Rochester Q-Jet Secondaries
Thread Starter
Le Mans Master
Joined: Oct 2001
Posts: 5,943
Likes: 3
From: Wildwood IL
Rochester Q-Jet Secondaries
While trying to straighten out 'ol Bubba's vacuum hose handywork, (headlights T'd off distrib vac advance) I noticed things are missing from the carb. Shop manual has tons of pics of Holly...but only partial drawings of Q-Jet. I'm not familiar with this carb so here goes the questions.
#1 Do the vac secondaries work off a round vac unit (passngr side tword front of carb) ?
#2 Choke vac and linkage missing. Would that be a vac unit on pasngr side rear of carb?
When I work the plunger on the secondaries...linkage pulls forward. Opposite of what it should to open the secondaries. WTF????
#3 Is this vac module mounted backwards???
Please enlighten me..... pics if you can.
Eddie
#1 Do the vac secondaries work off a round vac unit (passngr side tword front of carb) ?
#2 Choke vac and linkage missing. Would that be a vac unit on pasngr side rear of carb?
When I work the plunger on the secondaries...linkage pulls forward. Opposite of what it should to open the secondaries. WTF????
#3 Is this vac module mounted backwards???
Please enlighten me..... pics if you can.
Eddie
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Rochester Q-Jet Secondaries (Silvr77)
The Q-Jet is not a vacuum secondary carb - it's mechanical. It uses a secondary airvalve to prevent secondary tip-in stumble, and this airvalve is controlled by a windup spring and a rod running to the choke pulloff diaphragm. The pulloff diaphragm does not pull the airvalve open - it produces a controlled opening rate - the airvalve is pulled open by differential air pressure on the secondary side. When the engine is running, the choke pulloff and the airvalve rod should hold the airvalve firmly closed. When the engine is off, the pulloff should be relaxed, and you can push the airvalve open with your finger. This is correct operation. You cannot induce the airvalve to open by flicking the throttle with the engine in neutral - you can't get manifold pressure to drop far enough to relax the pulloff. The engine must be under load and require the CFM before the airvalve will open.
Most Q-Jets use only a single choke pulloff mounted on the forward pasenger side of the carb. The linkage that runs up to the airvalve doubles as the choke pulloff linkage. A few model carbs used a secondary choke pulloff on the rear of the carb, but very few carbs had this configuration. Carbs using a secondary pulloff function just fine without it.
Most Q-Jets use only a single choke pulloff mounted on the forward pasenger side of the carb. The linkage that runs up to the airvalve doubles as the choke pulloff linkage. A few model carbs used a secondary choke pulloff on the rear of the carb, but very few carbs had this configuration. Carbs using a secondary pulloff function just fine without it.
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
Re: Rochester Q-Jet Secondaries (Silvr77)
I'll take a shot at some of these.
No, this vacuum unit is the choke pull off. It's what allows the car to idle more slowly when you blip the throttle after starting the engine cold.
I'm not sure about the 77s but on my 74 and earlier q-jets the choke is controlled by a coiled spring that sits in the intake manifold on the right hand side of the carb. this spring is connected to the choke mechanism by a rod. When cold the choke is on and as the engine warms the spring expands and pushes the choke off.
No it's supposed to be mounted on the front of the carb. The secondaries on a Q-jet open based on the velocity of the air that the engine requires. The opening rate is controlled by a spring mechanism on the right side of the carb. It's a small screw that is held in place by an allen screw hidden under the set screw. This controls the tension on the secondary spring and allows the secondaries to open quicker if loosened or slower if tightened. If you really want to know about Q-Jets i would recommend Doug Rowe's book on them. He goes into great detail on all aspects of the Q-jet including rebuilding. Hope this helps.
Whoops, Lars types faster than me. :lol:
[Modified by vettfixr, 10:48 AM 3/31/2003]
#1 Do the vac secondaries work off a round vac unit (passngr side tword front of carb) ?
#2 Choke vac and linkage missing. Would that be a vac unit on pasngr side rear of carb?
When I work the plunger on the secondaries...linkage pulls forward. Opposite of what it should to open the secondaries. WTF????
When I work the plunger on the secondaries...linkage pulls forward. Opposite of what it should to open the secondaries. WTF????
#3 Is this vac module mounted backwards???
Please enlighten me..... pics if you can.
Eddie
Please enlighten me..... pics if you can.
Eddie
Whoops, Lars types faster than me. :lol:
[Modified by vettfixr, 10:48 AM 3/31/2003]
Thread Starter
Le Mans Master
Joined: Oct 2001
Posts: 5,943
Likes: 3
From: Wildwood IL
Re: Rochester Q-Jet Secondaries (vettfixr)
Lars & Vettefixer: What would I do without ya?? Thank you both......
I'll do the old paper clip on the linkage trick next time I drive it, to see if the secondaries open. This old 72 has been butchered and Bubba'd, and the most difficult thing has been to figure out what I'm supposed to have and how it's supposed to work vrs what I have. Thanks again fellas.
Eddie :cheers: :cheers:
[quote]
I'm not sure about the 77s but on my 74 and earlier q-jets the choke is controlled by a coiled spring that sits in the intake manifold on the right hand side of the carb. this spring is connected to the choke mechanism by a rod. When cold the choke is on and as the engine warms the spring expands and pushes the choke off.[Quote]
Vettfixr: This is on my 72. Same as yours!!! Thanks
[Modified by Silvr77, 10:14 AM 3/31/2003]
I'll do the old paper clip on the linkage trick next time I drive it, to see if the secondaries open. This old 72 has been butchered and Bubba'd, and the most difficult thing has been to figure out what I'm supposed to have and how it's supposed to work vrs what I have. Thanks again fellas.
Eddie :cheers: :cheers:
[quote]
I'm not sure about the 77s but on my 74 and earlier q-jets the choke is controlled by a coiled spring that sits in the intake manifold on the right hand side of the carb. this spring is connected to the choke mechanism by a rod. When cold the choke is on and as the engine warms the spring expands and pushes the choke off.[Quote]
Vettfixr: This is on my 72. Same as yours!!! Thanks
[Modified by Silvr77, 10:14 AM 3/31/2003]
