The following photos show the change from the AFB to the Q-Jet, which took place in August of 2004.
This is the old Carter 9625 AFB that someone once gave me in 1987 for free. Notice there is no choke assembly, which makes it somewhat tricky to start even during the summer. At least I didn't have to worry about my wife sneaking off with it.
Edelbrock spreadbore to squarebore adapter. This adapter is made to mount spreadbore side down. Notice the bump on the gasket on the bottom right-hand corner. It corresponds with a bump on the adapter's bottom left-hand corner. I'm sure it was one of GM's better ideas but the carb does seem to seal well in spite of it.
Offenhauser makes a spreadbore to squarebore adapter (PN 5816-R) too but theirs is designed to be mounted spreadbore side up as we are intending to do. If I would have seen the Offenhauser catalog when I started this project, I might have used one too.
Spreadbore adapter mounted on the Offy intake. This adds 3/4" to the height of the carburetor but keeps a separate pathway to the intake manifold for each carburetor barrel. There are open plenum adapters available too but I wanted to keep the plenum volume as small as possible to minimize any chance of off-idle bog.
I had intended to install a 1/2" insulator between the adapter and the carb but found that I just don't have the head room for it with my Corvette air cleaner. The Offy-supplied mounting plate is 7/16" thick so you would gain an extra 1/16" clearance over the plate I am using.
I like the Quadrajet carb because of the small, double venturi primaries and because they were so plentiful in the junkyards. I happened to have this one surplus from a 1977 Pontiac I converted to propane in January of 1998.
The other thing I like about this carb is the small central fuel bowl. I found that the AFB was sensitive to fuel level in the bowl and had a loss of power on hard left turns. Of course, after I raised the fuel level, this problem improved somewhat. The as-installed Quadrajet has no apparent sensitivity to turns in this orientation.
I was at a speed shop back in the 80's when some kid replaced the stock air cleaner on his Corvette with one of those shiny chrome ones. I gave him $10 (Canadian!) for it and have been waiting for an opportunity to use it ever since.
With everything assembled, I have about 1/4" of clearance between the hood and the top of the air cleaner lid. I knew the kids' play dough would be useful for something other than for making a cat's birthday cake.
When I first took the car out for a spin, it seemed to run at least as well as the AFB and this is with the carb set up for a Chev 350. Since the carb has been sitting in my garage since 1997, a good rebuild probably couldn't hurt. I know the sound the carb makes when the secondary air valves open up on the 350 but I didn't hear it anything like it on the short drive I made to test it. I got out onto the QEW and took it up to about 90 mph but ran out of highway as I only went from one interchange to another.
Notice the two marks on the hood as the air cleaner was rubbing on it. If you look at the previous photo, you can just see where the paint was worn on the air cleaner lid. Looks like I'll have install a lower profile air cleaner.
I extended the air cleaner hold-down stud up to the hood and marked the contact point. From that point, I measured a radius of 6¼" to the hood bracing which means a maximum allowable diameter of 12½". The height of the the Corvette air cleaner is 3-3/8". For aftermarket, I think you would be safe using a Mr Gasket 9" diameter assembly which is only 2¾" high but I haven't verified if this would work. There are smaller 6" diameter by 2" high ones too but think these are more restrictive.
I took a walk through one of the local junkyards and found an extremely short air cleaner in a 1984 Firebird. The lid diameter is over 14" so it hits the bracing.
The Firebird air filter diameter is less than 11-3/8" and the height is 2½". After an afternoon with a saber saw and some black paint, I got an open-element air cleaner with a 12" diameter lid. There is plenty of hood clearance now with the Firebird air cleaner so I will try adding some heat dissipators later.
The air cleaner I found had a collar on the inside diameter of the air cleaner pan. I think its only purpose was to limit horsepower on the Firebird so be sure to remove it for a free-flowing air cleaner.
I got the electric choke conversion working but found that there wasn't a good ground from the thermostat housing. A short length of wire to one of the carburetor studs fixed that problem.
Notice the blue hoses going to the bottom of the intake manifold. I got my prototype intake manifold heater installed at this time too.
A close-up of the intake manifold heater. This device has completely eliminated the off-idle bog. The Quadrajet seems to pull a lot harder than the AFB but this is my seat-the-pants feeling. I'll have to go to the drag strip to know for sure.
With the open air cleaner, I can hear the air valves opening up now. With a flow capacity of 750 CFM, the sound is not quite a loud as I was used to with the much larger Chevy. They do, however, seem to crack open quite frequently while I'm cruising on the highway.