Weber Carburetor Idle Jets

[Peter-Paul]

I have a 1977 911 with a 1982 3.0 liter engine that has been converted to Weber 40 IDTP carburetors.

To make the engine somewhat run as it should, I need to back out the mixture screws between 4 and 5.5 revolutions. I read that 1.5 revolutions is more the norm.

The idle jets in the carbs are ‘60’ right now. It appears that these are not correct and that I need a larger size.
I think that normally I would go up 2 sizes, hence go to ‘70’, but I wonder if anyone more familiar with carbs could comment on what size might be more suitable?

Thanks

[Dpmulvan]

What’s the history of the carbs?

[pocv0]

Lots of variables, but you may have mains that are too small. I'm running 140 mains and 65 idle on a 2.7 with 10.3:1 CR and Elgin E cams, 36mm venturi's, 180 air correction. Mixture screws are out approximately 2 full turns.

[pocv0]

I suggest you pick up a book by Pat Braden, Weber Carburetors. Great book, invaluable source for getting your carbs sorted out.

Howard

[Dpmulvan]

Paul abbots website check out the tech sections. Performance Oriented

[Peter-Paul]

Dpmulvan, I do not have any history on the carbs other than that I cleaned and rebuild them with the jets that were in there to begin with. Anything in particular that would be important for me to check?

pocv0, you are right, quite a few variables. Thanks for the details about what you run on your engine. Not sure how that would compare with my 3.0, but it is certainly something to think about.

[Peter-Paul]

Thanks guys I will check on the website and consider the book as well.

[pocv0]

My 2.7 liter setup would be close to what you would need on your 3.0. Your carbs may have been on a 2.2 prior to the 3.0, but since you have no history you'll need to check the size of the venturi's as a starting point. 3.0 liter engines( 500cc per cylinder require 36mm venturis) for proper airflow. Then you can calculate the size of the mains needed ( 36mm venturis x 4 = 145 mains) as a starting point.

Paul Abbot is THE Weber guy for rebuilds, etc.

Howard

[stownsen914]

Here's a summary of jetting Webers for different 911 engines. Note that yours might be a little different due to being IDP instead of IDA. I believe PMO published this summary. It doesn't include 3.0L for some reason but might be helpful anyway.

[Dpmulvan]

Originally Posted by Peter-Paul

Dpmulvan, I do not have any history on the carbs other than that I cleaned and rebuild them with the jets that were in there to begin with. Anything in particular that would be important for me to check?

pocv0, you are right, quite a few variables. Thanks for the details about what you run on your engine. Not sure how that would compare with my 3.0, but it is certainly something to think about.

Theres some hidden gallery’s that could be clogged. Paul’s website covers them.

[icarp]

Weber 40 IDTP , was designed for 2.0 914-6
The carbs have to go through a special rebuild , enlarging fuel pathways
It is an undersized carb for anything over 130 hp with out the mods . EPA special
It is the same body as the IDA just undersized in all drilled pathways

Ian