Re: Holley 2 barrel running rich (long)
10" at cruise (loaded) seems a bit low.
Try something - rev it to about 3000 RPM - out of gear (unloaded.) See what the vacuum is there. -- should be at least 16, preferably 18 or more.
Then idle again, now snap it hard, make sure the vacuum goes to near 0, then snap it closed once the R's are up. Vacuum should bounce way up on deceleration, like 19 or 20, then back to whatever's normal at idle.
If it doesn't go up to at least 16 at 3000, and/or bounce up after a fast snap closed, suspect plugged exhaust.
A plugged exhaust can blacken the plugs making it seem rich. It's the incomplete combustion that causes the blackness.
Also timing way retarded can also cause the blackness, again misleading.
Let us know what you find please.