Homemade propane bbq problems

I have a problem. I have been assigned the task of taking a homemade charcoal grill and converting it to propane gas. The guy before me tried and while it works it scares the hell out of me and it doesn’t work very well.
The grill is a 20 inch pipe that is 6 feet long. It has a flexible gas line and comes out of the ground to a steel ball valve. The line then splits to go to each end of the bbq where there is another ball valve. On the end of that there is a home made contraption built out of a two-to-one steel pipe reducer that I assume lets the gas mix with air. There is a brass orifice inside of the reducer. Then a ¾ inch steel pipe goes into the bbq then splits two run two ¾ inch pipe burners.
They problem with the whole deal is while you do have a flames in the bbq it is not an even flame and there is gas back flowing out of the reducer. Due to uneven cooking they would like me to add a third pipe burner in the center of the two existing ones.

Does anyone have any ides?

Thanks
Jeff