How to Tweak Your Rochester Quadrajet for More Power and Better Mileage
Words And Photos: Jeff Smith
A few months back, (Read it HERE) we detailed how JET Performance restored an absolutely desperate looking Q-jet to the point it looked like it had just rolled off the Chevy parts department counter back in 1970. Since carburetors have to perform better than they look, the restoration included replacing the primary throttle shaft bushings and resealing the lower plugs that often leak fuel from the bowl directly into the intake manifold. With our carb restored, it came time to install it on our small-block.
It might be a good point here to also mention something often lost when discussing Q-jets. Every factory Rochester Quadrajet flows a minimum 750 cfm, and there were a few units rated at 800 cfm. The advantage of the Q-jet is its small primary throttle venturis with very quick-responding boosters that offer excellent throttle response, making it an excellent street carburetor. And with those small primaries, a 750 cfm Q-jet can work very well even on a tiny small-block like a 305 or even a 283.
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.
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.
For a street performance engine, don’t be afraid to put the initial timing up around 10 to 14 degrees initial. A total timing of around 34 to 36 degrees is a great place to start. Don’t forget to include the vacuum advance, as it can really improve the engine’s part-throttle response. Our mild 383 was previously dialed in with 14 degrees initial and 36 total, taking full advantage of the vacuum advance. We’re working on a separate story for tuning with vacuum advance, so look for that in an upcoming post on PowerPerformanceNews.com.
While most guys want to head straight to wide-open-throttle (WOT) tuning, frankly, that’s the easy part. Instead, let’s attack the idle and part-throttle first because that’s where street engines spend 98 percent of their time. We’ll start by setting the idle mixture screws and idle speed. We’ll deal here with automatic transmission cars because the added in-gear load can sometimes create problems.
Start by completely warming the engine and setting the idle speed where it’s comfortable, and the engine will still idle in gear without experiencing problems. First, check to make sure both idle mixture screws are adjusted equally. Next with the transmission in Park and a vacuum gauge hooked to read manifold vacuum, adjust the idle mixture screws in small increments until you can get the highest vacuum reading with the leanest idle screw setting. Remember, turning the idle mixtures screws in (clockwise) creates a smaller passage and a leaner idle mixture setting. With your foot on the brake pedal, check to make sure the engine idles comfortably in gear. If not, the carb will need further tweaking.
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.
There are several ways to meter primary fuel flow. The variables are jet size, the power tip and taper on the metering rods, and the power valve spring tension. For our 383, JET configured this Q-jet with a 76 primary jet and 39B tapered metering rods combined with the medium spring. Valdez opted for a V hanger with DA rods (0.044-inch power tip) for the secondary metering. The best place to start for tuning would be to install the lightest power valve spring. By reducing spring tension, less manifold vacuum is required to keep the primary metering rods in the jets — which leans the mixture by reducing fuel flow.
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.
Another useful tuning point on later Q-jets is the adjustable part throttle (APT). We’ve included a photo showing where this adjuster is located on late model carbs. There is an APT for early carbs, but it is located in the baseplate and more difficult to access. Adjusting the metering rods deeper into the jets leans the mixture, while counterclockwise adjustments richen the setting.
These adjustments alone can keep you busy for weeks if you are serious about tuning your Q-jet. The best plan is to make a minor change and then drive the car for several sessions to evaluate the change and see if it improves the engine’s response to throttle. Now, we can take a look at the secondary side of the Q-jet.
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.
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.
So, how do you evaluate your WOT changes if you can’t get to the drag strip? A simple way to measure change would be to choose an isolated, safe location away from all traffic or intersections and bring a friend along as timekeeper. The test will be simply to accelerate in a given gear (like second or third with no tire spin) between 4,000 and 5,500 rpm and measure the time it takes to run through this curve. Do a minimum of three runs in the same direction and average the times. Then, make a secondary metering change and perform the test again. If the time is longer, do the opposite of our first change and test again. If both the leaner and richer runs resulted in slower times, then the original metering was the best tune. Record the numbers and procedure in a tuning book. You can use it for reference.
This short tuning reference has only just scratched the surface of what you can do with a Q-jet. Do your best Q-tune, and you will be amazed at how much better your engine will perform.
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