Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

weird vacuum leak

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Marx3:
I rebuilt a 17056230 for a stock Eldorado. spiced it up with a few modifications to give it more fuel, 44 rods, DA's etc.
Nothing radical.
installed bushing on the throttle shaft, centered the plates.
When I fire the car up it idles at 500-600 rpm if I want it to. It purrs like a kitten. Take it out on the road under normal driving, it still purrs like a kitten idling at about 600 rpm.
It seems I can do this forever and the idle never changes.
Go WOT and the idle now becomes erratic, almost like it has a misfire on one or two cylinders.
I have been figthing a vacuum leak right around the passenger side front carb to mainfold-bolt. Tried several different gaskets, both thick and thin, but the problem just wont go away. As soon as the engine has seen WOT, this area develops a vacum leak.
I have wonder what possible things in that area could create a leak. Other than the throttle shaft I cant think of anything.
The carb was born with an electric choke, so it would not have a vacumsource for the choke housing.

Anyways, I have been fighting this problem for a long time and I am propably starring myself blind here.

ANy suggestions as to what could cause a vacuum leak after going WOT ?

carmantx:
To start, I would verify that the secondaries are fully closing after WOT and allowing the primary side to fully close.

Marx3:
good idea. I will check up on that. I just dont see how this would cause a vacuum leak at the passenger side front corner of the carb... ? right around the front and the fast idle cam.

carmantx:
I had a customer complaint about a similar vacuum leak.  I could not find an issue.  I built a replacement baseplate and swapped them out and issue went away.
I used that same baseplate on another build and it had no issues. 
check closely for gasket material or other foreign objects on the base or main body.

Marx3:
I actually tried two different baseplates before. The engine suffered from high idle for a while, untill I put in a basepalte from a 77 425 carb. Both baseplates are rebuilt with bushings and plates centered etc.
I am going to put in the other baseplate and see what happens. I´ll report back in a couple of days once its done.

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