you know i am not smart enough to determine if it's a ignition (detonation) or fuel issue from a single data point (car slows down in better air).
since you didn't give us any O2 data, I will assume you don''t have a sensor, can you tell me what size carb and jetting along with type of cylinder heads and your ignition total timing.

Joe

Without Data, you’re just another guy with an opinion.