Q-Jet Secondaries
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Q-Jet Secondaries (Stewart's74)
Stewart -
The Q-Jet secondaries are purely mechanical. If the lockout lever on the passenger side is fully retracted (when the choke is hot), the secondaries absolutely positively will open upon engine demand. The secondary airvalve is forced open by differential airpressure - there is nothing that prevents it from opening.
It is difficult to get the secondary airvalve to crack open by "winging" the throttle in neutral - for this reason many people doubt the operation of the airvalve system. But it does work... you can often just barely see it if you slam the throttle wide open briefly: as the rpm quickly zips up through 3500-4000 rpm while holding the throttle wide open, the airvalve will just barely crack open and a little fuel will often spill out onto the airvalve. Be sure to release the throttle before you pass through redline...
Another way to verify operation is through a seat-of-the-pants check: Let the engine warm up so the lockout lever retracts. Then do a wide-open run through first gear and note how the car feels and sounds. Then, use a piece of wire to engage the lockout lever to prevent the secondaries form opening. Do the run again. I'll bet you'll notice a difference....
One thing you might want to check, though (very common problem) is your throttle travel - most Vettes I work on do not obtain wide open throttle with the gas pedal to the floor. With the engine off, have someone push the pedal to the meatl and see if the throttle at the carb is really wide open...
[Modified by lars, 3:36 PM 4/4/2003]
The Q-Jet secondaries are purely mechanical. If the lockout lever on the passenger side is fully retracted (when the choke is hot), the secondaries absolutely positively will open upon engine demand. The secondary airvalve is forced open by differential airpressure - there is nothing that prevents it from opening.
It is difficult to get the secondary airvalve to crack open by "winging" the throttle in neutral - for this reason many people doubt the operation of the airvalve system. But it does work... you can often just barely see it if you slam the throttle wide open briefly: as the rpm quickly zips up through 3500-4000 rpm while holding the throttle wide open, the airvalve will just barely crack open and a little fuel will often spill out onto the airvalve. Be sure to release the throttle before you pass through redline...
Another way to verify operation is through a seat-of-the-pants check: Let the engine warm up so the lockout lever retracts. Then do a wide-open run through first gear and note how the car feels and sounds. Then, use a piece of wire to engage the lockout lever to prevent the secondaries form opening. Do the run again. I'll bet you'll notice a difference....
One thing you might want to check, though (very common problem) is your throttle travel - most Vettes I work on do not obtain wide open throttle with the gas pedal to the floor. With the engine off, have someone push the pedal to the meatl and see if the throttle at the carb is really wide open...
[Modified by lars, 3:36 PM 4/4/2003]
