I rebuilt the pumps a couple nights ago. I was able to detect a small amount of leakage through the valves of the return pump. On the stand that night I started and stopped the kart numerous times without issue.
I brought my daughter out to an industrial park to check it out today. It started on the first pull. After warming the motor I shut it off and push started it with my daughter. This was the last time it would start without issue. We ran the car through the gears and then stopped/restarted it roughly 10 times. Every time, the motor would not start unless I blew through the vent lines to force fuel back to the tank. The motor would then start in gear immediately or after continuing to push in gear for a short time.
Would it make any difference if the pumps were mounted lower than the carb. Since there is no float, could fuel be running down into the bowl when the motor if off.