When I built the TOTW 1742 Long Land Pattern kit, I drilled a 5/64" vent. Start there, shoot it, and see if you get miss fires. As stated previously, you can always go up a size up if you have to. I do get an occasional flash-in-the-pan with the 5/64 vent. I'm going to shoot mine some more and see if the vent enlarges itself before I go up a size.