Weber Carburetors


40IDA Weber Carbs


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This link provides jet sizes for various Porsche engines.  These are intended to be a starting point in selecting jets – the actual sizes you will need may be different than this table suggests as each engine is in a different state of tune, is driven differently & is maintained differently in different environmental conditions.  Also,  some sizes can be calculated using the formulas at the bottom of this page.

Weber Jet Settings


PMO has provided written instructions about adjusting and tuning the Webers which I found to be very helpful.


- Weber Tuning, Adjusting and Synchronizing instructions - 

There are 2 sets of instructions, each slightly different but with the same result.

The 1st set consists of 3 pages and is for a conversion from Fuel Injection to Webers.

The 2nd set  contains 5 pages and is useful when converting from Webers to PMOs.  However, I found it useful to read the info in both sets of instructions.


A diagram of the 40IDA-3C1 carb is also provided below.


40IDA-C1 Diagram

CIS to Weber Page 1

CIS to Weber Page 2

CIS to Weber Page 3

Weber Carb Adjust Page 1

Weber Carb Adjust Page 2

Weber Carb Adjust Page 3

Weber Carb Adjust Page 4

Weber Carb Adjust Page 5


Misc Comments From PMO






We recommend NGK BP5ES Spark Plugs.

The Spark Plug Gap should be .036”


The Main Jet size can be decreased .05MM for each 2500’ in Elevation increase.


Unfortunately, a previous owner may have installed the incorrect jets.  Some do this just because they don’t know what the proper size is or they assume the Webers they just purchased on eBay will have the right size jets in them.


If your 911 engine has been modified or if you want to make sure the Venturi’s installed in your Weber carbs. are the correct size, the following formula can be used to calculate the proper Venturi size:



20√ V/1000*N/1000


 V = the size of one cylinder in cc’s.

 N = the RPM where peak HP is reached.


Then, it’s √ of V/1000 X N/1000 *20.

For example, a 3.0L Porsche 911SC would be:




√ of 2.9441=1.715838

Times 20=34.31676 or a 34mm venturi is called for.


The above formula was kindly provided by Bruce Anderson from his excellent book, “Porsche 911 Performance Handbook” which is an excellent addition to any Porsche lovers bookshelf.  Please visit for information on his book as well as his excellent training classes.


To calculate a starting point for the Main Jets, use the Main venturi Size /0.21 such as 34/0.21= 162.  In this case the 160’s or 165’s should be tried to determine which works best for this particular engine.



The Air Correction Jets can be calculated by using –

Main Jet Size X 1.1 

So, a Main jet of 135 would suggest a AC Jet of 135 X 1.1 or 138.5 making a 140 Air Correction Jet an appropriate starting point.


If you have any comments or questions about these formulas please feel free to contact me using [email protected].  Additionally, if you have any formulas or information you believe other Porsche owners may find useful please let me know.


If you would like to Purchase Weber Carb. Parts or a Oil Analysis Kit, please visit:

Or if you’d like to review the Test Results, Program Description or Info about where Contaminates may originate, please visit:



I hope you find this information useful!