Quadrajet Idle Speed/Mixture Adjustment
Thread Starter
Instructor
Joined: Feb 2003
Posts: 164
Likes: 1
From: Houston Texas
Quadrajet Idle Speed/Mixture Adjustment
Hi Everyone! I am trying to fine tune the idle speed and mixture on my Quadrajet.
I read through quite a few threads about the specifics of this carburator and of the model year (CEC valve, etc...). But I can't find the answer to my question.
I have disconnected the vacuum connection from the carb to the distributor and plugged it. I made sure that all vacuum ports are either connected or plugged, except for one, next to one of the two mixture adjustment screws, which I connect to my vacuum gauge.
At normal operation temperature, I get an idle speed of 1,100 rpm and only 2 to 2.5" of vacuum. This reading looks very low to me. I was expecting a reading between 10 and 18. I checked for any leak in th system. I couldn't find any, in the vicinity of the carb anyway.
The car runs just fine, so I don't want to mess too much with it. But all suggestions are welcome.
Athanase
1971 LS5 - 4 speed
I read through quite a few threads about the specifics of this carburator and of the model year (CEC valve, etc...). But I can't find the answer to my question.
I have disconnected the vacuum connection from the carb to the distributor and plugged it. I made sure that all vacuum ports are either connected or plugged, except for one, next to one of the two mixture adjustment screws, which I connect to my vacuum gauge.
At normal operation temperature, I get an idle speed of 1,100 rpm and only 2 to 2.5" of vacuum. This reading looks very low to me. I was expecting a reading between 10 and 18. I checked for any leak in th system. I couldn't find any, in the vicinity of the carb anyway.
The car runs just fine, so I don't want to mess too much with it. But all suggestions are welcome.
Athanase
1971 LS5 - 4 speed
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Hi Everyone! I am trying to fine tune the idle speed and mixture on my Quadrajet.
I read through quite a few threads about the specifics of this carburator and of the model year (CEC valve, etc...). But I can't find the answer to my question.
I have disconnected the vacuum connection from the carb to the distributor and plugged it. I made sure that all vacuum ports are either connected or plugged, except for one, next to one of the two mixture adjustment screws, which I connect to my vacuum gauge.
At normal operation temperature, I get an idle speed of 1,100 rpm and only 2 to 2.5" of vacuum. This reading looks very low to me. I was expecting a reading between 10 and 18. I checked for any leak in th system. I couldn't find any, in the vicinity of the carb anyway.
The car runs just fine, so I don't want to mess too much with it. But all suggestions are welcome.
Athanase
1971 LS5 - 4 speed
I read through quite a few threads about the specifics of this carburator and of the model year (CEC valve, etc...). But I can't find the answer to my question.
I have disconnected the vacuum connection from the carb to the distributor and plugged it. I made sure that all vacuum ports are either connected or plugged, except for one, next to one of the two mixture adjustment screws, which I connect to my vacuum gauge.
At normal operation temperature, I get an idle speed of 1,100 rpm and only 2 to 2.5" of vacuum. This reading looks very low to me. I was expecting a reading between 10 and 18. I checked for any leak in th system. I couldn't find any, in the vicinity of the carb anyway.
The car runs just fine, so I don't want to mess too much with it. But all suggestions are welcome.
Athanase
1971 LS5 - 4 speed
To read manifold vacuum you must use one of the 3/16" nipples in the aluminum base; not the nipple up higher because that is so-called "ported" vacuum.
Instructor
Joined: Dec 2011
Posts: 189
Likes: 55
From: Virginia
Check this link: https://www.corvetteforum.com/forums...nnections.html
Sounds like you hooked your vacuum gauge to the ported vacuum rather than the full manifold vacuum port.
Sounds like you hooked your vacuum gauge to the ported vacuum rather than the full manifold vacuum port.
Thread Starter
Instructor
Joined: Feb 2003
Posts: 164
Likes: 1
From: Houston Texas
Check this link: https://www.corvetteforum.com/forums...nnections.html
See the yellow arrows on the picture below:
The two ports I had my vacuum gauge connected. Wrong?<br/>Which one is the "ported" vacuum?
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Are you sure your gauge is accurate? I would think either of those two bottom nipples would be manifold vacuum. Check your gauge by plugging it into one of the fittings on the #8 manifold runner or the big one going to your PCV valve.
Burning Brakes
Joined: Apr 2008
Posts: 1,222
Likes: 93
From: shelbyville West Michigan
this may help: ported vacuum is typically above the throttle blades, but can be down at the base as well if it is internally passaged to above the throttle blades. manifold is typically below the line of the throttle blades. Not familiar with that carb enough to tell, but if unsure, use the manifold fitting directly on the intake...
