After overheating my engine trying to get off a sandbar (stupid) my maximum WOT dropped to 4000 RPM's. Immediately afterwards I changed out my exhaust manifolds and risers only because of their age. Other than the loss of RPM's at WOT the engine does start and run good/smooth. Prior to writing this message I did check the butterfly's in the carburetor to insure that they are fully opened at max throttle. They were. I then attached a vacuum gauge to the carb to check the vacuum at various RPM's. 800-900rpm - 16in HG, 2000rpm - 20in HG and 3000rpm - 14in HG. At all rpm's gauge fluctuation was + or - 1/2in HG. Short of checking the engine timing I have run out of troubleshooting options and could sure use a little help. Forgot to mention the compression in all cylinders is normal with #1 being on the low side @ 125psi. Any ideas?