A couple ideas: One, I'd guess that by going from a 54 to a 64, you've gone from a too-small jet to a too-big one, and that could be causing your speed problems. I have a 64 on my Newport, but that's with a high-flow filter and a biturbo, you might do well with something like a 60.
Two, check for air leaks where the carb clamps onto the intake. Mine has a leak in the little cutout at the top of the carb, right where it says "bing." It's been a constant headache for me, still haven't figured out a good solution. Spray some carb cleaner around there and see if it makes your engine die. If it does, you've got an air leak.