[ffcagle34]

How does the secondary butterfly work on a rochester Q-Jet. I see the mechanics of how the valve closes, but how does it open. Some guy told me that it is opened by an air valve. What air valve. Right now my engine is bogged down with it closed. If I open it by hand my 350 sbc runs cleaner and rpms are up. Is this how it is suppose to operate. I have a ton of literature that identifies the parts, but none tell me what they do. Arrrrrg!!!

[davidpozzi]

There is a small vaccum can on the right front of the carb that releases tension when the engine is floored.
The choke blade looking butterflys on the secondaries is the "air valve" you were looking for.
The secondary air valve has the shaft offset to the front of the carb. The rear is sucked down by air flow and it is held back, or counterbalanced by a spring that is wound around the air valve shaft on the underside of the right rear of the carb where the vaccum can linkage connects.
Notice a small screw you can see through the linkage slot there? That is a tension adjuster to adjust air valve opening rates.
It is locked in place by a small allen screw pointing straight up just underneath the adjuster screw.
If you release the allen too much, the screw will be pulled by the secondary air valve tension spring and your secondaries will open too fast and you'll have a big bog.
It is best to adjust the valve opening with the air cleaner in place when making test runs. The air cleaner can make a big difference in opening speed.

If the choke is on even a little, the secondary butterflies are locked out from opening on the right side of the baseplate.

To the front edge of the secondary air valve, there is a hole on each side that provides fuel like an accelerator pump.
fuel is pulled out by the fast moving air and vaccum into the secondaries. If the valve just pops open, no fuel can come out and the engine will go lean. The fuel is stored in secondary fuel wells that store a small amount of fuel replenished by a very small jet. This gives a shot of fuel like an accelerator pump but just for a short time, then the wells are empty.
Doug Roe has a great book on Q Jet carbs. It's by HP books.
David

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67 RS 327 original owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer