I am running a mallory regulator with a return line to my carb, and am still using the tbi pump. At idle i am between 6 and 7psi, if i get on it more than half throttle it drops to 4psi. The real problem is when i floor it my pressure immediately goes to 0. I have a nordskog digital fp gauge to monitor my fp. What could be causing this, what should i check? All opions and suggestions are apreciated.

I know the manufacturers always say to locate the reg as close to the carb as possible for best results. I run an expensive $95 Aeromotive fp reg with a return line and don't have problems. Also if you're dead heading it instead of having a return line you may see the pressure variance

If the pressure sender is between the carb and the regulator like is should, either the regulator isn't working or the pump isn't keeping up with carb demand.

That's the short answer. Inadequate pump.