Rochester Quadrajet Power Piston & Primary Metering Rods
Technical Reference Info
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 2 basic types of primary rods and they are not interchangeable. 1974 and earlier rods and 1975 and later 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 Q2581 which is used with 1974 and earlier rods and the Q2582 which is used on 1975 and later rods.
Dont interchange early and late rods!
The 1975 and later rods are .080” shorter than the 1974 and earlier rods. Do not mix and match parts.
Primary Metering Rod Lengths - (Tip to Tip)
Early Rods, "A" & "B" (1974 & Earlier) 2.465" to 2.480"
Late Rods, "K" & "M" (1975 & Later) 2.395" to 2.400" (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
• Power tip: small diameter nor 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 (Q3622). 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 # Q3621.
Primary Metering Rod Differences
Year Style Taper Power tip dia.
1965-67 “A” Single .026”
1968-74 “B” Double .026”
1975 & later “K” .026”
1975 & later “M” .036”
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 postion, 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 pistonis moving up and down at idle, the engine rpm will also be unstable and will rise and fall. Power piston spring assortment kit #Q3601 contains 4 color coded springs of various tensions, one that will work for almost any application.