Weber tuning suggestions needed from the experienced ones!

[Cornerlot]

Looking for suggestions, I'm rather a Weber novice. I don't currently have selection of Weber tuning parts, and I need to refine what I have without buying a large selection of parts in advance. I would like to get a suggestion on what jets, emulsion tubes, etc. would match the way the engine is set up now. The current 40IDA carbs are right off a stock 2.4L and seem lean low-end (random pops out the exhaust and intake below 2500 RPM).

Currently displaces 2.8L with a 2mm overbore, ported twin plug heads and ignition, 10.8-1 forged pistons, Elgin ‘Super C-2’ cams, and SSI exhaust through GT3 style muffler. Mid-year 911 with a 915 trans, 99.9% street driven.

Thanks.

Steve

[Trackrash]

Probably should call PMO. He is the authority on Weber setups.

A starting point would be:
34 mm venturis
F3 emulsions
135 mains
145 air correctors

It would help if you post a shot of your carbs. So we know the exact model you have.

[T77911S]

this is THE place to go.
talk to
1quicks

Performance Oriented

[zedsn]

Paul Abbott is the man when it comes to weber IDA carbs. Give him a call at performance oriented and he will be more than happy to help you on parts needs and advice. He is a great and helpful guy.

[1QuickS]

Hi Steve,

That is one strong street engine, especially in a 914/6. To provide good power without sacrificing throttle response:

• 36mm main venturis
• 155 main jets
• F3 emulsion tubes
• 170 main air correction jets
• 60 idle jets

I recommend also to have a look at my Tech1 web page for tuning information:
Performance Oriented

[toddu]

I have 60 idle jets in my 46 PMOs. Engine is 3.2ss, mod-s cams, 10.5:1. Runs strong, but seems a bit rich cruising around town. I admit that I need to run through tuning. Is the advantage of larger idles the transition to the mains?

Todd

ps - don't mean to hijack

[Cornerlot]

Originally Posted by 1QuickS

Hi Steve,

That is one strong street engine, especially in a 914/6. To provide good power without sacrificing throttle response:

• 36mm main venturis
• 155 main jets
• F3 emulsion tubes
• 170 main air correction jets
• 60 idle jets

I recommend also to have a look at my Tech1 web page for tuning information:
Performance Oriented

Thanks Paul, this will be real useful. When I pulled the intakes I noticed the CIS injector notch in the intake ports was right at the edge of the gasket coverage and may have contributed a slight vacuum leak. I will get the conversion insulators for this online.

This setup engine is in my '77 911S; my 914/6 has MFI on its 2.7L. That one needs sorting too.

Steve

[1QuickS]

PMOs have the ability to adjust idle air correction jets to tune the idle circuit for mixture strength during progression. Larger idle jets affect this mixture across progression while the idle air correction jets perform similarly to the main air correction jets; larger idle air correctors lean out progression toward transition and shorten duration of progression; smaller idle air corrections provide the opposite effects. I would be surprised if you could use smaller idle jets than 60 but possibly you could tailor your progression with adjusting idle air corrections.

If you are not fouling your plugs then that is good. If you try 55 idle jets & low speed operation suffers then you have a strong indication that 60 idle jets are required. I recommend trying some 55 idle jets & go for a test drive, it won't cost much or take too long to have the data you need and once that test is complete you can re-install the 60s & be back to where you are now.

[1QuickS]

Hi Steve,

I assumed 914/6 from your avatar.

The OEM intake manifolds do not reliably cover the CIS notch. PMO manifolds can be purchased that do support this interface reliably. Alternately you can have some aluminum plates made that will reliably support gaskets in this area but you will probably need longer studs to install. Also, using PMO insulators is recommended to minimize fuel percolation but without support these will age & deflect which will subject you to air leaks in this area. Richard Parr will not sell insulators to you if he thinks you plan to use OEM intake manifolds on CIS heads & use his insulators as a "adapter" due to his liability to engine damage from air leaks.

[Cornerlot]

Originally Posted by 1QuickS

Hi Steve,

I assumed 914/6 from your avatar.

The OEM intake manifolds do not reliably cover the CIS notch. PMO manifolds can be purchased that do support this interface reliably. Alternately you can have some aluminum plates made that will reliably support gaskets in this area but you will probably need longer studs to install. Also, using PMO insulators is recommended to minimize fuel percolation but without support these will age & deflect which will subject you to air leaks in this area. Richard Parr will not sell insulators to you if he thinks you plan to use OEM intake manifolds on CIS heads & use his insulators as a "adapter" due to his liability to engine damage from air leaks.

This engine does already have the PMO/REP manifolds, but no provision for covering the CIS notch. Not sure how old they are, maybe new ones have this change. ************ lists the insulators for this.

Steve

[1QuickS]

I recommend you get the type designed for CIS heads, I think Richard sells 12 varieties of manifolds.

By the way:

• PMO stands for "Porsche Mail Order"; appropriate for when there was no Internet
• REP stands for "Richard E. Parr"

[toddu]

Originally Posted by 1QuickS

[*]PMO stands for "Porsche Mail Order"; appropriate for when there was no Internet

Well, I think it's still appropriate today :-). Joking aside, Richard is a great guy!

Todd