Best Mechanical Fuel Pump

67Dart273 said
This IS odd.
...But the odd thing is that WOT seems to trigger it. Mechanical pumps "pump" by means of the spring in the pump. That is, the cam "cocks" the pump arm, and the actual "pump" is when the spring pushes the arm back down. What this means is that WOT should NOT affect pump pressure.

Sorry I'm going to hijack, but hopefully this helps us understand the problem.

I had this same issue as I described in one of my previous post in this thread. I'm using the same FBO pump, with 3/8 pickup and 3/8 line all the way to the carb. The reason I believe it happens at WOT throttle is not because of pressure increase, but because of the increase of flow at the rear bowl's needle and seat. Normal driving the rear bowl float never opens, but when you nail the accelerator the flow increases at that point (needle and seat). Couple this with a poor sealing needle and seat, and/or no vent "whistle" and the inertia of car slowing or stopping quickly forces extra gas up and out of vent tube.

I removed needle and seat, cleaned it really good with carb cleaner and adjusted the rear float just below the opening. It took a lot of trial and error to get the float adjustment correct because to check it I had to drive it and open the throttle all the way. Idling in the garage and cranking open the rear butterfly helps get it close but a test drive is the only way to tell if its really fixed.