There is a thing called gas flow science that is being ignored here. When you cut, bend, weld, or otherwise modify an engineered system, you change the characteristics of that system. When you are dealing with gas pressure that is a fraction of 1 psi, it's not hard to imagine that you likely have turbulence in the welded area that is affecting or impeding the gas flow. I don't think dumping more gas into the burner is the answer bacause at some point you may encounter a flashback situation...but like you, I'm just guessing and I don't really know. But if there ever is an incident, it's all documented right here about how you modified an outdoor gas appliance.
Would it be possible to for you to reconsider the burner situation?