secondary air valve
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Joined: Nov 2007
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From: Chicago IL
secondary air valve
Should you be able to open the air flaps over the secondary's by hand? From my searching, it seems that they open due to rpm and load, but I also saw someone mention that you can open them to see the throttle plates. mine seem to be stuck in the closed position. I have a 72 350 with what I was told was the original carb. thanks
Safety Car
Joined: Mar 2007
Posts: 3,652
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From: Bridgewater nj
You have to make sure the choke is fully open and linkage must be opened to WOT wide open throttle. The the secondary flaps can be pushed open.
With the car hot and wrapping the throttle open the secondary flaps will barely open. On the passenger side under the secondary pivot is a small Allen screw. It puts spring tension on the secondary flaps. Too little and they flop open and it bogs. Too much and they don't open all the way.
This is a very brief explanation of what's going on with your secondary's.
With the car hot and wrapping the throttle open the secondary flaps will barely open. On the passenger side under the secondary pivot is a small Allen screw. It puts spring tension on the secondary flaps. Too little and they flop open and it bogs. Too much and they don't open all the way.
This is a very brief explanation of what's going on with your secondary's.
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Joined: Nov 2007
Posts: 60
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From: Chicago IL
You have to make sure the choke is fully open and linkage must be opened to WOT wide open throttle. The the secondary flaps can be pushed open.
With the car hot and wrapping the throttle open the secondary flaps will barely open. On the passenger side under the secondary pivot is a small Allen screw. It puts spring tension on the secondary flaps. Too little and they flop open and it bogs. Too much and they don't open all the way.
This is a very brief explanation of what's going on with your secondary's.
With the car hot and wrapping the throttle open the secondary flaps will barely open. On the passenger side under the secondary pivot is a small Allen screw. It puts spring tension on the secondary flaps. Too little and they flop open and it bogs. Too much and they don't open all the way.
This is a very brief explanation of what's going on with your secondary's.
Le Mans Master
Joined: Mar 2008
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From: Oxford MA-----You just lost the game!!!!
http://www.underhoodservice.com/Arti...rburetors.aspx
Go down to photo 9, and you'll see and read about the choke lock out that has been described here. Also, a bunch more info about the q-jet here. Hope this helps you out,
Scott
Go down to photo 9, and you'll see and read about the choke lock out that has been described here. Also, a bunch more info about the q-jet here. Hope this helps you out,
Scott
Melting Slicks
Joined: Oct 2005
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From: Redruth Cornwall
The air valve is independant to the secondary throttle plates. The air valve should move easily with finger pressure. As has been said above, the secondary throttle plates will only open when the choke is fully open and the lock out has moved.
Ref Scottys post,
Ref Scottys post,
Race Director
Joined: Jan 2000
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From: San Jose CA
With the car running at an idle, one or possibly both (if there are two) of the choke pull offs will prevent the secondary air valves from opening more than maybe 1/8". Test them with the engine off and they should be fine.
Although, there are secondary lockouts on some carburetors that lockout the air valves. I have only seen them on Pontiac carbs, but I have not seen many Chevy 4M carbs, I stick around with M4Ms mostly. This lockout would be attached near the link from the choke rod (the one that goes from choke assembly to air flap door). The only time I've seen it was on a 74 Pontiac 455 carb, a 4MC (yes, 4M with an integral choke).
Although, there are secondary lockouts on some carburetors that lockout the air valves. I have only seen them on Pontiac carbs, but I have not seen many Chevy 4M carbs, I stick around with M4Ms mostly. This lockout would be attached near the link from the choke rod (the one that goes from choke assembly to air flap door). The only time I've seen it was on a 74 Pontiac 455 carb, a 4MC (yes, 4M with an integral choke).
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Thread Starter
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Joined: Nov 2007
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From: Chicago IL
Any other suggestions?
Thanks for all the replies
Race Director
Joined: Jan 2000
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From: San Jose CA
The choke will release the lock on the Secondary throttle plates, but I still cannot open the air valve flaps. I looked around the flaps and I cannot find anything, other than something in the carb below the flaps that could be possibly holding them closed.
Any other suggestions?
Thanks for all the replies
Any other suggestions?
Thanks for all the replies
If not, 1977L48 raises a good point, it could be that the throttle valve doors are binding. There should be a clear gap around them.
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Joined: Nov 2007
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From: Chicago IL
I have tried it both on and off. If I loosen those screws on the flaps, is there any chance that something can fall into my intake, like nuts or something from below the flaps ?
Race Director
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From: San Jose CA
If you do decide to loosen them, be careful. The screws thread into the shafts that they ride on. Do not try to back them out particularly far; they are *likely* staked and you will damage the shaft if you loosen them up more than a little bit. (aside from breaking them loose, do not try to turn them past where they start to feel tight again)
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From: Chicago IL
Thanks for the suggestions, unfortunately my car is stored about 30 min from me and have not tried any of this yet. If the air flaps do loosen when I break those screws loose, is there a proper way to tighten them back up so they don't bind again, or is something warped or damaged? Also if those flap are shut all the time, would this make a big impact on performance?
Thanks
Thanks
Safety Car
Joined: Mar 2007
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From: Bridgewater nj
Thanks for the suggestions, unfortunately my car is stored about 30 min from me and have not tried any of this yet. If the air flaps do loosen when I break those screws loose, is there a proper way to tighten them back up so they don't bind again, or is something warped or damaged? Also if those flap are shut all the time, would this make a big impact on performance?
Thanks
Thanks
Going back to the choke lock. That pin can move on the shaft. Sometimes it will move enough to catch on the choke linkage. You can tap the pin with a punch and hammer to get it to move. If you secondary flaps barely move, I would say that's still your problem! If the flaps are binding, they tend to move a little and get stuck in that position.
That pin has to swing out of the way to let that linkage pass and the flaps will open. Or just bring it to any guy over 40 that works on cars and hell show ya
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From: Chicago IL
Okay, broke all of the screws loose, the air flaps will still stuck, but with some light tapping they came loose! I tightened the screws back up and they still seem to move freely. Test drove it and it feels like it pulls a bit harder. The whole reason I noticed that these flaps were stuck, is because I wanted to see if my secondaries are opening all the way, and now that I can see them, it looks as though they are only opening maybe 1/4 to 1/2 way when someone puts the petal to the floor. It was hard to see the primaries, but it looks like those open all the way. Any suggestions?
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From: Chicago IL
Thanks
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this is from a paper by Lar's
Secondary throttles adjusted to not open fully:
Results in: Poor WOT performance.
Comments: The secondary throttle shaft is actuated by a link off the primary shaft that hits a lever on the secondary
throttle shaft. It is common for this to be misadjusted so that the secondary throttle blades do not fully open. Some
factory cars & carbs were intentionally set up to limit secondary throttle opening (like 1st-generation 400-powered
Firebirds) in order to limit horsepower for one reason or another. Check for this whenever the carb is disassembled
and adjust it by bending the contact tang on the primary throttle shaft.
Secondary throttles adjusted to not open fully:
Results in: Poor WOT performance.
Comments: The secondary throttle shaft is actuated by a link off the primary shaft that hits a lever on the secondary
throttle shaft. It is common for this to be misadjusted so that the secondary throttle blades do not fully open. Some
factory cars & carbs were intentionally set up to limit secondary throttle opening (like 1st-generation 400-powered
Firebirds) in order to limit horsepower for one reason or another. Check for this whenever the carb is disassembled
and adjust it by bending the contact tang on the primary throttle shaft.