My Suburban was just like yours.
The Pilot ran fine and when I turned the gas valve to "ON" the flames came out the big holes at the base of the main burner, not out of the slots in the top.
It turns out that the burner is a two-part assembly screwed together.
You can CAREFULLY use some pliers or vice-grips to unscrew off the part with the slots from the lower part that is bolted to the base tin sheet.
I found a spider nest in there as well as in the main burner orifice.
When I cleaned that all out (along with the wasp nests in the intake and exhaust) the furnace
started running fine.