Quadrajet Secondaries Not Opening Fully

[OneShot]

Good evening all,

I have been tuning my Q-Jet for the last several days as I drive it to campus and such. Haven't DDed the truck in a while and am getting it used some just to keep it from sitting.

I tuned the choke / high idle cam first then the mixture a few days later when the engine was warm. Runs great on the primaries now.

Engine bogs at wide-open (secondaries engaged). Got around to checking the secondaries next and noticed the following:

-Upper air doors open freely: check
-Upper air doors have good spring tension on close: check
-Throttle linkage engages secondary throttle valve lever on driver's side of carb: check
-This linkage ^ starts opening secondaries at ~70% throttle: check

Problem is, when I peg the throttle with my hand, I see the secondary throttle valves are only opening ~5 degrees instead of the ~80-90 degrees that they should be opening. I observed this when I crawled inside the engine compartment, operated the throttle with my hand, and held the upper air doors open with my finger so I could see the throttle valves rotate. On a warm but shut down engine.

I understand this may be unclear - I will get a video to post tomorrow after work.

Why are they not opening more? Is there a linkage or something I have forgotten?

Thanks!

 

[75gmck25]

The linkage for the secondary throttle plates is mechanical, so if they don't open at full throttle there is a mechanical linkage problem. The upper air doors are vacuum actuated by the engine, so you cannot see them operate unless there is enough full throttle vacuum load to pull them open. You can't operate them by just running the throttle up while looking at the engine.

Also check the kickdown cable adjustment for your TH350 or the kickdown switch for a TH400 to make sure you are getting a drop to 2nd gear when you need it.

I've had issues with my throttle linkage cable not pulling far enough back on the linkage, and it did not engage the secondaries. Can you pull the linkage back by hand and see the secondaries go fully to 90 degrees? If it works when you pull it by hand, you need to adjust the cable.

Once you get the secondary throttle plates going to 90 degrees, then try adjusting the air door tension. Its relatively easy if you have the right allen wrench to loosen the set screw. A secondary bog might mean the tension is too light and its getting a big gulp of air too early when the secondaries open. IIRC, base tension should be set to about 3/4 turn after the little spring rotates and just touches the rod. More tension will open it up later.

Bruce

 

[OneShot]

The linkage for the secondary throttle plates is mechanical, so if they don't open at full throttle there is a mechanical linkage problem.

When I operate the linkage by hand I can only get them open 5 degrees or so, but it seems that the linkage rotates all the way, as it should. The "knob" on the rear driver's side of the carb will rotate 90 degrees... maybe I'm just winding up the spring and not actually turning the shaft.

I added a 1" aluminum spreadbore carb spacer last summer. Hopefully that's not interfering. I will investigate this further.

Also check the kickdown cable adjustment for your TH350 or the kickdown switch for a TH400 to make sure you are getting a drop to 2nd gear when you need it.

It had the TH400 in it but it has the SM465 in it now. Good idea though.

A secondary bog might mean the tension is too light and its getting a big gulp of air too early when the secondaries open. IIRC, base tension should be set to about 3/4 turn after the little spring rotates and just touches the rod. More tension will open it up later.

The air doors snap back fine by hand, but I'll try resetting the tension to to 3/4 turn of engagement just to be sure.

Maybe I will stick a gopro under the hood with the air cleaner off so I can actually see if that's the problem or not!

 

[OneShot]

Interesting, at 8:45 in this video:

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Tyler says that some of the later Q-Jet carbs (hey, I have one of those...) are spring loaded in the secondaries to allow primary operation if the secondaries are stuck.

Thinking my carb spacer may be the issue, or the secondaries are just stuck and need worked back and forth. The troubleshooting continues...