I'm going to try and answer a few questions here.

Cowl on or off, and can it make a difference? Unfortunately that's a yes and no answer. Nothing wrong with running with the cowl off. When the plane is on the ground it makes no difference at all and makes things a lot easier to tune. This is why I do about 98% of the tuning process in the first half tank of gas on the ground. Then the cowl goes on.

Some planes create an issue with pressurizing the diaphragm once the cowl goes on. usually, not always but usually, it's a plane with a round cowl. That's a separate issue and not handled with tuning. Requires a little work with a diaphragm tube.

Burbal good or bad? It's not good, but it may be safe and acceptable depending on the degree. Most of this can be removed with tuning (needles), but sometimes not all of it. There are methods of eliminating the burbal but few people want to purchase spring accessory kits in order to experiment with spring tension. For practical purposes most should limit internal cheacks of the carb to things like metering needle height and the correct installation of the pump and check valve diaphragms.

Fuel line? Type F Tygon works extremely well. So do other grades of Tygon but they need be replaced more frequently. That frequency is reduced when a clunk heavy enough to do the job is used. Viton is very good. Neoprene is very bad. You can get as fancy and expensive as you want but you should still be checking a possibly replacing fuel line once a year. if you're not doing that you're either cheap, lazy, or both. No fuel line is going to help you if that's the case.

Size fuel line and brass tubing? 1/8" fuel line and plumbing will handle the biggest engine most of you will ever fly. So will that Walbro filtered clunk I always end up talking about. Call back when you bust 290cc's and we'll talk about other sizes.