I had this same basic problem on a 220S with a 250S engine it it. One carb ( the rear one ) worked great but the front was running super rich. I tried everything and eventually we changed it to another carb body. This seemed to work better but I had a very pronounced flat spot right off idle. I fixed that by bending the small arm that pushes on the accelerator pump piston. There should be no free play between the arm and the piston should move instantly.

Well....... this same car showed up again last week after winter storage. I had it running OK when it left last summer but it was now flooding the front carb, and of course, all the plugs were fouled out.

I looked it over and removed the top of the carb. The needle and seat wasn't tight enough so I found a new seal and installed it tightly - it doesn't leak now.
After some minor adjustments and a fresh set of plugs it now runs better that it ever has. Starts right up and idles smoothly. Pulls well and doesn't stall anymore.

Not easy to get right, that's for sure.