Help! Carburetor is backfiring!

[pjmoreland]

When I rev my engine up slowly, the carburetor starts backfiring repeatedly. It makes a pop-pop-pop-pop noise in rhythm with the RPM of the engine.

It just started doing this today when I was driving down the highway. At first it only happened with the throttle barely open. It got progressively worse over the course of about 10 miles to the point where I thought I wasn't going to make it home.

I thought that maybe a valve had gone bad so I disconnected one spark plug wire at a time to see if there was one cylinder in particular that was causing the problem, but in no cases did the popping go away.

I checked my timing and it is set at 10 degrees. Just as a test I tried retarding the timing to about 2 degrees, but the popping persisted.

The manifold vacuum is around 15 at idle.

I have a mild 350 engine with a four-barrel Quadrajet and an HEI distributor. I triple-checked my spark plug wires. They are definitely arranged in the 18436572 order.

Any ideas?