AFR slowly richens at idle

George Jets said
Sounds like you have an electric fuel pump, here is a test for you.<snip>
See if this helps straighten out the idle, and see if the AFR stays around 13.5

Thanks! I didn't think of that, but I should have ;) I have a toggle switch on the dash so I can turn it off. No need to disconnect anything.

Also could be your float levels could be set too low, not shutting off the fuel when the fuel bowls are full.

Shouldn't that be "float level set too high"? The fuel will still shut off when the float rises to the set point. (With a running engine, there is just a trickle always coming in that exactly matches the trickle going out) :) Next thing to double-check: fuel level not rising continuously when the engine is off but the pump on!

You were saying this is an old Holley.
Ethanol Fuels are hard on carb internal parts too if it has ever been run with ethanol ?

I did a complete rebuild this winter, converted to 4-corner idle (Quick Fuel baseplate) and adjustable brass (drilled setscrews). Everything's clean inside. Other than one tankful recently, it's always had ethanol-free pump premium. It's only got 149 miles on it, too.