Q jet question

[Onehit]

Ok so the rebuilt carb is on and it runs great finally. I'm curious about the secondarys, are they always under constant vacuum? I've noticed that even when the car is full warmed up they are still under vacuum. Is this one of the things controlled by the computer?

 

[pontiacgp]

The computer has no control over the secondaries. I'm not sure what you mean the secondaries are under vacuum,

 

[Onehit]

I did a little more digging and found out the the rear butter flys open gradually at wot. Carbs are still foreign to me, other then taking one off and putting it back on

 

[84 W40]

Sounds like the tension spring for the secondarys butterfly is tight. Did you take it apart when you rebuilt it ? The computer controls MC solenoid on the primary side.

 

[MrSony]

I don't think he had an issue, he was just wondering. The choke pull off on the front passenger side of the carb has a green rod connected to it which in turn is connected to the secondary air valves/doors. The choke pull off opens up the choke flap a lil bit when the engine is first started and stays engaged while running. Under WOT when there is no vacuum, the pulloff releases and lets the air doors flop open. There is a 3/32 allen set screw and a flathead adjustment screw to adjust the tension of the air doors. If it runs good and doesn't bog when you mash it, leave it alone.

 

[pontiacgp]

I did a little more digging and found out the the rear butter flys open gradually at wot. Carbs are still foreign to me, other then taking one off and putting it back on

If that is happening while you are reving the car by hand then that sounds normal to me. The butterflies open differently under load.

 

[Bonnewagon]

To answer the OP's question, the secondaries are always "seeing" vacuum on their underside because they are exposed to full manifold vacuum by virtue of their attachment to the intake manifold. What they are NOT seeing is any mechanical action controlled by vacuum with the exception of the choke pull off Mr. Sony mentioned. The secondary throttles only open when the primary throttle reaches about 3/4 of the way open. Then the primaries open the last 1/4 of the way and the secondaries progressively open fully. Do not confuse the upper air valve with the lower secondary mechanical valves. The upper air flaps open upon air flow demand by the motor so even if you force the mechanical air valves open too soon- the upper air valves will not open in concert. This is why the air valve spring tension is so important. The common reaction is to loosen the tension which is a huge mistake. That way the air valve will open way too soon and cause a bog- and that is how the Q-jet earned the nickname "Quadra-bog". The correct spring tension will restrict the air valve from opening until the primaries are flowing almost fully and when the secondary throttle valves do open the motor is then demanding WOT air flow. At that point as the secondary throttle valves open air flow demand is sufficient to open the air valves which in turn lift the secondary metering rods which allow WOT fuel supply. So the correct spring tension is so tight that the primaries are wailing before the secondaries even think about opening. The secondary air valves are also restricted by the green rod Mr. Sony mentioned. It is attached from the air flaps to the choke pull off. As manifold vacuum drops under throttle opening the pull off relaxes slowly acting as a damper. Again, manifold vacuum controls this and if the motor can't use the extra fuel/air mix then the air valve can't open. There is also a mechanical lockout mechanism so that the secondary throttle valves can not open until the choke has fully opened. As Mr. Sony said- if it works- leave it alone.

 

[pontiacgp]

I wasn't thinking of the manifold vacuum when the op asked about constant vacuum......

 

[Onehit]

Oh trust me I'm leaving it alone. This thing has never started so easy. All I have to do is lightly tap the gas when it cold and is starts right up and idles down by it self. Now I know that the choke never worked right either before the rebuild.

 

[pontiacgp]

It would help you if you got the Doug Roe book and /or the Cliff Ruggles book on quadrajets.