[Chad Johnson]

For what its worth:
1. The picture of the carburetor looks like there has been backfire(s) through it -------- new power valve for sure. Other than that my guess is electrical.
2. Check all your plug wires for continuity (including the coil wire).

3. Check around the coil tower and the distributor for signs of arching (deterioration of the metal contact).
4. Make sure that the plug wires are on the correct plugs (match the engine firing order)

You or your mechanic have probably checked all of these, but just in case.

Good luck with your problem.