Cracked primary burners

Sounds like the burner is cracked and it is lighting back to the injector

You have a blocked burner lack of air causing yellow flame >I would also check to see if burner is cracked as this would cause light back .

Take the burners out and check for a blockage in the valve orifices, then check the burner ports for clockages, and the inside of the burner also.

Hello,
It sound like your back burners have a partial blockage or possibly are rusting or falling apart or have cracked seams. Remove the cooking grids and burner shields, and examine the burners closely. If you can't find a blockage inside (spider web, bugs nest, etc) then their condition is probably deteriorating. Let us know if you need more assistance.

Is it still under warranty? I'd call Viking or a qualified repairman to fix it. If your burners are clogged, the flames should be coming out the venturi tube and flaming towards the gas valve knob. If the flames are coming out of the gas valve itself, then it will need to be regreased. If the burners can be removed without too much trouble, soak them in the sink with hot water and dishwashing detergent. Be sure to remove the knobs, so no one turns the gas on without the burners connected. If it's difficult, call a repairman.

Have you clean the grill!

yellow flame is lack of air so burner jets are blocked they need cleaning out all the flame will be the same size along the burner

Remove the cooking grate so you will have unrestricted access, remove the flame tamer (it should be able to come up without effort) to expose the burner. At the back of the grill there is a screw that attaches the back part of the burner to the grill. Remove the screw, then grip the burner and pull it towards the back of the grill, this should release it from the gas valve in the front. To put the new burner in, just do all these steps in reverse...let me know if this works for you, if not I can walk you through it via chat.

if the unit is running on natural gas, this is not unusual. if running off propane tank something is wrong with the regulator

Remove the burner and check the flexible tubes for kinks or blockages. The flexible tubes usually just slip over the backs of the valves. Spiders love to get in there and block them. Also check that the holes in the burners are not clogged. Be careful when reseating the flexible tubes to be sure that they are properly seated on the back of the valves. Failure to properly seat the flexible tubes can cause leakage there, and the gas can ignite at the back of the valves and not reach the burner. Be certain that the replacement burner is designed to use with your grill and the flexible tubes mate properly with the back of the valves.