fuel delivery

Hi, I have a mikuni 22 that I'm having some problems with. When I try to feed it fuel, fuel doesn't even go down the line into the carb. But the weird thing is, that when the carb is attached to the manifold it does this, but when it's off, it flows perfectly fine. I tried cleaning it twice...

Got a GX200 Clone, has header, GX390 carb, 18LB. springs, air intake. Just added Mikuni fuel pump because engine bogged on bottom end (I figured this was due to poor fuel delivery). Engine runs fine with choke on and rides fine until you slow down to near stop and it just cuts out. I do not...