Quadrajet ID and adjustment information

[GulfCoastFighter0]

Sent: 7/18/2008

I have a 1980 Minnie Winne who's original 400 has been replaced with a 350. It seems as if most of the accessories were kept from the original. My question concerns the carburetor. Then only info I can find on it is the Quadrajet and GM logos. It is a four barrel model. I have the shop manual, but the adjustment section reads like a law book and has no illustrations for the location of each different adjustment screws.. With all the hoses and lines around I can?t locate the mixture adjustment screw(s). I have experience tuning Holley carbs, but this is my first Quadrajet. Any help finding the model ID and mixture tuning would be of great help.

[Lefty]

Sent: 7/18/2008

Most Quadrajet carbs came with the mixture screws set at the factory, and sealed with a plug. Unless you are totally rebuilding the carb there is no need to remove them. If  you determine that you do need to access them, heres how.
Locate the two aluminum plugs (They will be about the size of a pencil eraser head), They will be located on the forward facing side, down low near the base. There is usually a large vacuum port right between them (often this is the PCV hose) Use a small drill bit and drill a hole in the center of each plug. Then thread a sheet metal screw into the hole. Take a pair of vise-grips, and grip the screw, pulling the plugs out. The mixture screws will be behind them. You will need a "Double-D" Mixture Adjustment Tool (Available at any auto parts store)
As a general rule of thumb, to set the mixture for initial running you first run the screws all the way in, then back them out 1-3/4 turns. This will usually be close enough for the engine to start and run at a decent idle. Then you can "fine tune" it from there.
With the engine running at proper idle speed (750rpm), fully warmed up (physically verify the choke is open fully, Rochesters are prone to sticking), and with the ignition timing properly set, you can adjust for proper air/fuel mix.
Slowly turn each screw 1/4 turn at a time (turn one 1/4 turn, then turn the other screw 1/4 turn, alternating) IN, until you hear the idle speed drop approximately 50rpm. Then, back each screw OUT about 1/2 turn. This should give you the most fuel efficient adjustment, while still allowing full power. Then, reset the idle speed if necessary to 750rpm.
The idle speed adjustment screw is located on the left (Drivers) side, where the throttle cable is attached. It will be the one that is facing forward, and is a slotted screw (Flat Blade).

There is no reason that your carb off of the 400 shouldn't be just fine on the 350. If you went from a 454 to a 305, then you might have an issue. But the 400 and 350 are close enough in size that it won't matter. And honestly, I doubt that you would be able to tell the difference if yo removed a properly tuned carb off of a 400 and stuck it on a 350, without re-adjusting anything.

[GulfCoastFighter0]

Sent: 7/19/2008

Thanks for the info! The engine is running rich, as the exhaust is showing some smoke and smells of raw fuel. I?ve performed a tune-up already. I guess I?ll start with checking the float level and go from there. I also found the model code, located on the driver side of the float bowl.
No doubt about the carb being fine, they used this same carb on 6cyls and mild big block applications as they claim it?s a variable cfm carb design. I included that info to assist with ID only.

[GulfCoastFighter0]

I had a "professional" rebuild the carb but it made no difference.  I got it back just in time to install it and test for our camping trip tomorrow but it was smoking / flooding worse than before.  So I popped the top, adjusted the float and re-installed it.  Not much difference so I make my own adjustments and made a huge difference!  I finally got it running good an now the front driver side brake caliper is not working!  I bled it, took it off and nothing.  No time to fix it before the trip so thank God for close friends.  We have borrowed out friends '97 Fleetwood Class C so we can still go camp with the wolves!.