Gating and reservoiring in this manner will allow for your cast to "directionally" chill. What you have going on is... your main sprue, once the mold has been filled chills virtually just as fast as your riser did. Your rounded hump section on top chilled (with some puckers of its own, by the way) second. Your main chunk of cast product chilled last while sucking what it could from its smaller appendage. I wouldn't be surprised to see a crack between that main pucker and the sidewall of that humped appendage just beneath the surface of your casting.
If your flask will not allow sufficient depth to create a reservoir that size, pack a plastic bowl/tupperware with sand, whatever will give you room to carve the reservoir you need, first outlining it's dimension onto your packed sand mold surface and carving a recessed imprint of the bowl into the surface of your sand mold, then flip that packed bowl over and onto your carved out niche. Tap the bowl to free the sand, remove the bowl, gently (but firmly - without crushing your packed mold) press that sand block onto your mold surface (do not apply talc to either, by the way). This is called an "upset". It will give you the depth needed to carve a decent reservoir into and give your product some head pressure. At this point, using a narrow spoon, compress some hand distributed sand at the seam between the mold surface and the upset side wall. Next, Carve out what you need for reservoir size, then punch your spue gate, trowel down the inside of the reservoir with a spoon to smooth and pack loose sand. Have waiting a stand you can lift that cope section of mold and set onto so you don't have to flip it onto its side to clean things up before closing the mold. Simply blow diffused air from a compressor onto the mold face and onto the mold's top where the upset is located. Remove the pattern from the drag, clean that half of the mold up, then close the cope and drag together. Bet that does the trick
Another option would be to gate with a reservoir (as described above) directly into the center of that largest section and riser out direct center mass of that upper hump section. You will need to have your reservoir higher than the surface of that raised hump section. And in both cases described, your reservoir feed into the cast piece has to be as short as you can get it (and still give you room to put a saw blade to for later removal) and it also has to be wide enough to ensure it will not chill till your cast product does. Guaranteed, with this method, that riser will chill first, that hump section second (suck'n what it needs from that large piece) and your main section last draw'n what it needs from the reservoir. Clear as mud??? HAHAHA.... if you need a sketch let me know.
