[StanJ]

Mat, does the engine seem rich only at idle, or all the way through the RPM range? Curt C.'s comments are correct; a power valve(s) with the proper vacuum rating (with 7" of manifold vacuum at idle, I'd suggest 4.5 - 5.0 in your case) will help clean up the A/F ration at idle/low load conditions but will not affect WOT...unless your present power valve is damaged. How old is the carburetor? Most Holley's come with power valve protectors these days, but this hasn't always been the case. If the carb is 5 years old or more (regardless of recent rebuilds)...and if the engine has "sneezed" (backfired through the carb) hard since the power valve was last replaced, the power valve diaphram might be ruptured, which would allow a considerable amount of raw fuel to drip into the engine under all conditions. One tip-off is that engines with this problem are usually very difficult to start (a prepetual state of "flooded").

Also, is the carb clean? The (stock) main circuit air corrector orifices for your caburetor are only .028 I.D., and are exposed to all the potential "road grit" that might be getting by your air filter (you are running a good, clean filter on this engine, aren't you?). If even one of these orifices gets plugged with dirt, your carb's A/F ratio will dive toward the rich side.