C3: Quadrajet secondary adjustment

[Dan Pepper]

Working on tuning my '69 350/350 QJ carb. It doesn't seem to be kicking in the secondaries like it should (like my C2 L36's Holley anyway). When driving and flooring gas, there is no distinct secondary kick-in - more of a smooth rise in RPM's w/o the familiar secondary howl.

With the car parked and in nuetral I've reved the engine via the throttle cable trying to open the secondaries but the butterfly hardly moves. As it is, the butterfly cracks no more than 1/4" open. Does the engine need to be under load to create more vaccuum to open the secondaries wider? Is the vac actuator shot? Or is that all I get from Quadraflush?

[Jack H.]

Re: C3: Quadrajet secondary adjustment

You'll know if your QJet secondaries are operational or not! First, you have to be driving the car (vs. twidling the throttle linkage in the garage). Then, IF the secondaries are actually NOT OPENING, use a whip, say a prayer, you'll NEVER get the tach to climb above roughly 4000 RPM. If you can accelerate up toward red line, I assure you the carb's secondaries ARE opening up!

[Dan Pepper]

Re: C3: Quadrajet secondary adjustment

She will redline - but not with the ooomph I'd like. There is no distinct "kick-in" like I feel with the Holley 4650. Shouldn't there be the same sound/sensation with the QJ?

[Mike M.]

Re: C3: Quadrajet secondary adjustment

Three things control the opening of the secondary air valves on the Q-Jet:
1) air flow across the air valve plates
2) the spring tension (wind up) on the secondary air valve (spring to close)
3) the choke pull off diaphragm (interconnected via link to the secondary air valves).
All of the above is to works together (highly damped system)to bring a Q-jets secondarys in slowly to prevent the huge bog you would get if the 2 1/4" secondaries opened too quickly. It is probably the interconnection to the choke pull off that is not being satisfied (vacuum is not reduced low enough for a long enough period of time) that keeps the air valves closed while you blip the carb sitting in the driveway.
The only interconnect in most Holley's is the secondary diaphragm/spring combination. All in all this is a much less "damped" set up as compared to the Q-Jet, and because it is so, you will get the "kick" when the secondaries engage. I've never felt the same "kick' from a Q-Jet, but you sure as heck can hear when those secondaries begin to flow air and the engine responds accordingly.
In factory tune most Q-jets are highly damped, however, if you understand how the system works, it can be adjusted for less damping. But the size of the secondaries create a narrow line between smooth power transition and an over rich bog.
Mike

[Dennis D.]

Re: C3: Quadrajet secondary adjustment

Had the secondaries lock out on my 70 L-46. Air valve issue. Roe's Q-jet book walks you through the whole carb and tuning.

[Mark B.]

Re: C3: Quadrajet secondary adjustment

Dan,

Back in the old days (geez, I sound like my old man), when I would terrorize the streets of Dallas, Texas in my 67 GTO, we would modify the Q-Jet by adjusting the secondary air valve spring with a little less tension to allow it to begin opening quicker. Not too much, or you'll create a bog. Also, we would remove the secondary metering rods and file a taper down one side of the rods to allow more fuel to enter when they began rising out of the secondary jets. It didn't hurt the normal running of the car as the secondaries only came on line under WOT when the air valves began to be sucked open. You certainly knew when the secondaries came on board. What a sound!!!! What a GTO!!! Good luck.

[Dan Pepper]

Thanks for the tips, guys. *NM*