I’ve had similar problems recently on 2 different O-320’s I’ve built, just don’t have enough leaning authority. If you have a fuel flow it might be helpful to post those numbers. In my case both engines had to have their carburetors modified to get enough fuel flow with one going through multiple carbs. Having dyno’d several 320’s I can tell you carbs with the same model # flow differing amounts of fuel right out of the box. It seems the new “billet” carb bodies flow different than the traditional cast bodies, with the cast ones generally flowing more fuel. This seems to be happening more and more with the new stuff. I’ve heard fuel formulation is part of the problem as well. Bottom line-you need more fuel and will have to have the carb modified by a shop familiar with the problem. I’ve read posts hear where individuals have had success drilling out the main jets on the MA-4 SPA’s themselves.
When doing some carburetor swapping on the the dyno I found the -5217 to work better than some of the others.
I feel your pain !