Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Engine suddenly running terrible

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Cliff Ruggles:
The PCV system is a controlled vacuum leak.  Not a good place to disconnect or tie into for a vacuum gauge either.....

ourkid2000:
Very strange. This issue has reappeared once again.

Yesterday I had the car up and running and warmed up. Then I adjusted the mixture screws to max vacuum. It seemed to be running great, the vacuum was up around 19" and running smooth.

Started it up today and it seemed to be running pretty good when it started up but when I backed it out into my driveway I could feel an off idles hesitation but it was still running alright. I let it warm up in the driveway for a bit and it just seemed to get worse and worse. Sure enough, it started to behave exactly as it had before.....running terrible, barely idling, low vacuum (13-14") and a terrible stumble when you give it gas. It seems to try to rev up if you feather the gas a bit and you can almost hear it clear up until it starts to die again. It almost sounds to me that a bunch of cylinders aren't firing.

I adjusted the mixture screws back in and then out 3 turns but it made no difference at all. Note that my vacuum gauge is attached to the port that runs the headlights so I think I can rule out a leak in that system.

The choke was completely off as well when it was warmed up (butterfly was completely straight up).

77cruiser:
Wonder if you have some dirt that plugs an idle tube.

Kenth:
Have the leaking bottom plugs in float bowl been fixed?

Cliff Ruggles:
One of the reasons I do NOT like 65-66 Q-jets.  The "conversion" for them from the original parts to a N/S assembly and blocking the bypass to the fuel bowl is problematic.  If you can't find any problems elsewhere like a vacuum leak or sucking fuel from a leaking bottom plug (all of those leak and what gave the Q-jet a BAD reputation for that issue) take a gander down into the carb while it's acting up and see if the fuel level is getting too high.  You can usually see the fuel migrating up to the gasket surface where the secondary metering rods go thru it.  It could be cutting fuel off and low fuel level as well which is more difficult to determine.

I'll also add that 100 percent of those units I've taken in for problems was leaking at the bottom plugs and most were also leaking at the bypass plug allowing fuel to enter the bowl and they would load up at idle.  For this reason I stopped using the plus and "O" ring and started filling that entire passage with lead and sealing it off permanently.  I'm probably helping out some of the folks who rebuild those but absolutely and for sure the bypass plug is USELESS for that purpose.

What happens, and it happens quickly is that the "O" ring seal things off when first placed in service, then fails after being in contact with fuel.  This can take days, weeks or even months but they ALL fail sooner or later. 

Not sure if that's what's happening hear but that issue will rear it's ugly head sooner or later.  Those carbs also develop leaks at the lead plug under the fuel filter housing which mimics a gasket leak at the housing typically causing the owner to over-tighten the nut and strip out the threads, which weren't that great right to start with. 

Not trying to put a big black cloud over your project, but if I owned a vehicle that used one of those carburetors I would amputate it off the engine and shelve it permanently and go to a 1969 or later side inlet carb with all the factory upgrades.  I've done that for hundreds of customers over the years who struggled with those early units and that move ended ALL of their issues once and for all.....FWIW......

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