weird vacuum leak

[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.

[Hillbillyenginering]

A couple of question.
What year is the intake manifold?
How did you verify the vacume leak is there?
Does the manifold have the internal heat tubes in it like some of the Buicks did?

[Marx3]

the car / engine is from 1976 and completely stock.
I have been wondering if the intake is uneven or something, but this problem has never been there before. The carb was rebuild previously. never any problems. Then I took it off to recalibrate the idle and primary systems. then the problem started. Originally I overdid the idle calibration( too much fuel and alot of bypass ), which made it impossible to get the idle below 800 rpm. otherwise the car pulled like a freighttrain with the new, more generous jetting.

 got the calibration down to stock again. now I encountered this problem. smooth idle at 5-600 rpm right untill the car has been driven at wot. Then the idle goes bad and climbs a bit.

I veryfied the vacumm leak by spraying brake cleaner around the baseplate, very carefully. Right around the front passenger side bolt, the idle is clearly affected, when psrayed with brake cleaner.
 
No these late 500 engines did not have the Buick-type heat passage.

[Hillbillyenginering]

I don't want to sounbd like D BAG.
But did you check the base for warpage?
I might try purissian blue on it and bolt it down and see if it is contacting correctly.
Thick grease will work also.
Maybe it is two problems.
Vacume leak and sticking power valve  or too strong of a spring.?

[Ethan1]

I don't want to sounbd like D BAG.
But did you check the base for warpage?
I might try purissian blue on it and bolt it down and see if it is contacting correctly.
Thick grease will work also.
Maybe it is two problems.
Vacume leak and sticking power valve  or too strong of a spring.?

 I agree. Check for warpage. Use some machinist dye and see if it contacting everywhere correctly. Also, use a straight edge as well.

[Hillbillyenginering]

Something else that you can try.
Get some liquid machinests dye, dykum. Or that liquid from bingo robbers.
Put it in a small syringe, when you have verified it is having the vacume leak, squirt a little on the area. When you see it get sucked in, shut the engine off. Let sit for 10\ 15 minutes to let it dry. Remove and it should show the path.

[Marx3]

I never checked the baseplate for warpage, cause the carb ran perfectly without leaks before I re-calibrated it.
I'll check up on it though.

Good idea with the machinst dye. I have some of that stuff in blue.
sticking power valve? The spring is stock. everything is stock, besides the jet and rods.

[Ethan1]

Something else that you can try.
Get some liquid machinests dye, dykum. Or that liquid from bingo robbers.
Put it in a small syringe, when you have verified it is having the vacume leak, squirt a little on the area. When you see it get sucked in, shut the engine off. Let sit for 10\ 15 minutes to let it dry. Remove and it should show the path.

 I like your thinking!

 Definitely worth a shot.

[Hillbillyenginering]

You said that normal driving you never have a prob!em. After WOT it has an idle issue.
Prior to WOT,the power valve is usually down from vacume. Wot takes away vacume. Hence spring pushes the valve all the way up. If it sticks, vacume will not be able to seat it. Causing more fuel to enter the idle circuit.
If it was just a vacume leak, you should see a lean ssue on four cylinders, not two. Also it should have higher rpms.
But maybe you are having a primary or secondary ignition issue.??
I know it seems odd. But I fought a carburation issue for years. Even swapped manifolds on my toro.
Finally the old Mallory coil took a dump. Changed coils and carb issue went away. But of course I bought a MSD distributor first.

[Hillbillyenginering]

Just throwing this out there.
Havew you used a timing light when it is tuning rough, after WOT. And see is one of your weights are sticking. Causing way too much advance at idle?

[Marx3]

I am not sure the idle circuit gets more fuel when the needles are raised... I mean, that not how it works?

Ill check up on the movement of the powerpiston.

The carb is completely rebuild and is in overall good shape.

It IS a vacuum leak. Idle quality can clearly be affected, when spraying break cleaner at a certain area between the manifold and the baseplate.

Timing has been under suspicion. It is rock steady in idle and the distributor, like the carb and the rest of the car, is in good condition. 
It seems the problem only starts after a WOT situation.