Turn the PUMP up all the way as high as it will go. Then use a regulator as close to the carbs as you can get it and set it to your desired number. As I say you don't need as much pressure with a 2x4 as you do with a single. But use whatever you prefer. Then test and see what you get. If nothing changes then proceed to check the pumps flow rate etc But start by turning the pump back up to max.