The Secret Behind Quadrajet Carburetor: How it Works 

They are decent carbs for the era. I like the small primaries with large secondaries for economy cruising. Due to their attachment to the low performance malaise era, Quadrajets have a bad reputation but if kept clean, they are actually a pretty decent option.

I have never heard of that trick. I do know there are a couple of different filter assemblies available. One with a check valve built in to prevent backflow and one with no check valve. I use the ones with the check valve whenever I change out a filter.

From my experience, leaving that out can cause intermittent stuck float valve. Especially the rubber tipped valves

Don't. As the gasoline evaporates or drops, the float pulls the needle out of the seat. Otherwise it can get stuck in the seat from the "varnish" in the fuel.

I had an '87 S-15 with a fuel injected Iron Duke in it and agree whole heartedly.

There isn't really much adjustment on you primary metering rods. Check to make sure the secondaries are not stuck partially open. Aside from that, I would be suspicious of the float level as it may be dumping too much fuel down the throat.

Not as a service, no. I have a couple of carbs from early 50s flathead Fords I will be digging into for an upcoming video series but I only do them for myself. The Carburetor Shop (I think) specializes in rebuilding Quadrajets. They are near Minneapolis, MN. www.thecarburetorshopus.com/ They were mentioned in a Vice Grip Garage video a couple of years ago when Derek sent out a Quadrajet for a rebuild. Sorry I can't be of any more help than that.

As long as you have access to the adjuster screws it is fairly straight forward to adjust Some smog era carburetors had the screws capped. A good starting place on an otherwise functional engine is 1.5 turns out. Start the engine and warm it up. Get your timing and idle set in the ballpark for your vehicle. Next slowly turn one adjustor screw clockwise till the engine begins to stumble a bit then back it off till smooth. Repeat for the second screw. Go back and forth till you get the engine to run smoothly with the screws in as far as possible and you should be pretty close with your idle mixture.

The big concern I would have is jetting. Quadrajest were used on everything from V-6s to 455 big blocks so it really depends on what the '78 carb came from. If its off a 305 it might run but be rich, if its off a 350 or bigger, it might be too rich to even run.

@@DougsMessyGarage thank you much! i will give it a whirl...

@@Inthefoxhole I have always had good luck with Quads. I am not a racer, just street use. I want to be able to turn the key and go for a cruise and the Quadrajet has always been good at that.

One possibility is corrosion on the mechanism. There are several levers and linkages on the secondary lockout and they need to move freely. Another issue may be a cracked vacuum line to the release vacuum pot (or a defective pot). The lockout should release once the pot has vacuum.

Choke has to be fully open, adjust the choke spring to fully open when the engine is warmed up, just watched another video explaining it😁

Yes you should get an accelerator spray on each of the front two barrels. Depending on the configuration of the intake manifold, the front barrels almost act like two separate single barrel carbs so each requires its own accelerator spray.

@@DougsMessyGarage Hey Doug 👋 Thanks 🙏 for the reply . I have a 5.7 chev that has been converted to a marine motor . The engine ran flawless but this season when I pulled her out and un winterized her well she was super gutless and bogged down when giving throttle and wouldn’t get up on plane . When I give her throttle there is a strong jet in the right front ( pump side mechanical ) but the left is just a slow drip as if clogged. My question is . Can a guy get in there with a pick or carb cleaner and blast that back open ? Thing is I really don’t wanna tear the carb off the block with all the different shift linkage and being a newb it’s pretty daunting and don’t want to ruin my boat season Also if I’m not getting juice in that left Venturi will that cause the big back barrels to not function properly?

@@buffordevans6942 Try adding some Seafoam to your fuel tank and some directly into the float bowl vent. If you are getting at least a dribble out of the second side, that might free it up. If not, you may be able to work a pick or wire strand into the accelerator port and free up the blockage.

Keep in mind that I am not a certified mechanic, just a backyard tinkerer but here are my ideas. High idle suggests you have a vacuum leak (could also be a sticky throttle too). Basically put, an engine is an air pump. The purpose of the throttle plate is to govern the speed of the air pump. If the engine can get air someplace other than through the throat of the carburetor, it will rev up even though the throttle plates are closed. I would get a package of vacuum line caps and start capping off each vacuum port one at a time trying to figure out which vacuum circuit is leaking. It is probably one of the big lines at the base of the carburetor, either the evap or brake booster line. A concern with a vacuum leak is that the engine will be running very lean in this condition because the incoming leaking air is not passing through the venturi and drawing the proper amount of fuel. It is also possible the base gasket for the carburetor could be leaking but if the carb hasn't been off the engine, a bad vacuum line is more likely. Dieseling is caused by a carbon buildup or even a metal edge (spark plug electrode, valve edge, etc) inside the combustion chamber getting hot enough to act like a glow plug. The excessive hot condition is often caused by running lean which ties back to the above vacuum leak condition. I believe heat buildup can also be caused by engine timing being way off but given you seem to have a vacuum leak, I'd start there. In my mind, step one: Find and eliminate the vacuum leak. Step two, if step one didn't also cure the dieseling, try a 'Tune-up-in-a-can' like Berryman's or Marvel Mystery Oil, or even some automatic transmission oil. Get the engine running and up to temperature and feed the cleaning solution down the throat of the carb operating the throttle by hand. Give the engine enough throttle to keep it running while you pour in the cleaner. The cleaner will reduce any carbon deposits in the combustion chamber. Also check you spark plugs to make sure they are the correct heat range for the engine. A too-hot of a spark plug can contribute toe Dieseling Hope this helps. I am no expert, just someone who has played with these carbs a bit over the years.

Keep in mind that the secondaries on a Quadrajet carb are run by vacuum rather than by mechanical linkage. The secondaries will not open unless the engine is running and the throttle is opened up fairly wide. If the secondaries are not opening when the engine is running and you are at full throttle, check for vacuum leaks. Make sure either all of the emission lines are properly connected or capped off. Check the line going to the vacuum actuator on the passenger side of the carb to make sure the line has not rotted off. Also, make sure the carburetor base gasket is not leaking. Hope these tips help.

Nope, I haven't sold it. I have been getting the parts together to get the brake system functional. More Corvette content coming soon.

Stuck needle and seat maybe? Could be a completely collapsed filter too.

Fantastic feedback sir, thank you for taking the time to watch and comment