The Quadrajet's primary venturis are in use at all times and it is crucial that the primary metering circuit is properly calibrated for all engines. For smaller engines (like the slant six) where the secondary air valves rarely open, it is especially important to get this right. Although the primary jets interchange between all years of manufacture, there was design change in 1975 and the primary metering rods from 1965-1974 (PNs starting with 70) are 0.080" longer than those from 1975-1980 (PNs starting with 170). Since the Quadrajet has been out of production for decades now and it is not as popular as modern squarebore aftermarket carburetors for performance applications, performance parts aren't as available as they used to be.
Compared with the old Carter Strip Kit (metering rods & jets), jetting a Quadrajet is much simpler because all of the metering rods have the same tip (power setting) diameter (power setting) of 0.026". Only the newer M-series metering rods (ie, 48M, 49M, ... 55M - truck applications & Edelbrock Quadrajets) and the 50D have a tip diameter of 0.036" along with Edelbrock's . The part throttle cruise section of the metering rod comes in a variety of diameters and tapers (transition between between lean and rich).
Quadrajet Tuning Steps
Lars Grimsrud (LG) is very knowledgeable about Quadrajets and has a Quadrajet tuning paper that is available upon request. He may be reached via the Corvette Forum and I would ask him for his paper before trying to tune your Quadrajet. Assuming you are starting with a freshly rebuilt carburetor, LG's Quadrajet tuning procedure has the following steps:
- Verify that that the float level is correct. Adjust as required.
- Determine main jet size.
- Determine main metering rod size.
- Set secondary air valve spring initial tension.
- Determine secondary metering rod hanger length
- Determine secondary metering rod size.
- Adjust secondary air valve spring tension.