Am wondering whether an ample supply of clean fuel is going into the carbs.
Crud inside the tank may hover suspended in the fuel. When running, the suspended crud may be drawn toward and collect on and obstruct the petcock fuel intake parts fitting up inside the tank. The sensation when riding feels like running out of gas. In this situation, switching to reserve sometimes adds a little more flow from inside the tank via the petcock reserve intake and then on through the petcock outlet and fuel line to the carbs.
Would suggest the following to avoid re-contaminating the carbs (despite repeated efforts, they don't stay clean):
Remove petcock from the tank and clean the area that fits up inside the tank.
Disassemble and clean petcock mechanicals.
Drain fuel tank and flush out with fresh gas to remove existing loose crud, grit, etc. Add fresh gas and physically shake the tank and rapidly drain it out the gas cap filler hole. Repeat several times until only clean fuel comes out.
Attach petcock to fuel tank.
Test flow of fuel through the petcock.
Use new fuel line or use existing line only after cleaning and blowing the line clear with compressed air.
Install a new in-line fuel filter.
The above steps should assure an ample supply of clean fuel going into the carbs. And until so assured, other testing and adjustments are problematical.
Realize this has been an exasperating ongoing problem for quite some time, and hoping to help get your bike back in business.