Make shure the floats are set right.
At idle do you see any fuel dripping out of the venturi nozzles? They should be dry.
Usually it's one and one half turns out to start with on the idle mixture screws. Some are backwards metering type on the spread bore Holley carbs. The more you turn the screw in the richer it gets.
Most Holley carbs are the opposite, screw in, gets leaner. You want them in as much as you can without the engine slowing down at idle.
If floats and timing are right, check for a bad power valve. If you have had backfiring it may damage the power valve diaphram and cause it to run rich. It still would not run rich at idle. It would run rich at part throttle when just cruzing along. Fuel mileage would go south too.
The metering block gaskets can leak, or power valve can be loose and leak into the power valve vaccum area.
If you have the timing or advance wrong, it will carbon up the plugs too.
If you aren't good at fooling with Carbs, take it to a good performance shop.
Good luck, David
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