1981 Other Mitsubishi Models

Hi, i know my carb pretty well... i've rebuilt it many times including just recently but i'm not sure what to make of this.

On the accel pedal, i press to accel but theres a flat spot in accel. The engine hesitates for a moment. So i checked the accel pump. works great, good strong jet of fuel BUT only on the release stroke of the throttle. For example, i open the throttle, when it closes i get the good jet of fuel. I've looked and looked at it but can't figure out why its doing this. Any ideas??

Thanks,

mike