carb question
Thread Starter
Melting Slicks
Joined: Sep 2007
Posts: 2,387
Likes: 123
From: CA.
carb question
i have a quadrajet on my 72. there are 2 air screws on bottom front. when i turn them in or out there is no difference in engines running. do these only work when secondaries are open? my mechanic says i need rebuild. car runs good but stalls sometimes at idle.
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
The 2 idle mixture screws affect idle & transition into pri., not sec. Unscrew the idle mixture screws & take out, clean out & reinstall. Check fuel filter.
Do you have a BB Q-J?
Do you have a BB Q-J?
Le Mans Master
Joined: Jan 2007
Posts: 5,485
Likes: 7
From: Peekskill NY
Try what Ganey suggested first.
Chassis service manual says disconnect & plug dist vacuum line, turn idle mixture screws in until lightly seated, then back out 4 full turns. Adjust idle speed to 775 rpm manual, 630 auto. Adjust idle mixture screws equally to obtain 750 rpm, 700 rpm auto, then reconnect dist vacuum line. If there's no difference then pull the carb for rebuild.
You can do it your self with a rebuild kit.
I just got the latest copy of Corvette Fever and there's a Q-Jet rebuild article. Check it out, plus lots of info on the forum & web.
Glenn
Chassis service manual says disconnect & plug dist vacuum line, turn idle mixture screws in until lightly seated, then back out 4 full turns. Adjust idle speed to 775 rpm manual, 630 auto. Adjust idle mixture screws equally to obtain 750 rpm, 700 rpm auto, then reconnect dist vacuum line. If there's no difference then pull the carb for rebuild.
You can do it your self with a rebuild kit.
I just got the latest copy of Corvette Fever and there's a Q-Jet rebuild article. Check it out, plus lots of info on the forum & web.
Glenn
Team Owner
Joined: May 2001
Posts: 87,302
Likes: 1,583
From: Western NY
You should most definitely see a change in engine speed (at idle) if you make any changes to these screws. As "ganey" mentioned, maybe the passages that these screws are in may have "gummed up" and not allowing a change in the mixture.
I would remove the mixture screws, and spray some carb cleaner in these passages, engine off of course! When you put the screws back in, turn them in all the way until they seat (GENTLY!!!!), then back them out 1.5 turns. That's usually a good starting point for fine adjustments.
I would remove the mixture screws, and spray some carb cleaner in these passages, engine off of course! When you put the screws back in, turn them in all the way until they seat (GENTLY!!!!), then back them out 1.5 turns. That's usually a good starting point for fine adjustments.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
If the idle mixture screws don't seem to have any function, it is because your air/fuel mixture is way off at idle. It's most likely due to a vacuum leak. Pull the hose that feeds the headlights/wiper system off the intake manifold fitting and plug off that outlet. Now try it again. If you still get no adjustment functions on the idle screws, you may have an air leak into the carb or into the intake manifold via a gasket 'break'. You just have to get your distributor timing and vacuum system working and set properly before those screws will give you the 'fine tuning' adjustment you need.
Thread Starter
Melting Slicks
Joined: Sep 2007
Posts: 2,387
Likes: 123
From: CA.
its a big block auto. at 1st i did have a air leak through the manifold because bolts that hold carb became loose. i bought new gasket and it ran better but still stalls. the manifold gasket i have is maybe a quarter inch thick and very hard. is this correct one? do i need to put a sealer on it 1st? i will try what you all said. thanks.
Team Owner
Joined: May 2001
Posts: 87,302
Likes: 1,583
From: Western NY
its a big block auto. at 1st i did have a air leak through the manifold because bolts that hold carb became loose. i bought new gasket and it ran better but still stalls. the manifold gasket i have is maybe a quarter inch thick and very hard. is this correct one? do i need to put a sealer on it 1st? i will try what you all said. thanks.
As "71Vette" mentioned, you might have a big leak somewhere else, which is negating your attempts to make "fine" mixture adjustments with the idle screws.
Le Mans Master
Joined: Jan 2007
Posts: 5,485
Likes: 7
From: Peekskill NY
its a big block auto. at 1st i did have a air leak through the manifold because bolts that hold carb became loose. i bought new gasket and it ran better but still stalls. the manifold gasket i have is maybe a quarter inch thick and very hard. is this correct one? do i need to put a sealer on it 1st? i will try what you all said. thanks.
Several catalogs show a insulator gasket for 71-81 w/ Q-jet on all applications. The insulator gasket appears to be the thick one.
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Thread Starter
Melting Slicks
Joined: Sep 2007
Posts: 2,387
Likes: 123
From: CA.
yes, the sticker is still on the firewall. i do have service manual also. thanks. one more question. i just put carb back on and when i manually race engine the secondaries are barely opening at high rpms. should they?
Last edited by nutsy; Feb 20, 2010 at 11:10 AM.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
The upper airvalve on the secondaries will not open by hand manipulation of the throttle linkage/cable. It may move a bit until it hits the stop, but it won't open. The choke needs to be warm enough to open fully, or there is a lock-stop that will prevent the secondaries from opening. Once the choke is fully OFF they will open, but only if the throttle cable and linkage has enough travel and only under actual operating conditions with the engine fully loaded. Revving it by hand won't do it...ever.
Le Mans Master
Joined: Jan 2007
Posts: 5,485
Likes: 7
From: Peekskill NY
The secondaries will open up when manually pushing the carb throttle arm, when it's warmed up while it's running. I've done it many times. You need to keep the upper air valve manually open so you can look down into the carb. As stated, make sure the throttle cable is in the correct position. It's only the last 1/4 inch or so of movement that engages the secondary pushrod. There's a soft metal tang that engages the pushrod to the secondaries as the throttle is opened. It's very easy to bend (slowly) for adjustment until the secondaries open fully. You can do this adjustment with the engine off. Look down into the carb and you'll see when the secondary plates start to open.
When I got my vette, they didn't open at all, and Dennis here on the forum made short video showing this movement. I adjusted the tang. Once they opened, it was an amazing difference.
Do a search for my old thread. I think the title was "Secondaries don't or won't open"
Glenn
Here's the thread link.
http://forums.corvetteforum.com/c3-g...m-opening.html
When I got my vette, they didn't open at all, and Dennis here on the forum made short video showing this movement. I adjusted the tang. Once they opened, it was an amazing difference.
Do a search for my old thread. I think the title was "Secondaries don't or won't open"
Glenn
Here's the thread link.
http://forums.corvetteforum.com/c3-g...m-opening.html
Last edited by GD70; Feb 20, 2010 at 11:52 AM.
Race Director
Joined: Apr 1999
Posts: 11,520
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From: CORVETTE 77 385 C.I. TEXAS
nutsy
Yes it's possible to see the sec. valve open some- it did on a Q-J I setup on a high power 383. So on a 454 if the throttle is pulled hard enough... Don't concern yourself about that now, that's not how sec. adj. is set.
Yes it's possible to see the sec. valve open some- it did on a Q-J I setup on a high power 383. So on a 454 if the throttle is pulled hard enough... Don't concern yourself about that now, that's not how sec. adj. is set.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
Yes, the lower throttle plates will open up with manual [or accel pedal] actuation (which is what you really want to test), but the airvalve won't. You can hold the airvalve open to check throttle plate function, but you can't properly test the airvalve operation by revving the engine.