Not sure how well you designed your intake for the venturi, but it can make a massive difference. Simply put, the best end design is a doughnut (torus). Split it in half like a bagle. Then lay it cut side down on the table. Looking down at it now, the air going into the center of that is going to flow better and have the least problems. And since you are only concerned with what goes into the hole, you only need the inside 1/2 of that form to make the air flow properly. The typical bell reducer is a poor choice, but can be made to work as you've already seen. You can look at any commercial central heat system using gas and see the same form. Now there is also a slight taper to the tube at times which you can research if really want to get a great burner design. In the meantime, eat the doughnut and ponder the possibilities.
If I can remember later I'll send out a sheetmetal version that can be bent to tube for the proper flame holder and intake for a venturi. Just need to know the inside dimensions of the forge you are trying to heat and the insulation thickness. Really should make that into a program to save time.
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