I don't have a problem running without a filter on the track, I keep the two metal screens in place and just remove the foam. The BIG issue is having to change the jets for optimum performance, thats a PITA.
The 64's with or without the air cleaner element "felt" about the same, power wise. BUT a big ass stumble with the 64's and no element. Clearly to lean.
Maybe I'll try 66's and a filter, I'm getting pretty fast at taking the carbs off and tearing them down.
OH, 6.5 power valve is the ticket for my setup. Which is interesting because the "rule of thumb" for a PV is half of whatever vacuum signal your idle makes. My vacuum signal is 12", half that, 6.0. But I got a BIG cam, 6.5 feels real good.