|Performance Tune Your Q-jet Secondaries|
Written by: Damon Nickles
THE SECONDARIES!! Best place to start in search of more performance quickly!
These mods focus on what happens above about 2/3 throttle- that's when the secondaries do their work. The secondaries on a QJet are very very easy to work on:
Secondary air door: Push it fully open with your finger. How far does it open? If its about 80+ degrees, you're OK. If it only opens about 55 degrees, you are losing performance cause the thing isn't delivering its full airflow potential (750-800CFM is what the carb can REALLY do if properly prepped). To get your air valve fully open go to a junkyard and find an older QJet that DOES allow at least 80 degrees of air valve opening. Take the secondary air valve door shaft out of the "donor" carb and install it in yours. All you have to do is take the air valve doors off (they unscrew, 2 screws apiece) and then slide the shaft out the side. Make note of how the little tension/closing spring hooks around the secondary throttle shaft underneath and to the side that you slide the shaft out of. Make sure you hook it back up the same way when you put it onto your carb. Once your carb opens about 80 degrees, you can gradually file down the little stop on the side until you get to about 85-90, where it reaches max airflow. Then just set the tension spring at about 1/2 to 3/4 turn from the zero tension point. Its all the carb you will ever need for street performance. Difficulty level: 2 on a scale of 1 to10.
Now that you have full airflow from the secondaries, you need to MATCH IT WITH THE RIGHT FUEL FLOW! This is the job of the secondary metering rods and hanger. They are located above the secondary air valve doors between the two secondary bores. A single screw holds the hanger on. Unscrew it and lift the hanger and rods out to change them.
(Here's the real scoop on secondary rods: Very few are still available that are any good for performance). A few sets that you might try to find are: DR, CP, or BA. Don't ask why or how, just GET ONE OF THESE SETS and put them in. Here's a co. that sells QJet parts like these- The Carburetor Shop (909)481-5816. That's where I go for all my stuff. ($25/pair). FYI- These Secondary Rods make a bigger diff in performance than all the other mods combined. Stock rods are usually WAY WAY too lean on initial air valve opening and you get the infamous "Quadra-bog." These will fix that. Get 2 hangers while you are at it: B and G. B is full rich, G is just slightly leaner- one of them will be about perfect. ($5/hanger.) UPDATE: They're out of business unfortunately. Some of these rods are still available through GM parts counters but you'll need the part numbers: DR: 17053659, EE: 17081879, CP: 7045842, BA: 7034337, ED:17081878, DH: 7048992. They come individually so you'll need to order 2. I dunno which rods are still valid part numbers and it changes every few months so..... Any ways, any of the above rod sets will do just fine- they're all identical in every way within .0003" (3 TEN-thousanths!) .
Lastly: The secondary air valve "damper." Its the little vacuum can on the side at the front of the carb that also serves to slightly open the choke when the engine first starts. It takes too long to fully open for performance. Most stock ones take about 2-3 seconds (push it in and release while timing with a stopwatch). A hi-po motor should take just under 1 second. Some of them you can modify by drilling out the TINY TINY little hole that acts as the "restriction" in the inlet where the vacuum line plugs on. We're talking a few thousandths of an inch can make a diff. Be real careful and get the right set of (jewelers) drills if you want to try to modify it. OR you could buy one already ready to go from the Carburetor Shop.
That's basically it. Do these mods, with the right rods, and you will definitely be smiling. It'll hit like its got FI and pull much faster out of the hole and a little better on the top end. I would say at least a 1/4 second reduction in ET from these mods alone! That's why I use them- dirt cheap HP if you're willing to spend some time instead of cubic yards of dollars.