Sticking Open Secondaries on a Quadrajet

[rpaldrich]

Quadrajet secondary air valves are sticking open. I haven't had the boat out on the water yet, but when i push them open with my finger, they catch in the open position and don't close. (not the butterflies below, just the air valves)

If I understand correctly, these air valves help meter the air flow to the primaries while the secondaries are closed, so under load if they get stuck open and I throttle back, will the engine will run poorly?

Any Q-Jet gurus out there seen this problem before?, I searched but the only post I saw was 10 years old and not much help. Is it an easy fix, or easy places to look for the binding? I've tried spraying carb cleaner on all the hinge spots. btw-the spring that helps the secondaries close works fine, up until you open the secondaries all the way and they bind on something.


Engine-Twin 1987 Mercruiser 260hp 350 Thunderbolt IV mated to Alpha One outdrives.

 

[dan t.]

Re: Sticking Open Secondaries on a Quadrajet

The air valves on a q-jet have 2 things to close them, 1 an adjustable spring on the shaft, 2 a vacuume pot on the side with a link running to the lever on the shaft. Possibly the spring is broken or the set screw came loose and the spring unwound.

 

[rpaldrich]

Re: Sticking Open Secondaries on a Quadrajet

Thanks Dan---if I open the air valves halfway or even 3/4, the spring works fine and shuts them all the way...its when I open the air valves all the way that the spring can't overcome what it is binding on.

Anything specific it could be binding on? and..if I take it apart, the manual talks about using "legs" to aid in the repair. Are these just straight vertical shafts that go through the carb bolt holes that help you align all the parts on?

 

[Jlawsen]

Re: Sticking Open Secondaries on a Quadrajet

Remove the link from the dashpot and re-check for binding. If I recall correctly the air valves do not open fully at WOT. The dashpot/vacum pots on Q jets do fail and it can be difficult to find a replacement. Q-Jets are also notorious for casting sand holes. Over time, the thin piece of casting over the hole gives way and all of the sudden your engine starts running badly. The only cure for that is a new carb.

 

[dan t.]

Re: Sticking Open Secondaries on a Quadrajet

The legs are just a rebuilding stand,handy but optional. You can make one by using the base gasket as a template, drill a piece of 5/8 or thicker plywood to put 4 5/16 bolts thru, countersink the heads so they sit flush. Run 4 nuts on for stops. Viola, you have a rebuilding stand. Now for the air valves,hook up a vacuume pump to the secondary dashpot or just a hose and suck hard and see if they still stick.

 

[John_S]

Re: Sticking Open Secondaries on a Quadrajet

I just checked a qjet I have, It's been a while. I have a newer elec choke model. The air door has a bend in it. It should open, from pressing with finger so the front lip is nearly perpendicular. The larger part of the valve will be at an angle, like 75 degrees. If yours is binding, check shaft, plates, hangers, spring wind-up, etc for wear. Should be somewhat obvious what/where the binding is occuring. The vac dashpot looks to only prevent the air valve from opening under high vacuum, ie idle. It should not impede on travel sitting at the bench.

 

[rpaldrich]

Re: Sticking Open Secondaries on a Quadrajet

Thanks--I disconnected Vac dash pot link and it still sticks. It only sticks in when I push it full open so maybe it'll be fine. I'll see How it Runs from WOT back to mid range on the water and have more details
Does the Vac dashpot only close the air valve at idle or
Will it work at mid rpms?

 

[John_S]

Re: Sticking Open Secondaries on a Quadrajet

Thanks--I disconnected Vac dash pot link and it still sticks. It only sticks in when I push it full open so maybe it'll be fine. I'll see How it Runs from WOT back to mid range on the water and have more details
Does the Vac dashpot only close the air valve at idle or
Will it work at mid rpms?

I don't believe it will force the air door closed, but would have to re-check the linkage. It prevents it from opening. Not sure what the vacuum has to drop to, so it can open.

While something is wrong, the 350 does not have enough breath to open that 750cfm carb to its fullest.

 

[BaileysBoat]

Re: Sticking Open Secondaries on a Quadrajet

Check the secondary metering rods. They are operated by the secondary air valves. If rods or their linkage is bent this will cause the rods to bind in the secondary metering jets.