Carburetors, what can go wrong?

[david craft]

1. The throttle blade shaft can wear in the bore. This allows air to leak past at idle causing poor atomization. Need to replace base or put a bushing the bore.

2. Idle screws can be over tightened and destroy the lands where they seat. You'll never get a good adjustment.

3. Quadrajets can leak fuel at the bottom center of the main body at the plugs. Old hot rodders use to sand this area and epoxy.

4. The base can be bolted down so tight that the throttle blades wont close.

5. Choke coils can burn out, choke kickdown linkage bent, a whole host of other stuff.

Be sure to check rebuilts out of the box, I've seen sloppy shafts that never should be on the shelf. Folks use too much return spring because of throttle linkage friction or bad geometry. Ideally the spting should be 180 degrees from the linkage and just enough to return to idle.

A random simplified carb adjustment procedure;

Bring engine up to temp. Change oil if dirty. Make sure choke unloader had functioned and choke is off. Plug vacuum line to distributor and set timing. Be sure that RPM is below 1000. Reconnect vacuum line. Kill engine. Count how many turns (full and partial) to lightly seat idle mixture screws and return to initial setting. Record data. Fire engine. Adjust engine idle to factory setting. Slowly adjust screw inward till rpm just begins to drop and record. Open back up to full rpm and add a quater turn. Adust idle down to specs again if it has increased and repeat process. Do both sides. There should be at least a turn or so between lean drop and rich stumble, otherwise the screw seats may be compromised. This thread is open to comments, questions, and fine tuning...lol

[david craft]

P.s. 1/8" gap at the choke blade at 70 degrees, adjust at the choke coil.

[Jim Stoltz]

Funny you should post this....

The Quadrajet on my RV is a "new" rebuild. The eBay seller has nearly 100% positive feedback, and offered to take the carb back as a result of the issues I'm about to describe. But I figure it's not rocket science and I can figure it out. Plus I'm not 100% sure the problems are with the rebuild.

First, I can't pass emissions with it. HC and CO are really high. It's an idle test. A GM mechanic friend of mine said it should be easy to get it through by adjusting the idle mixture. The problem is even when I turn the screws all the way in, they don't affect the idle. So it must be getting air from somewhere. Shafts are tight and it's not leaking at the base (new gasket there, intake is not warped or anything). Curiously, and perhaps related to #3, if I look down the "horn" on top of the carb, near where the air cleaner stud is, I can see a pool of fuel. Also, when I shut it off I get a very strong fuel smell for a while. Last, MPG is really bad - maybe as high as 5 MPG but I suspect it's even worse than that. 

Second, I never did figure out how to get the choke to work. I put an electric heater on it and it would set but not release. If I backed off the tension adjustment, it wouldn't set. Sometimes the fast idle will work, other times it won't, and sometimes it won't kick down. I gave up and adjusted the spring so it's wide open all the time. It's tough to start in the cold, but what can I do?

When I change the oil, it's not diluted with fuel. So while it's obviously running rich, it's not so rich that it's washing down the cylinder walls. The carb is allegedly correct for my application ('84 454, truck spec). I still have the Quadrajet that came with the engine (it's a GM Performance crate engine that I bought used). I'm not sure what the carb is off of, but it has the opposite problem - too small and not enough fuel. The "new" carb really woke it up, but obviously there are still issues with it.

Oh yeah - it "runs on", too, after I shut it off. Particularly when it's hot.

Ideas? (sorry that was so long)

[Russell E Johnson]

I had the quadrajet rebuilt on our 87 454 and for the money it cost me I could have gotten a brand new Edlebrock 650CFM cheaper.  Wish I had gone that way, but I thought the rebuild would be cheaper.  Now my engine floods everytime I try to start it, hot or cold.  It runs great after it is started, but getting it started is a real pain.

[Jim Stoltz]

Is that what the stock Quadrajet is? 650 CFM? I might go that route, or at least slap the old carb on to get through emissions inspection.

[Russell E Johnson]

I don't know if the quadrajet is a 650 or not, all I know is that the 650 Edlebrock is quite sufficient and they work straight out of the box.  I have used them on our 66 (327) & 78 (360) jeep wagoneers and on a 76 Ford F250 with a 351 Windsor and never had a problem with any of them.

[Jim Stoltz]

I might have to do that. I've been dong some research and the Quadrajets are trouble-prone because of their age. Even the rebuilds can have warped air horns or bases. I'd be willing to bet a warped air horn is my problem. 

I just searched on my '84 Quadrajet specs and it's actually 800 CFM. Edelbrock makes an 800 CFM carb with an electric choke for $400. I'll have to save my pennies and pop one on in the Spring. I'm going to take my Quadrajet back off in the meantime and put a straight edge across the air horn and base.

[david craft]

Try this...one full pump to the floor. Then half way to set the fast idle cam. Then just crank with the foot off the pedal.

[Russell E Johnson]

OK, I will give that a try and see if it works, thanks.

[david craft]

Ok...as ive posted before drivetrain is not my forte. I just wanted to clarify that its been a couple decades since I have rebuilt one of these. So, if the idle screws have no effect, your leaking fuel or off the idle circuit into another circuit. I'm worried that: metering rods didnt like shipment, you'll void warranty opening carburator. Heck, those phenolic floats get old and saturate as well. You have to pull it and weigh it to tell. That rebuilt should have worked out of the box. Personally I used to just rebuild my own. But, carb soak is expensive if you can even get it. Are you using a tach for the idle speed? Make sure your off the fast idle cam...maybe send it back and go with a local builder or get an economaster. If they still make these Ive had good success

[Jim Stoltz]

Thanks. It looks like everything is new, including the float. Since the wrong carb for the application came with the used engine, I figured I'd buy the right one already rebuilt. As Russell has said, I'd have been better off spending another $100 for a brand new Edelbrock.

I've opened up the carb already trying to fix it so there's no warranty anymore. I replaced the air horn gasket and the tang from which the secondary metering rods hang (it was bent). I may spend a little more time with it before I snag a brand new one.

[david craft]

a old rodder with q-jet experience would be good to have. they really are a good carb when all is well. other things to check. fuel lines dont contact coolant lines or other sources of heat. carb preheat valve on exhaust manifold is functional. you prolly read about the retarded timing for engine run on. on a side note, check the advance weights on the HEI. when then engine misfires, the spark can go right down these and try to weld them.

[david craft]

Sheesh...I lost a post...lol. Check vacuum line to brake booster, booster diaphram. Maybe just cap this line to check for changes. If clogged, the PCV valve might force you into an off idle circuit as well.