Power Piston & Primary Metering Rods

Rochester Quadrajet Power Piston & Primary Metering Rods 
Technical Reference Info 
By QuadrajetParts.com

Power piston/spring/rods/bushing- 
Under actual operating conditions the Quadrajet power piston alters the primary fuel flow by raising and lowering the metering rods in the primary jets. Primary Metering Rods connect to the power piston and are retained by a retaining spring.  The rods have a tapered end that fits in the primary main jet. The tapered portion have 2 sizes that you need to be concerned with, the top of the taper will be the larger diameter of the rod. The large portion of the rods taper is used during light throttle driving giving a leaner fuel mixture for better economy. The bottom portion of the rods taper will be a smaller diameter; this smaller diameter of the rod is used during heavy throttle acceleration, giving a richer fuel mixture.

There are 3 basic types of primary rods and they are not interchangeable:

1974 and earlier rods

1975 and later rods

Computer controlled rods.

All 1974 and earlier primary rods have .026” (power) tip diameter with different larger diameters, the difference in primary rods is the upper portion which is the larger diameter. For example a 44B rod will have an upper diameter of .044” and a tip of .026”. 1975 and later primary rods can have either a .026” or .036” power tip.  Also note that the retaining spring comes in two styles, the #2581 which is used with 1974 and earlier rods and the #2582 which is used on 1975 and later rods.  Computer controlled (Electronic) carburetor primary rods have .026” (power) tip diameter with different larger diameters; the difference in CCC primary rods is the upper portion which is the larger diameter. For example a 62U rod will have an upper diameter of .062” and a tip of .026”.

Don’t interchange early and late rods! 
Although they look similar, the 1975 and later rods are .080” shorter than the 1974 and earlier rods. Do not mix early and late rods.  

Primary Metering Rod Specs:

EARLY RODS
-  "A" Single Taper ,026” power tip (1974 & Earlier) 2.465" to 2.480" (Overall length)

     Originally used on 1965 thru 1967 Quadrajets, (OK to use "B" rods as a substitute.)

-  "B" Double Taper ,026” power tip (1974 & Earlier) 2.465" to 2.480" (Overall length)

      Originally used on 1968 thru 1974 Quadrajets, also used on later year truck and marine applications.  (OK to use "A"    rods as a substitute.)

-  "C" Triple Taper ,026” power tip (1970 Oldsmobile and maybe others) 2.465" to 2.480" (Overall length)

LATE RODS

-  "E" (1975 & Later) 2.395" to 2.400" (Overall length)

-  "J"  Medium Length Taper .026" power tip (1975 & Later) 2.395" to 2.400" (Overall length)

-  "K" Full Length Taper .026" power tip (1975 & Later) 2.395" to 2.400" (Overall length)

-  "L"  Short Length Taper .026" power tip (1975 & Later) 2.395" to 2.400" (Overall length)

-  "M" .036" power tip (1975 & Later) 2.395" to 2.400" (Overall length)

-  "P"  Full Length Taper .026" power tip (1975 & Later) 2.395" to 2.400" (Overall length)

-  "S" Log Length Taper .036" power tip (1975 & Later) 2.395" to 2.400" (Overall length)

Electronic "CCC" Rods (Computer Command Control) also known as Feedback-carburetors

-  "U" .026" power tip (1981 & Later) 2.124"  (Overall length)

Primary metering Rod selection- 
When selecting primary rods you need to aware of the following parameters: 
• Early (1965-74) rod or Later (1975 & up) rod (Non CCC carburetors)
• CCC (1981 & up) rod (ONLY CCC carburetors)
• Later Style Power tip:  .026”/small diameter or .036”/large diameter size 

When using a 44B rod (1965-74 style), during light throttle driving, the .044” portion of the rod should be in the primary jet, giving a leaner Air/Fuel mixture. When you accelerate, the power piston raises and the .026” portion of the rod is then located in the primary jet, giving a much richer fuel mixture. The motive force for the power piston movement is engine vacuum and the resisting force is the tension of the spring located underneath the piston. If vacuum is too low for the spring tension then the power piston will be pushed upwards, which raises the rods in the jets and richens the fuel mixture. This is normally what happens at WOT (wide open throttle) acceleration. However, if the spring tension is mismatched to the engine vacuum (camshaft) or if the metering rods are mismatched to the jet sizes, then performance will suffer.

Power Piston Retaining Bushing 
Most power pistons have a plastic retainer bushing at the top of the power piston. Years ago we used to stake the main body casting surrounding the power piston to help the retainer stay in place, but now there is a much better way. Use a replacement split bushing retainer (#3622). It holds great, its easy to install and eliminates the chance cracking the power piston bore casting. Some early style power pistons did not use the plastic retaining bushing; they were designed to use a metal retainer ring, part # #3621.   Note: Power piston retaining bushings are ONLY used on Non-CCC carburetors.

Testing a power valve- 
You can easily test your Rochester Quadrajet power valve spring to see if it is too strong. At idle, the engine vacuum needs to keep the power valve fully seated (down position, lean position). Take a normal plastic drinking straw and place it in the top vent of the airhorn, by leaning the straw on a slant you should be able to feel the power piston as you press the piston down and up. With the engine shut off, place a mark with a marker on the straw showing to top height of the vent. When you start the engine, the straw should pull down and stay down. If you see the straw move up and down, you know that the power piston valve spring is too strong for the vacuum produced by the engine at idle and needs to be replaced with a lighter (weaker) spring. Also if the power piston is moving up and down at idle, the engine rpm will also be unstable and will rise and fall. Power piston spring assortment kit #3601 contains 4 color coded springs of various tensions, one that will work for almost any application.

#3601 Power Piston Spring Colors:

Yellow ---- Silver ---- Black ---- Blue

<--Weakest to Strongest Spring--->

Spring / Inches of Vac. / Length
Yellow: / (2-7) / 0.950"
Silver: / (4-9) / 1.020"
Black: / (5-9) / 0.950"
Blue: / (8-12) / 1.450"

 Rochester Quadrajet Secondary Metering Rod Chart

Power tip lengths determine when full enrichment occurs: 
---S (Short power tip) Full enrichment occurs at 90 degrees air valve opening. 
---M (Medium power tip) Full enrichment occurs at 80 degrees air valve opening. 
---L (Long power tip) Full enrichment occurs at 70 degrees air valve opening.

ID Code / Power Tip Dia. / Tip Length 
----- Richest ------- 
AD 0.0222 S
CB 0.0279 S
BV 0.0297 S
DX 0.0300 M
DC 0.0300 M
CC 0.0300 L
BY 0.0317 M
DU 0.0337 S
DS 0.0337 M
DG 0.0337 M
DF 0.0337 M
CF 0.0337 M
DM 0.0393 S
BP 0.0397 S
BW 0.0397 M
CJ 0.0397 S
BM 0.0397 M
BG 0.0397 M
BH 0.0397 M
AX 0.0397 L 2600-AX
BB 0.0397 L
BF 0.0397 S
BJ 0.0397 S
BK 0.0397 S
CA 0.0397 M
CS 0.0397 S
CM 0.0397 M
CE 0.0410 M 2600-CE 
BN 0.0410 S
BL 0.0410 S
BE 0.0410 S
DA 0.0440 M 2600-DA
CY 0.0440 M
AU 0.0527 L
CK 0.0527 L 2600-CK 
CV 0.0527 L
AH 0.0527 M
EJ 0.0540 S
BU 0.0547 S
EA 0.0547 S
CR 0.0547 S
CX 0.0567 L
EK 0.0567 S
ED 0.0567 M
EF 0.0567 S
EG 0.0567 S
DZ 0.0567 S
DH 0.0567 S
CP 0.0567 M
CN 0.0567 S
CH 0.0567 S
CD 0.0567 L
BZ 0.0567 L
BA 0.0567 M
AP 0.0567 M
AZ 0.0567 L
AY 0.0567 L 2600-AY 
AL 0.0567 M
AJ 0.0567 M
AV 0.0567 M
AR 0.0567 M
AK 0.0567 S
EE 0.0570 M
DV 0.0570 S
DR 0.0570 M

BD 0.0577 M
BC 0.0581 M
DN 0.0584 S
BT 0.0597 M
EC 0.0610 M
DJ 0.0647 S
CL 0.0667 L 2600-CL 
AT 0.0667 L
DL 0.0679 S
EH 0.0686 S
DP 0.0686 S
BX 0.0697 S
AN 0.0697 S
DB 0.0697 S
DT 0.0700 M
CT 0.0774 M
AS 0.0774 M
CG 0.0774 M
DY 0.0874 L
DW 0.0874 L
DE 0.0874 M
BR 0.0897 L
AW 0.0905 M
BS 0.0947 L
CZ 0.0947 L
EB 0.0947 L
DK 0.0997 L
DD 0.1047 L 

----- Leanest -------