Gas Grill Burners with yellow flames coming from beneath them

I've got a Char-Broil Commercial Series LPG grill, model #463240904. All 3 cast-iron burners had massive yellow flames near their back ends. At first I thought the flames were coming from the burners' side edges, where rust had eaten away enough metal to connect up to 4 side burner holes on each burner.

Today I patched the rust holes along the side edges on all 3 burners with standard JB-Weld (company rates product for 600 degrees). After the J-B Weld set I drilled new side holes, matching the diameter of the ''normal'' holes nearby (The holes get larger going from front to back along the sides).

When I put the burners back in the grill and fired it up I had the same large yellow flames near the back ends of all 3 burners. Looking closer I saw that the flames were actually coming from BENEATH the burners, not out the side holes. Each burner has a slot on the undersuface about 3'' from the back end, extending across the entire width of the burner. The slots looked ''normal'' to me, not a series of holes connected by rust damage. So much LPG is flowing out these slots that the flames come out each side of the burners about 3 times the length of the blue flames coming out the side holes. The gas flow is too great for the oxygen supply, so the flames are yellow. We're going through LPG like crazy, which I'm assuming is due to the LPG flowing out the undersuface slots that run the full width of the burners and are almost 3 times wider than the largest side holes near the back end of the burners.

My questions are:
1) Are there supposed to be slots beneath the burners that span the entire burner width, or are there supposed to be a series of small round holes like the ones along each side edge?
2) I don't know if the slots have to be present for ignition of LPG at start-up. If I seal the slots with J-B Weld and don't drill holes like I did along the side edges, will the burners function properly with only the side holes?
3) If the under-surface slots are sealed, is there any danger of excessive back-pressure in the burners causing a flashback of flames back to the venturis at the front ends of the burners?

Thanks in advance for anybody with any ideas and/or experience with yellow flames shooting out these slots.