Power Jetting the Edelbrock Performer Carburetor

The Art & Science of Carb Jetting and Set-Up


Now that you have read, fully understood, and adjusted your carburetor according to Section 1 and Section 2 it is time to make the adjustments that separate the backyarder from the perfectionist.

Following the instructions in the previous sections will provide a fairly good compromise, but the accuracy of a stop watch or "it feels faster" is questionable.  This is where analyzing the air/fuel mixture with a simple oxygen sensor can result in huge gains.

To fine tune the jets and metering rods and get the maximum power and throttle response you have to be able to identify the air / fuel ratio during idle, cruise, power, and acceleration is by using an AF meter.  You don't have to buy an expensive AF ratio meter, my solution is to install an oxygen sensor in the header collectors and measure the voltage with a DVOM (digital voltmeter).  

An Oxygen sensor is a chemical generator. It is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no Oxygen in the exhaust manifold, a voltage is generated. The output of the sensor is usually between 0 and 1.1 volts. The ideal mixture for gasoline this is 14.7 parts of air to one part of fuel, but to protect the engine and maximize power and throttle response this level needs to be varied slightly. When the engine has more fuel than needed, all available Oxygen is consumed in the cylinder and gasses leaving through the exhaust contain almost no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen is 0.2 to 0.7 volts. 

The sensor does not begin to generate it's full output until it reaches about 600 degrees F. Prior to this time the sensor is not conductive. It is important to locate the sensor close to the engine and exposed to a full exhaust stream.  The easiest solution is to install the sensor into the stock location, if your exhaust manifolds or headers have a bung, if not it is easy to have a bung welded into the header collector or front exhaust pipe.  The O2 sensor is constantly in a state of transition between high and low voltage.  It is important to remember that the O2 sensor is comparing the amount of Oxygen inside and outside the engine. If the outside of the sensor should become blocked, or coated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible.

 The following reference chart identifies the normal voltage an oxygen sensor generates in relationship to the air/fuel mixture.

Minimum Voltage

Maximum Voltage

Air / Fuel Ratio

0.0 Volt

0.1 Volt

17.0 : 1

0.1 Volt

0.2 Volt

16.0 : 1

0.2 Volt

0.3 Volt

15.5 : 1

0.3 Volt

0.4 Volt

15.0 : 1

0.4 Volt

0.5 Volt

14.7 : 1

0.5 Volt

0.6 Volt

14.6 : 1

0.6 Volt

0.7 Volt

14.5 : 1

0.7 Volt

0.8 Volt

14.2 : 1

0.8 Volt

0.9 Volt

13.2 : 1

0.9 Volt

1.0 Volt

12.5 : 1

The Edelbrock calibration reference charts used in the previous sections to set-up the carburetor tend to take large "steps" between setting. Because the fuel metering on the Edelbrock Performer carb use primary main jets and a stepped metering rod inserted into them to provide two stages of fuel delivery just changing a metering rod can dramatically alter the fuel ratio.  Calculating the correct combination of jet and rod can be confusing (the total fuel the jet will flow is determined by the jet area minus the rod area).  To simplify this task I have calculated all of the jet / rod combinations in the table below.  

Using a DVOM connected to the O2 sensor warm up the engine to operating temperature. Take voltage reading at idle, wide open throttle, part throttle cruise, and part throttle power modes.  The ideal voltages you should obtain are:

Carb Circuit

O2 sensor Voltage

Air / Fuel Ratio

Idle

N/A

Use lean drop adjustment*

Wide Open Throttle

0.9 - 1.0 V

12.5 : 1

Part Throttle Acceleration

0.7 - 0.8 V

14.0 : 1***

Part Throttle Cruise

0.25 - 0.4 V

15.5 : 1**

* It is unreliable to use an O2 reading to adjust the idle mixture, use the
   lean drop procedure in the manual for adjusting idle mixture. 
** Most carbs are set far to rich at part throttle cruise, a leaner mixture
   here will provide excellent throttle response and increase fuel 
*** Ensure that the mixture at WOT and Part Throttle Acceleration are
    greater than 14.0 : 1 or serious engine damage can result

After you have recorded the O2 sensor readings refer to the table below.  The table identifies the jet size, the metering rod diameter, and the potential fuel flow (from leanest to richest).  Note your current jet rod combination and find a combination from the table that will provide a leaner or richer mixture determined by your test runs.  Using this table you can very precisely alter a singular carb circuit easily and accurately.

 
 

Jet Size

Metering Rod Diameter

Jet - (minus) Rod = Area

Lean

to

Rich

.086

.075

.0013909

.086

.073

.0016235

.089

.075

.0018033

.086

.071

.0018495

.086

.070

.0019603

.089

.073

.0020356

.086

.068

.0021771

.092

.075

.0022297

.089

.071

.0022619

.089

.070

.0023724

.092

.073

.0024623

.086

.065

.0024904

.089

.068

.0025892

.095

.075

.0026703

.092

.071

.0026883

.086

.063

.0026914

.086

.062

.0027896

.092

.070

.0027991

.095

.073

.0029027

.089

.065

.0029028

Lean

to

Rich

.086

.060

.0029812

.092

.068

.0030159

.089

.063

.0031038

.098

.075

.0031250

.095

.071

.0031289

.086

.058

.0031666

.089

.062

.0032020

.095

.070

.0032395

.086

.057

.0032569

.092

.065

.0033292

.098

.073

.0033576

.089

.060

.0033936

.086

.055

.0034328

.100

.075

.0034361

.095

.068

.0034563

.092

.063

.0035302

.089

.058

.0035790

.098

.071

.0035836

.092

.062

.0036284

.100

.073

.0036686

Lean

to

Rich

.089

.057

.0036693

.086

.052

.0036850

.098

.070

.0036944

.095

.065

.0037698

.092

.060

.0038200

.089

.055

.0038452

.100

.071

.0038947

.098

.068

.0039112

.086

.049

.0039229

.095

.063

.0039708

.100

.070

.0040054

.092

.058

.0040054

.095

.062

.0040690

.086

.047

.0040739

.092

.057

.0040957

.089

.052

.0040971

.100

.068

.0042222

.098

.065

.0042245

.095

.060

.0042606

.092

.055

.0042716

Lean

to

Rich

.089

.049

.0043353

.086

.042

.0044232

.098

.063

.0044255

.095

.058

.0044460

.089

.047

.0044860

.098

.062

.0045237

.092

.052

.0045238

.100

.065

.0045356

.095

.057

.0045363

.095

.055

.0047122

.098

.060

.0047153

.086

..037

.0047334

.100

.063

.0047366

.092

.049

.0047617

.100

.062

.0048348

.089

.042

.0048353

.098

.058

.0049007

.092

.047

.0049127

.095

.052

.0049642

.098

.057

.0049910

Lean

to

Rich

.100

.060

.0050264

.089

.037

.0051458

.098

.055

.0051669

.095

.049

.0052023

.100

.058

.0052118

.092

.042

.0052620

.100

.057

.0053021

.095

.047

.0053531

.098

.052

.0054191

.100

.055

.0054780

.092

.037

.0055722

.098

.049

.0056570

.095

.042

.0057024

.100

.052

.0057301

.098

.047

.0058080

.100

.049

.0059681

.095

.037

.0060128

.100

.047

.0061190

.098

.042

.0061573

.098

.037

.0064654

.100

.042

.0064683

.100

.037

.0067786

 

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