Carburetor Running Rough

[c3po]

My Quadrajet carburetor backfired a few times today even when it was warmed up, here is what I found:

1) Carb bolts were loose, retightened them
2) 2 vacuum caps were bad, replaced them with new ones.
3) Gasket on top of carburetor is ripped, going to get another 1

She is running better but I think the carburetor gasket up top that is ripped could be leaking some air in. I may also need to check out the carburetor base gasket. I just have a feeling that unwanted air is getting in somehow.

 

[Oldswagon]

Check the throttle shaft for play. The bushings will wear out and allow a vacuum leak. You'll have to take the carb base plate off, but a machine shop can rebush it for minimal cost.

 

[c3po]

Originally Posted By: Oldswagon
Check the throttle shaft for play. The bushings will wear out and allow a vacuum leak. You'll have to take the carb base plate off, but a machine shop can rebush it for minimal cost.


Thanks for the advice, this carburetor is only 4 years old, but I will check this out if she is running funny after I put the top carb gasket on.

 

[artificialist]

I'm wondering... Is there some electronic equipment attached to the carb? A failing O2S or mixture control valve could cause an incorrect air fuel mixture and than that could be the cause. Also a TPS or MAP problem could cause the spark timing to be incorrect, but a vacuum leak would have to be solved before the MAP sensor can be blamed for a problem.

That in mind, I would think that all the mechanical parts of the carb would need to be in working order before an electronic diagnosis can be performed.

At least I think it works that way. I learned automotive diagnostics when cars with carbs became a small fraction of the cars on the road. That, and most of the equipment found on electronically controlled carbs and spark are in TBI and MPI engines.

 

[flanso]

Look down the primaries, with the engine off. Open the throttle a bit and watch for fuel discharge from the accelerator pump orifices.

 

[c3po]

Originally Posted By: artificialist
I'm wondering... Is there some electronic equipment attached to the carb? A failing O2S or mixture control valve could cause an incorrect air fuel mixture and than that could be the cause. Also a TPS or MAP problem could cause the spark timing to be incorrect, but a vacuum leak would have to be solved before the MAP sensor can be blamed for a problem.

That in mind, I would think that all the mechanical parts of the carb would need to be in working order before an electronic diagnosis can be performed.

At least I think it works that way. I learned automotive diagnostics when cars with carbs became a small fraction of the cars on the road. That, and most of the equipment found on electronically controlled carbs and spark are in TBI and MPI engines.


I got rid of the electronic carburetor a few years back, it was going bad and a new Electronic Quadrajet Carburetor would have been $1100.00 I said no thanks.

 

[c3po]

Originally Posted By: flanso
Look down the primaries, with the engine off. Open the throttle a bit and watch for fuel discharge from the accelerator pump orifices.


I forgot about this 1, if it is squirting out a small stream than it should be fine, no stream of gas then it is bad.

 

[flanso]

Discharge per each accelerator pump orifice should be approximately 0.2 cc per stroke. Eyedroppers often have scales with graduations every 0.1 cc. Get one, seal the bottom hole and hold it with a wire in front of each pump orifice. Measure discharge 5 times and take the average. If it is not near 0.2 cc per stroke, the rubber cup on the accelerator pump is probably worn. On most Q-Jets, the pump has a floating cup; the cup itself can be replaced on the old pump stem. You probably won't need a complete new stem.

 

[c3po]

I checked the accelerator pump and it seems to be shooting fuel out fine.

I replaced all of the vacuum caps as a few of them had some cracks in the rubber. I am going to run this car tommorow so I will let you know what happens.

 

[SuperEd73]

c3po,

Running rough/backfires could be anything from a bad batch of gas to a clogged fuel filter to a gasket/vacuum leak to timing getting knocked out of whack somehow. Easy way to check for a vacuum leak around the base gasket would be spraying a bit of carb cleaner around the base of the carb with the engine idling. If the RPMs go up, then you know there's a leak present.

Ed B.

 

[c3po]

SuperED73, I can check the fuel filter tommorow since I have another one.

I did replace a gasket on the TOP of the carburetor where the air filter sits, I put it on yesterday as the old gasket was split in half.

How could the timing have been knocked out of whack. I did have to tighten my carburetor bolts, they were loose, could a bad gasket cause loose carb bolts.

Anyway, I will do the spray thing with carb cleaner.

On my intake manifold there is a cap with allen screws for the hot air choke which I do not have, so there is a cap, I took it off and there was some CARBON in there, could this be an issue and do I need to get rid of the carbon in the intake manifold.

 

[Oldswagon]

Which gasket did you replace? The round gasket that seals the air cleaner to the top of the carb? Did you remove the top of the carb (exposing the fuel bowl, accel pump, etc) and replace the top gasket? The round gasket at the top of the carb has nothing to do with driveability, it just seals unfiltered air from getting in. You can run without that gasket entirely.

Loose bolts may have warped or damage the baseplate gasket. Worse yet, unevenly loose bolts could have warped the entire carb baseplate. A base gasket is cheap, a baseplate is a more expensive and a little more involved to replace.

To me it sounds like you have a vaccum leak somewhere. Do like was suggested and check for leaks with spray. An accelerator pump won't cause rough running (typically it can cause hard cold starts, and hesitation under acceleration). Bad gas is also a possibility, but it never works out the easy for me. The fuel filters on a Q-jet are dirt cheap, and super easy to change (I change mine annually). So put a new filter in regardless.

Carbon probably doesn't help either. Get some GM Top engine cleaner and run it through the engine. It works wonders to clean out the engine.

 

[c3po]

I replaced the round gasket that seals the air cleaner, I will spray the base plate area.

I do not think it is the accelerator pump because it starts up fine.

If the base plate area is leaking I will pick up another gasket, I have been thinking vacuum leak this whole time.

What would carbon have to do with all of this, incomplete combustion.