I have a '71 350/270 with the original Quadrajet. At what rpm should I expect the larger secondary air valves to open?

The car is new to me and I am trying to make sure the carb is working as it should. So far I have babied the car but would like to know how it will respond to full a full throttle.

thanks

 

The air valve opens as a function of air flow. If they are working properly, you will feel very linear response without any hesitation or a sudden surge. You won't know the answer to your question until you have done full throttle accelerations, partial throttle accelerations, throttle chops and immediate re-accelerations.

When you've got it tuned (check Lar's posts on this), you will be very happy.

 

There is an easy way to find out. I would have tried it the first drive, adjusted if needed & there would be many per drive thereafter.

 

There are 'lock-outs' on the secondary system. It will not open until those lock-outs will allow it. The first one is on the choke side. Until the choke is completely off, there is a brass pivot lever that will prevent the secondaries from operating. That lever has a notch at the bottom which engages a pin. Once that lever is 'off' of the pin, the secondaries are operational. Also, the secondary throttle plates will only begin to operate near the very end of travel on the throttle cable; it's that last little bit of travel that opens them to 'full open' very quickly. So, you have to be off of the choke and you have to get full throttle action from the accelerator cable for the secondary air valve to work. If you have someone in the car working the accel pedal, then you can be under the hood [manually] holding the choke lock-out off the pin while also opening the secondary air valve and looking down into the secondaries [engine OFF, of course]. Then have your helper depress the accelerator completely to the floor. If the lower throttle plates do not open...or do not open all the way, you need to adjust the throttle cable to obtain full travel on the carb linkage. This is a very common problem with Q-Jets because of the lack of knowledge on how they work, along with the delayed action of the secondary butterflies built into the linkage design. Finally, when you do get that all working properly, the secondary air valve will open as it needs to based on airflow demand by the engine. Very few engines require the complete opening of the Q-Jet secondaries, as they do not need 750 CFM of airflow. Only the stoutest 350-383's with very good cylinder heads, cam and headers could ever demand enough flow from a Q-Jet. There is a tricky little adjustment on the secondary air valve damper spring that determines the rate at which that valve opens; and it should be adjusted to meet your engine's needs. If the spring is too tight, the valve won't open quickly enough and the engine power will be weak. If that spring is too loose, the valve will open too quickly and the engine will "bog down" when WOT is executed. Once you get all that stuff set right, your carb will operate as well as...or better...than it did when it was new.

 

They don't open according to engine speed, strictly by load. Don't expect them to open just by revving the engine in the driveway.

 

Thank you very much.

This is what I need.

 

And if you loosen the set screw too much it and the spring will fall out (it is screwed in from the bottom).

These are very small parts so use extreme caution.

cc

 

You will not see the upper secondary air valve open when revving the engine in neutral and whilst stationary. They only open as said, under load ie under actual driving conditions. A common complaint about secondaries not opening is due to the misunderstanding of how they operate. The upper section is not mechanically connected to the throttle linkage so will not open at all just by operating the throttle to WOT.

 

another thing from i have heard is that on smaller engines the secondaries may never open all the way. supposedly thats why they put the 750 qjet on such a large range of engines so would it be safe to say that a stock 305 may never even get halfway open while a hot 427 may go all the way.

did i read that correctly?

 

Well, I can't imagine GM installing an [expensive] 4-bbl carb on an engine that will never open the secondaries...especially a Q-Jet with relatively small primary venturi(s). You can definitely tell {by the sound} when the secondaries open up on a Q-Jet. I have a 'little' 350/270 base '71 engine and mine have absolutely no problem opening under WOT. {I am sure that they do not fully open, however.}