[ankostis]

The distance the metering rod tip travels in the jet is determined by the hanger. Each hanger is stamped with a single letter – B through V. The B hanger will pull the rods out the greatest distance, the V hanger the least. If your metering rods have a long power tip, it’s not necessary to have a more aggressive hanger (like a B) since the tip will have achieved its maximum metering long before the door is fully open. In fact, it could be an advantage to keeping the mixture somewhat on the lean side in that short amount of time the air valve is transitioning to full open. This is where a good wide-band oxygens sensor and a data logger could be very useful in setting up your Q-jet to be just right.

[fluffy]

The Q–jet’s small primary venturis combined with those highly efficient booster means very small changes in throttle will affect fuel metering, starting with a tapered primary metering rod that moves up and down within a primary jet. The rods are positioned by a hanger that moves with the primary metering piston that is pulled by engine vacuum. Underneath that piston is a spring. At low load, the high manifold vacuum overcomes the spring pressure and pulls the hanger and metering rods deep into the primary jets. This reduces fuel flow, reducing the amount of fuel delivered. As you step on the throttle, load increases, vacuum decreases, and spring pressure pushes the rods out of the jets, increasing fuel flow.

[Michael33]

Of course, this also means heavier part-throttle load may make the engine run lean — which is not good. A car with a taller rear gear (like 2.73:1, or numerically lower) may not like a softer spring as well as a car equipped with a deep gear like a 3.73:1 (numerically higher) because of load. This is why you should make small changes and evaluate them over multiple driving events before making your next change.

[Myst]

Before we get started, we will assume the engine is in good condition and the entire ignition system and ignition curve is dialed in. Make sure the engine is not suffering from a vacuum leak; you can’t tune a carburetor properly if everything’s not sealed up. When tuning an engine with an unknown ignition curve, start your tuning efforts with the ignition system first. Carburetors are often blamed for problems that are caused by ignition system issues. [links]

[EvgenijM86]

If you supplied JET with the proper specs on your car, their long experience with the Q-jet combined and all the different engine and camshaft combinations would have your Q-jet probably very close. But let’s say you have a decent Q-jet you’d like to try on a completely different engine and transmission combination. It would probably be best to give JET a call just to get some basic tuning parameters, but let’s say the carb works well and you’d like to get it dialed in a little closer. more

[Sweet]

Secondary metering changes are far easier than on the primary side because we don’t have to remove anything except the air cleaner to make changes. The secondary metering jets are fixed so the variables to experiment with are the metering rods, the hanger, and the secondary opening rate. Q-jet secondary metering rods are tapered with three areas: a power tip, the tapered section, and the straight section at the top that occupies the jet for the first 4 degrees of air valve opening. The most important part of the rod is the power tip. This is the area of the rod still in the fixed jet when the air valve door is 100 percent open.

[Marsianin]

Re: Rochester Quadrajet tuning.

[alystair]

If your not sure that you have the correct gaskets, go to http://www.quadrajetparts.com, you can search for your rebuild kit based on yoour carb # and verify your gaskets with the detailed pictures of the gaskets in the kits. here

[laczoka]

Before you even fire the engine, make sure the choke is adjusted properly. more

[CalypsoL]

Tony's Quadrajet Tuning Tips and Articles

[lancommy101]

AIR VALVE ADJUSTMENT
A full 90 degreeing opening can inhibit fuel flow from the tubes. By 90 degrees I'm talking about the rear portion of the flap, the front will go past 90 degrees on a correctlyset up carb. The very best q-jets, Ram Air, Super Duty and Ho models were set so that the secondary flaps leading edge is 1.130" to the edge of the opening in the airhorn when fully open. Going beyond this can cause a lean spot as it effects how well fuel is pulled from the tubes and the lower edge of the flap nearly blocks the openings. here

[MCNARS]

The primary barrels of the carburettor are tiny compared to most four-barrel designs, but this is what gives the Quadrajet its gas milage edge. In contrast, the secondary barrels are huge, providing a performance edge. During normal driving the primary barrels are adequate for cruising speeds. The beast comes out when the pedal is depressed further. The secondaries open and there is the slightest amount of delay as the accelerator pump richens the mixture. A Quadrajet carburettor car is often distinguishable from other cars by the sound of the engine as the secondaries open. There is a moment of quiet followed by a large increase in exhaust volume, sometimes described as a 'booming' noise. more

[freetx]

They're attached to the power piston (which fits into the hole you dropped the spring in). No matter what set you have, go out and get the following:

[Eivind Nag]

3. Connect the diaphram and see if the bog goes away. If not, tighten in 1/8 turns until it does. [links]

[sturle]

Quadrajets - most stock 4-barrel Olds's come equipped with one, yet most of us are reluctant to touch them, even when it comes to a simple rebuild. Why? Mostly because it is the least talked about, least understood carburetor mounted on cars. 4M, 4MC, 4MV, M4M; all these silly numbers, lots of levers, vacuum lines, and other gizmos all contribute to make that thing look overly complicated and impenetrable.

[BTCbh]

Now turn the carb over, install the rest of the screws and turn them in according to the sequence in the Chassis Service Manual. here