The Secret Behind Quadrajet Carburetor: How it Works

[@DougsMessyGarage]

I'm no expert but I have worked on these carbs a few times over the years. Feel free to ask me a questions if I didn't explain something in the video.

[@robertwest3093]

The Quadrajet carburetor is and will always be my first choice on a street engine. I’ve even used them on small block Fords before. They love them!

[@DougsMessyGarage]

I have generally had pretty good luck with Quadrajets too. Keep them clean and re-assembled correctly and they usually start better than fuel injection.

[@RedfishCarolina]

Had two of these I think, both were on Olds 307's (85 Riviera and 85 Cutlass Supreme). Never had a problem with either of them.

[@ericschrock8493]

Zero mechanical experience here so I appreciate the video! Working on one from a 72 Deville. Hard to really get into it when I don't even understand the theory of operation. Thanks for the run through!

[@DougsMessyGarage]

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.

[@setha360]

Always check the butterfly brass bushings by moving them up down for wear, Im currently waiting on bushings and remmer at the moment when bad they let air in and cause rough idling to lean condition.

[@Inthefoxhole]

I've built holleys and Quadrajets.If you want a good reliable carb with not much maintenance. Quadrajet is the way to go.I had a 73 chevy van 350 engine.350 turbo trans.373 gear Fastest van around.I ran 650 holley double pumper.Didn't perform anywhere near that Q jet in that application. I'm putting float well plugs in my Q jet as I speak.

[@DougsMessyGarage]

@@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.

[@danlaabs5090]

Some rebuilders say to remove the hanger off the needle so it seals to prevent back flow.

[@DougsMessyGarage]

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.

[@briankennedy5578]

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

[@Spiritof_76]

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.

[@sirlucius3016]

Hoping for some guidance. I have a 75 eldo 7045193. When i took off airhorn to start my process of rebuilding, there is a cone shaped dome with a flat head screw on top...looking at top view from front its at 7:00 . yours appears empty. Ive seen newer ones , with airhorn on, has a elictrical plug on top. Anyway, i tried to loosen the flathead screw...it turned but seemed stiff, so i left it. I took it off with the 2 other screws that hold this object to the metal itself. Looking at it, there is a rubber piece that would fit in that hole with a metering rod thru it. Also appears to be a jet at the bottom. The flathead i originally tried to turn im guessing controlled this rod and isnt meant to come out. So what is this ? I dont even know what its called. When i put it back on i bottomes it then backed off 2 turns. Just trying to figure out what this is, thanks

[@DougsMessyGarage]

Sorry, I don't have experience with that feature. I know Caddies tended to have a bunch of extras and GM loved vacuum actuation in the '70s so possibly your issue could be related to directing a vacuum signal? Thanks for watching and for commenting.

[@sirlucius3016]

@DougsMessyGarage i found late last night this is a rare option. Its called an 'aneroid bellow' ? Some sort of altitude adjustment. Thanks for reply

[@DougsMessyGarage]

@@sirlucius3016 That's cool. Thanks for bringing it up. That is a good piece of knowledge to add to the collection.

[@snowcrest7863]

Not an ideal carburetor for performance tuning, but when it comes to daily driving (back-in-the-day) it was the best for basic starting and driving. Many times growing up in Minnesota, the old '74 Impala Wagon fired right up at -20 below zero. When GM/Chevy came out with their first generation fuel injection in the early '80s, none of those cars would start below 0 F.

[@DougsMessyGarage]

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

[@willytv275]

Question - why is the linkage from the diaphragm connected to the slotted lever arm on the secondary butterflies? I thought this lever was just associated with the choke.

[@DougsMessyGarage]

Locks out the secondaries when the choke is active.

[@willytv275]

@@DougsMessyGarageI did some additional reading and the lever (via the diaphragm) keeps the secondaries effectively disabled during normal engine operation regardless of choke position. This is because the diaphragm is seeing vacuum generated by the engine. When the accelerator is pushed to the floor and the engine is under load at high RPM, vacuum goes to zero which releases the diaphragm (and linkage) allowing the secondaries to open. It’s all about vacuum and really nothing else.

[@randyweitzel5071]

Great videos, my 1987 monte ss 305 was rebuilt. Gas mileage is way down 10 miles per gallon from 17. What can I check. Maybe the primary metering rods need adjustment?

[@DougsMessyGarage]

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.

[@meadowpuckett2958]

Check your timing etc​@@DougsMessyGarage

[@randyweitzel5071]

Ok I'll see if a friend can help on your advice, thank you. Do you rebuild carburetors ?

[@DougsMessyGarage]

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.

[@831Batman]

My secondaries ain't opening up going up hill or when I hit the pedal. I can't figure it out what would be the reason for that please and thank you .

[@DougsMessyGarage]

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.

[@jtp682]

good video! I need some advice on something. I recently bought an 1986 chevy truck with a V6 4.3. It has a propane system on it and the original carburetor is nowhere to be found. I have a 78 quadrajet that I just rebuilt and would like to install it. the truck intake manifold is the original spread bore quadrajet carburetor. Also this engine has the electronic spark control distributor without the vaccum advance. if I put this 78 carb I got on it do you think it will work just fine?

[@DougsMessyGarage]

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.

[@jtp682]

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

[@josephsabia8396]

What would cause the secondary lockout to stay engaged when the engine is warmed up to operating temp? We’re having a problem getting the secondaries to open up

[@DougsMessyGarage]

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.

[@jimclarke1108]

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😁

[@buffordevans6942]

When giving this carb throttle should a jet of fuel be seen on both sides of the primary or just the side where the pump is ?

[@DougsMessyGarage]

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.

[@buffordevans6942]

@@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?

[@DougsMessyGarage]

@@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.

[@tomsteffs6511]

what would cause a high idle upon start up and violent dieseling upon shut down of the motor?

[@DougsMessyGarage]

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.

[@gabrielbryant892]

What I needed to know was how to adjust the air and fuel man

[@DougsMessyGarage]

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.

[@xkaosxvpx8165]

If gas can be sucked into the line but doesn’t make its way into the carburetor, what could be the issue? Thanks.

[@DougsMessyGarage]

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

[@jo-annmacdonald7260]

Makes sense to me

[@inell6]

So did you end up selling this car? Don't see anymore videos.

[@DougsMessyGarage]

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

[@dennistrahan8195]

You have NO idea how to properly disable a Q jet my freind, you ALWAYS remove the roll pin from the accelerator pump arm and the secondary metering rods before attempting to lift the airhorn top of the carb from the midbody so you don't BEND THE METERING RODS AND TUBES !!! thats why you fond the tube laying in the midbody of the carb !!! Heat has no bearing on these tubes falling out !!! Ive built hundreds of these carbs since the early 70's , NEVER have I twisted the top sideways to unload the acceleration pump lever to remove top of carb ! Dennis Trahan master technician 52 yrs experience experience .

[@DougsMessyGarage]

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