I used to have a sputtering problem near ½ tank and below before I fixed my gas cap. I simply took it off, tapped it with a hammer for a bit. Put in a bunch of WD40 and let it set for awhile (15 – 20 min) and then started tapping it again. I then smacked it a few more times a bit harder on the concrete floor in the downward position (oriented as if it were on the tank). Problem solved. Not only does it not sputter anymore at ½ tank… The bike overall runs better at about 7/8 of a tank and below.