Try making the head bob up and down; right now it's just kind of... frozen in place and looks odd. Try moving it down one pixel on your second frame, then back up on the next, down the next, ect.
there's only 2 frames really (technically 4 frames, but two sets of arms/legs), and im worried that a 1 pixel bob would look jittery at this speed. ive heard people use subpixeling? i can give him a 4 frame loop at most
edit: does this work?
Close. I'd try experimenting with lowering the helmet itself too, as you did with the head.
The problem is that the body is static and his feet never come off the ground. It should be every frame that the feet are fully extended, (one foot all the way forward, one back) he should come off the ground by one pixel. It might be helpful to see each individual frame, as well.
update!!
edit with olimar's
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Oh wow, really nice job!!! It's so hypnotizing to watch lol