When you pour there is a tendency for the sprue to stop taking the metal until it kind of burps. I’ve been attaching the sprue to the part at an angle. The sprue is vertical and the part slopes downwards. I am thinking that the sprue angled downwards at about 45 degrees would work better. The angle would allow the hot metal to flow down the bottom side of the sprue and under the foam allowing the hot gases to go up the sprue without causing a burp of metal. The part should slope the same direction so that the metal goes smoothly all the way to the bottom.