With this style of box carto's are really not ideal. The MB's feeding system is flush with the 510 connector, and for cartos you'd really want it to extend into the carto some (like the wetbox would). I can't really see carto's getting fed all that well with the system Tom's got going here (not that that's a bad thing).
But to help you some, if you look at the 510 connector (where you'd screw the carto into on the box) you'll see that it's got 4 little breather slots, this is where the air flows for the "draw." If by chance, the tube that comes from the juice bottle to the connector is going into the hole on your carto at all, it'll block that airflow. This is what I'm guessing is happening to you.
If that is indeed the case, you've got two options for a fix, the quick way that'd be not so great with carto's is just to simple snip that tube down a tad bit, till it's not obstructing the suction, but keep in mind you want it flush with the 510 connection. Or, if you'd rather use cartos, you could rig something (maybe a needle?) up to replace that plastic tube at the top of the feeder system with something of a smaller guage (so air can pass by around it), and leave it longer than the original tubing so that it can access the meshing inside the carto.
As always though, do this stuff at your own risk, I'm no Tom, I'm just trying to help based off my experiences with these things.
what about a 510-510 adaptor to raise up the carto?