What could be causing this Camaro to burn through a set of spark plugs every WEEK?

exhaust system, spark plugs
Dear Tom and Ray:

I am hoping you can help with a problem I am having with my wife's 1967 Camaro Convertible. She drove it daily to work until about a year ago, when this problem developed. Four different mechanics can't find the problem. It burns up a set of spark plugs in less than a week. All of the burned plugs are black with soot and they all look the same. The motor, which was completely rebuilt six years ago, is a 327 V-8. Two different people have rebuilt the carburetor, and then we bought two different rebuilt carburetors. I also had the valve seals changed, even though the car doesn't use oil. Two of the mechanics tried hotter plugs. I had a new coil and distributor put on. The compression is good. Any advice or direction would be greatly appreciated.

TOM: I'll bet you want some advice or direction, Richard. Especially now that you've spend a thousand bucks and 23 weekends trying to fix this thing.

RAY: I think you have an obscure problem called "manifold burn through."

TOM: I had that once after eating at my mother's house. I was chewing antacids for about a month after that.

RAY: In this particular engine, some of the hot exhaust is sent through a passage in the intake manifold. Why do they send hot exhaust through the manifold? Because it runs directly underneath the carburetor, and heats it up, which makes the car warm up faster and run more efficiently.

TOM: But when the engine gets old enough, that hot exhaust can burn a hole right through the manifold. All it takes is a pin hole. And then that hot exhaust gets sucked into the carburetor where it contaminates the fresh mixture.

RAY: And if exhaust is getting sucked into the carburetor instead of fresh air, the engine will suffer from oxygen starvation, and will behave as if the mixture is way too rich (too much gas, not enough air). And what's the sign of a too-rich mixture? Black spark plugs! So remove the intake manifold and check for a pin hole under the carburetor. It think you'll find one.

TOM: Brilliant! Brilliant, Raymond. What a brilliant deduction!

RAY: Well, it'll be brilliant if it's right. If not, what's another eight or ten weekends in the driveway, right Richard?
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exhaust system, spark plugs

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