I have a 71 Z with an L28 and 72 SU round top carbs fed by a holley street pump. Heres my problem, Normal driving, putting around town she drives fine. But when I get on it it starts to sputter/flutter type sound at around 3500 RPM. If I gradually build speed it's no problem only when I get on it is when it happens. Initially I thought I may have been running the fuel pressure to low. I had it set at 2.5 so I bumped to 3.5. Same problem. I played around with the mixture settings a bit but no change. Any ideas???