyone in the area who has a flow bench and could help please let me know. The car won't be on the road this winter anyway.

So in summary it sounds like the 32 venturi set up for CIS cams is the better starting point. I'll start gathering those parts...