Carb trouble?

[D-Rat]

Some questions for the v8 experts....

This is a 400 that was running great up until about a month ago. There's about 12k on the engine since it was rebuilt, balanced, blueprinted, etc. The engine runs like a dream, but seems to have recently developed a carb issue. The Holley was rebuilt at the time the engine was.

Symptoms:

Runs great until up to full temp, then acts like its running rich and "loading up". Will occasionally backfire out the exhaust. Driving on the freeway, I'm feeling some intermittent power loss as if someone was lightly tapping the brakes.

When I posted this in the engine forum, I was told to check my timing, which I haven't yet been able to do since I seem to have lost my timing light. I do ask how the timing could change that much when it hasn't been touched. It has an HEI distributor, and the wires are in excellent condition with about 12k miles on them.

Fuel flow is good, and the filter is clean. I haven't been able to detect any vacuum leaks on the lines and fittings, and the hoses are new - brake booster, vacuum advance for the dist, etc.

When I pull the air cleaner when the engine is warm, the butterflies are open, indicating the choke seems to be working as it should. It has an electric choke.

Carb appears to be a 730CFM Holley, but I'm not dead sure, as I've had some difficulty cross referencing all the stamped numbers on the thing.

I'm ready to buy another carb - looking at a new 600CFM Edelbrock, but want to rule anything else out first.