That's good that total timing is right.

You didn't mention the cam in the engine. If it's an aftermarket cam with more-than-stock valve overlap, you might need a lot more than 10* initial timing...like 15 to 18.

If that's the case, you might have the curb idle screw cranked too far open, exposing too much of the transition slot, causing the rich condition you describe.

Flip the carb over and take a look at the relationship between the t-slots and the throttle plates. The plates should expose enough of the t-slot that it looks like a square. Any more and it must be fixed, either by giving he engine more timing or other methods.