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Quadrajet won't idle after rebuild

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So the float valve kept stuck closed on my father's '76 454 big block one day. After that it became harder and harder to start. So I figured it was time for a rebuild. It's been 25+ years since the last rebuild and years of the truck sitting without being driven hasn't help.


Despite having rebuilt several Webers, I was mystified by the Quadrajet and decided to have it rebuilt by a local mechanic who has rebuilt hundreds of them. In fact he rebuilt this same carb some 25 years ago.


Dad picked it up from him and he said, "Just put in on, don't mess with it". I don't know what he meant by that because usually carbs will at least need the idle mixture adjusted after the fact.


So I put the carb back on and now the truck won't idle. In order to start it I have to pump the accelerator while cranking and hold the throttle at least 1/4 way to keep it running. If I let off the accelerator it will run about 2 to 3 seconds then stall. When this is happening it seems to idle too fast at that. So I tried adjusting the mixture screws which has no effect.


The engine will not produce black smoke out the exhaust which leads me to believe it's running too lean. So does this sound like a lean mixture?


So this got me thinking. When I initially removed this carb, all that was between it and the intake manifold was this black cardboard looking spacer. No paper gaskets.


When I got the carb kit it came with a new black spacer and two paper gaskets.


Since the engine ran fine with just the spacer before the rebuild, I decided to just use it. I was in a hurry because I hadn't had the time to put his carb back on so I really wasn't thinking to be honest. I did not look closely to see if all three were identical. That should have been a dead giveway to use them.


I'm starting to wonder if the problem is a massive vacuum leak between the carb and intake manifold due to the lack of paper gaskets. Does this sound logical?


If so them why did the carb work just fine initially without the paper gaskets but it's not sealing now? Maybe a slight difference in this spacer? I could see this on a 1500cc 4 banger but looks like it would have to be a massive vacuum leak to stall a 454 big block.


This weekend I'm going to install the paper gaskets and see if it idles. Maybe spray some copper coat sealer on them just for good measure?


Now that I have messed with the mixture screws, can someone tell me how to reset them to where I can at least get the engine to idle and stay running long enough to where I can adjust them?


On Webers, it's, turn the screw all the way in, then back it out 2 turns then go from there. Usually the engine will start to bog down from excess fuel once you turn the screw out too far.


How many turns outward should the screw be on a Quadrajet? I've never adjusted a carb with two mixture screws before. What's the best way to get both banks syncronized?



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How old is the fuel? Could there be condensation in the tank?



Well actually the fuel is at least one year old. Last spring I installed selector valve, fuel lines (20 feet) and filler hoses. Not an easy job. I poured what I had syphoned out of the tanks. Split it in half to each tank to check for leaks.


The truck was running good at the time then it started this float valve issue which lead to the carb rebuild.

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Make sure you have all vacuum lines connected, especially PCV and brake booster lines. This sounds like there is a blockage in one of the circuits in the carb. To check for leaness, use your hand as a choke over the top of the carb while it is idling. You may find it easier to have a helper with you, or run a jumper wire down to the starter to crank the engine from within the engine bay area. Finding a kid to help is easier.


You say the float stuck closed and it was harder and harder to start. That does not really sound right. If it is stuck closed, no amount of pumping the pedal or holding the throttle open will help it start. No gas is no gas. I think you may still have the same issue you had before rebuilding the carb, bad gas. Drain the tanks and replace the fuel, or, get a 5 gallon can of gas and run a fuel line from the can of gas to the input side of fuel pump. You will need to replace fuel in carb as well.


Mixture screws generally will work if wound all the way in (lightly bottomed out, don't crank them down tight), then backed out two full turns. Generally you will turn the screws when truck is idling about a half turn each in either direction to adjust mixture. Whatever you do to one screw, do the same to the other one.

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sounds like the choke is not set up properly



Choke is working properly. Closed when cold then I managed to keep the truck running long enough to where it opened up.


One thing I noticed that as soon as the engine dies, smoke or vapors are being emited somewhere around the base of the carb. Typically you might see this out the venturis when you shut the engine off but it appears to be coming from the base of the carb. That makes me think I have a big vacuum leak somewhere around the carb.


About the gas. Yes it's possible the fuel has gone stale. But if there were the case, then looks like the truck wouldn't even run at all. The truck started, idle and ran fine back in the spring before this carb issue occured. Keep in mind this truck has seen less than 1000 miles in the past 10 years if that gives you any idea how much it's been driven. That gas maybe a combination of several years worth of gas.


If adding the paper gaskets to the spacer doesn't fix the problem, I'm going to syphon the fuel out and start with new fuel just to rule it out. I can always burn that off in my lawmower. Or at least dilute it down with fresh fuel.


I normally fill several 5 gallon jugs of gas up whenever I run out for my lawnmowers and use it until it's gone. I'm using gas I bought in July of 2010 and my mower still idles and runs fine. Matter of fact I was trying to toubleshoot an issue with my 68 Fairlane and thought it was the fuel. I syphoned it out and put fresh fuel back in .Turns out it was my Grant steering wheel horn button causing feedback to the ignition system and headlighs through the steering column. I ran that gas in my lawnmower with no issues and it was close to 2 years old but had been treated with StaBil.


I haven't bought any gas for my lawnmower in over a year!

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  • 3 weeks later...

Fascinating...to me, at least. I haven't rebuilt a Quadrajet in more than about 15 years. I had a 400 smallblock in my 79 El Camino with a 3310 (780? cfm) Holley and was getting crappy mileage and throttle response, so I put a Quadrajet on it that I rebuilt and rejetted both the main and secondaries on as well as the power valve. Great carburetors, but not made for quick rejetting. But after working on it for weeks I could rejet any of the circuits on the side of the road with a screwdriver.


To your problem:

  • I'd look at the old thick heat isolator spacer/gasket and check the shape outside and inside compared to the new one. There are many configurations of Quadrajets. If you don't have the old one, look at the bottom of the carb and the spacer.
  • check the adjustment on the secondaries with the carb off the manifold. They should open and close smoothly and close completely. If they're cracked open, they can affect the idle. My recollection is there is a idle stop screw to allow them to close but not let the butterflies ride on the bores. They gotta close almost completely.
  • Various configurations of Quadrajets had different vacuum ports. Make sure all of the are hooked up or capped.
  • While you have the carb off the manifold, remove the idle screws, flip the carb over, and blow compressed air in the screw hole and observe that air comes out the idle port (small hole) just below the butterfly. Preferably do this with the carb top cover off to see if any crap blows back into the floatbowl, if you are comforable with removing the cover.
  • Do the fuel change recommended earlier.


I've run out of ideas. But I'd do all of those above before I ran it again. They don't take long. After that, consider returning it to the guy that rebuilt the carb, if you haven't already by now.


My opinion is that a Quadrajet is a much more sophisticated carburetor than people give it credit. It makes a Holley look like a garden hose and a barndoor. You said you rebuilt Webers, you know what I mean then. Webers and Quadrajets are closer together than Holley's and Quadrajets, in terms of types of circuits and adjustability. I had Webers for years on a 4 banger. Rebuilt them and others a number of times...long time ago. A few years back, I gave my older brother all my collection of Quadrajet parts and extra carbs. I had a pretty good selection of primary and secondary metering rods and power valves and whatever else is in those things. I had a book from one of the hod rod book publishers on the Quadrajet. It helped explain that complex carb pretty well.


Good luck

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IIRC you shouldn't need any gaskets with the spacer. One thing to check. Open the throttle by hand and see how much the throttle shaft movers up and down. If this carb has never had the shaft rebushed, it likely has alot of movement. This might not be your main problem, but it will cause idle problems.



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I finally got around to working on the truck today and it seems to be fixed!


I installed the previous spacer (no gaskets) and sprayed some Permatex Copper coat on both sides just for good measure. The truck fired right up and idled fine.


I believe I have the idle mixtures adjusted correctly. Unlike Webers I have delt with where you have to find a sweet spot between screwed all the way in and so many turns out. Too many turns out and the engine will start bogging down and possibly belch black smoke out the exhaust.


With this Quadrajet if I turn the mixture screws in, the engine will stall but if I turn them out so far, there doesn't seem to be any difference where the engine picks up speed and all the way out. In other words 6 turns out isn't any different than 2 turns out.


The only thing I can figure out was the spacer that came with the rebuild kit was a bit different. There is an indentation in the intake manifold. The old gasket was cut out around this indentation. If that makes sense.


Only issue now is if I open the throttle too quick, it feels like the engine want's to bog down for a split second. I had this issue with a Weber and it was the accelerator pump leaking fuel externally (no fuel going in the carb). But with this truck, if I open the throttle real fast I can see some black smoke come out the exhaust which leads me to believe the accelerator pump is working. Maybe it's pumping too much fuel? Is that possible?


I gave it a test drive up my driveway and I do believe the secondaries were not functioning because this truck will really move on now.

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Related to your bog. Sometimes increasing the spring pressure on the on the secondary air valve will help. It's been 25+ yrs but you should find a recessed screw head on the side top, near the back.There is also a set screw under this that the head points down. Loosen the set screw then tighten the other. This will slow the opening of the airvalve. It will decrease or eliminate your bog.



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I'm not sure what "too quick" means but if you mean accelerating normally from a low speed, it can be the idle jet settings. Try a 1/4 turn in or out. Also most QJets have two holes in the accelerator pump arm. Typically you want the rod in the inner hole.


If the problem is when you go WOT then markw8's advice may solve it.


Was this the original QJet for the engine and did it ever work correctly? If not, then it may have the wrong jets and rods for your engine.

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