If you experience a lot of spitting back through the carbs when cold ( not sure if you are running the original choke slides or not...??) and even if your slides have been sleeved or whatever.... you can still easily put the slides in the lathe and machine from say 20 to 30 thou off the bottom, then refit as per normal, and this will help fix the problem. I made up a soft alloy mandrel to slide neatly inside the slide so as not to crush the sides in the lathe chuck, and i generally take off 30 thou and refit.....air filters will always create some form of drag no matter how free breathing they "Look" and normally if you run them you may need to lean off the idle mixture a bit ( on 276/289 ) carbs, screw in is RICH and out is LEAN, and maybe drop the needle by one or two notches......Just do some spark plug colour checks after a good run.....let it cool off first though....!!!.......Greg.