Weber Carburetors
Hello, I am a distributor for PMO carburetors and Weber parts. Please use our secure shopping cart at www.youroil.net to place an order.
Valuable PMO info is available at --PMO Info
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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.
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.
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
Where:
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:
499/1000=.499
5900/1000=5.9
.499*5.9=2.9441
√ 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 911handbook.com
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 info@youroil.net. 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: http://www.youroil.net
Or if you’d like to review the Test Results, Program Description or Info about where Contaminates may originate, please visit: http://members.rennlist.com/oil
I hope you find this information useful!
